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Section 5. Development of an observational programme
Lead author: David Hydes
This review is necessary so that the most appropriate methods for measurement are
used during the monitoring programme. It will cover:1. The accuracy and precision of methods will be assessed with respect to those
required to detect change at the present rate of atmospheric CO2 increase and the
corresponding CO2 uptake into the coastal seas.
2. Existing measurement methods currently in use in the UK and abroad will be
reviewed in the light of whether they might be appropriately introduced into the
programme. The review will include considerations such as availability, ease of
deployment and operation, purchase and running costs, as well as maintenance
requirements.
3. The introduction of errors from the sampling and measurement process will be
assessed.
4. The reliability of determining pH on the basis of measurements of pCO2, salinity
and temperature will be assessed.
5. A key development in marine monitoring is the development of automated
monitoring methods and the use of chemical sensors. These developments will be
reviewed from the point of view of possible applications, and the timescale for
implementation in a UK coastal monitoring program.
6. The components of the carbonate system to be measured in a monitoring
programme will be clearly defined.
7. Quality control procedures will be defined.
8. Procedures for data reporting and the assembly of meta-data will be defined.
9. As part of the development of the strawman a review will be carried out of the UK
capacity for the collection of appropriate samples as part of existing observational
programmes and more continuous observations such as may be based on Smart Buoy
or Ferry Box technologies. The best ways forward for combining this capacity and
measurement technologies will be recommended.
5a A critical review of existing and developing technologies for measuring the
variability and change of pH.
Background
This review is necessary as there is currently no accepted, easily available, procedure
for the monitoring of the pH of seawater. To make a decision on the way forward the
accuracy and precision of methods available need to be assessed against the precision
needed to detect change in the pH of the marine system and the likely reliability of
their use in a monitoring programme.
The oceans are absorbing carbon dioxide (CO2) from the atmosphere and this is
causing chemical changes by making them more acidic (that is, decreasing the pH of
the oceans). In the past 200 years the oceans have absorbed approximately half of the
43
CO2 produced by fossil fuel burning and cement production (Sabine et al., 2004).
Calculations based on measurements of the surface oceans and our knowledge of
ocean chemistry indicate that this uptake of CO2 has led to a reduction of the pH of
surface seawater of 0.1 units. Because the pH scale is logarithmic this is equivalent to
a 30% increase in the concentration of hydrogen ions. If global emissions of CO2 from
human activities continue to rise on current trends then the average pH of the oceans
could fall by 0.5 units (equivalent to a three fold increase in the concentration of
hydrogen ions) by the year 2100 (Royal Society, 2005). At pre-industrial levels of
atmospheric CO2 (280 ppm or µmol/mol) the average pH of seawater is estimated to
have been 8.18, at the present level of CO2 (380 ppm) the average pH is 8.07. Few
measurements have been made to document this change against the natural variability
present in the oceans. The pH of the world’s oceans varies both on a regional basis
and in line with the annual cycles of temperature and biological growth (which
consumes CO2) and respiration (which produces CO2). “Figure 4” in Royal Society
(2005) shows that the global range in 1994 was about 7.90 to 8.20. If current trends
continue the atmospheric concentration is predicted to reach 700 ppm by the year
2100 (IPCC 2001). This corresponds to a decrease in the average pH of the oceans of
0.5 pH units. At the present annual increase of atmospheric CO2 of 3 ppm, pH is
decreasing, on average, by 0.003 pH units per year. This defines a target precision for
monitoring.
Recommendations
For immediate application, to achieve a resolution in measurements of pH close to the
annual rate of change of pH of 0.003, the best approach is to calculate pH from
measurements of other carbonate system parameters. This can be done using
established systems available in the UK and for which the quality of the
measurements and procedures needed are well documented and understood. This can
be done by the analysis of water samples to determine the total alkalinity and total
CO2 content of samples. This should achieve a precision approaching 0.006pH.
Combination of total alkalinity measurements with measurements of the fugacity of
CO2 (fCO2) from ships should allow pH to be calculated to precision approaching
0.003pH. Using both approaches “over determines” the system allowing the reliability
of the measurements to be assessed. The draw back of this approach is the high cost of
processing water samples (approx £30+k per year). We recommend that future
monitoring of ocean pH be based where possible on direct measurement of pH.
Within the proposed Defra monitoring plan for the UK (2008-2010) we propose that
an electrode based system developed for the Environment Agency be built and
evaluated. This system may provide a reliable way of making measurements with a
precision which meets the target of 0.003 pH units and, with the use of appropriate
calibration protocols, a similar degree of accuracy to the precision. These
measurements would be crossed checked against determination of the pH done
through calculations based on the measurements of other carbonate system
parameters. £50k would need to be invested to realise the potential of electrode based
measurements.
In the longer-term measurement of pH should be done using colorimetric
measurements. These methods are under development for use in monitoring. They
offer a higher precision (<0.003 pH) and the likelihood of being more reliable than
electrodes in routine use. To make such systems available by the end of 2008 would
require an investment of £200k. This would generate a saving in later years by
removing the need for laboratory-based measurements.
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pH scale
The total hydrogen scale, (also called the total proton Hansson pH scale, Hansson,
1973) is the recommended pH scale (DOE, 1994), with hydrogen ion concentrations
reported in mol kg-1. An appropriate programme for thermodynamic calculations of
the CO2 system is CO2SYS, which is available from the server of the Carbon Dioxide
Information Analysis Centre (http://cdiac.esd.ornl.gov/oceans/co2rprt.html). For
calculations the CO2SYS programme should be used; this uses the HSO4- constant
from Dickson (1993) and the carbonic acid constants from Mehrbach et al. (1973) as
refitted by Dickson and Millero (1987).
Direct measurement
Potentiometric pH measurements require relatively simple, low cost instrumentation
and can be readily automated. The main drawbacks of the potentiometric approach
can be the errors associated with the irreproducible liquid junction potentials,
electrode drift between calibrations due to ion adsorption on membrane surfaces and
both standing potentials and background electrical currents, especially during
shipboard measurements (Bellerby et al., 1993). The introduction of free-diffusion
liquid junctions has greatly reduced problems with potential errors, and provided high
quality pH seawater data with an overall precision of 0.003 pH unit (Whitfield et al.,
1995).
The application of seawater Tris buffers instead of NBS buffers has also resulted in
much improved seawater pH data using the potentiometric approach. Based on the
knowledge gained in the studies lead by Whitfield, an electrode based system was
developed by PML for the UK Environment Agency in the 1990s. In use, it has
achieved precisions approaching 0.003, although its long term stability (weeks to
months) has not yet been fully evaluated. We suggest this as the method of choice for
a UK monitoring programme in shelf seas waters where the system can be regularly
maintained and the quality controlled by reference to determination of other carbonate
system components.
Colorimetric measuring systems for determining pH which can operate autonomously
are currently being developed. Such pH systems based on spectrophotometric
measurements of a dye indicator provide pH data with an accuracy of ±0.005 pH
units and an overall precision of ±0.0004-0.001 pH unit. To date these system have
only been used for research purposes and they have been “one offs” (Friis et al., 2004,
Bellerby et al., 2002). They offer the promise of being more robust and more accurate
with wider applicability in terms of platforms than electrode based systems.
Investment of the order of £100k over one year would be required to produce a system
which could be cloned for less than £30K per instrument. Greater investment would
be needed to produce units that could be replicated for £10k.
Calculated pH
The UK has substantial experience and well established procedures for the monitoring
of the partial pressure of CO2 in seawater on a range of platforms, notably research
ships and ships of opportunity. The UK also has the capacity to carry out
determination of the Alkalinity and Total CO2 content of seawater samples to the
degree of precision required to determine the pH of seawater to better than 0.01.
We recommend that this capacity is used to:-
45

Determine the pH in all subsurface waters sampled by a monitoring
programme.

To provide verification of the direct measurements of pH made in surface
shelf sea waters
Supporting evidence
Definitions
pH is a master variable in solution chemistry, which exerts control over equilibria and
kinetics of a wide range of biogeochemical processes in the oceans. Oceanic pH is
controlled by the CO2 system, through buffering processes of carbonates and carbonic
acids. Four different variables of the oceanic CO2 system can be determined: Total
CO2 (also called dissolved inorganic carbon), the fugacity or partial pressure of CO2
(fCO2/pCO2), total alkalinity (TA) and pH.
pH (the definition is complex and described in detail below)
Total alkalinity (TA); TA= [HCO3-] + 2·[CO32-] + [B(OH)4-]+[OH-]-[H3O+] plus
other minor components - see later
Total inorganic CO2 (TCO2) (i.e., the sum of the dissolved CO2, the bicarbonate, and
carbonate ions TCO2=[CO2] + [HCO3-] + [CO32-]);
Fugacity of CO2 (fCO2) The partial pressure of CO2 (pCO2) is the pressure that CO2
dissolved in a water sample exerts on overlying air (pCO2). The pCO2 is defined to be
in wet (100% water-saturated) air and is a function of the solubility of the gas and the
dissolved CO2 concentration. The fugacity of CO2 (fCO2) is pCO2 upon correction for
the non-ideal behaviour of the gas. The fugacity is about 0.3% to 0.4% lower than the
partial pressure over the range of interest in natural waters.
TA and TCO2 are independent of temperature and pressure; while fCO2, pCO2 and pH
are not.
Determination of two of these variables (along with the temperature, salinity,
pressure, abundances of other constituents of seawater) will allow the calculation of
the other two, because the relevant equilibrium constants (K1 and K2) are well
established.
CO2 exchange with the atmosphere is controlled by Henry’s law
CO2(gas)
=
CO2(aqueous)
CO2 + 2 H2O = H3O+ + HCO3-
K1
HCO3- +H2O = H3O++CO32-
K2
The accuracy and precision obtainable in such calculations has been considered in a
number of papers (e.g. McElligot et al., 1998). Other work has gone into the
estimation of the second parameter where only one has been measured. This is
possible for alkalinity, which often follows a near-conservation relationship with
salinity (e.g. Lee et al., 2006). Unfortunately, this is not as easy as it sounds. Two
definitions of alkalinity are in current usage and they differ in how minor species are
treated. There are four different pH scales [total, seawater, free, and NBS (National
Bureau of Standards, now the National Institute of Standards and Technology)] in
current usage (it is even more complicated in the literature where the distinction
46
between the total scale and the seawater scale hasn't always been made). There are
several different formulations of K1 and K2. Therefore it is critical that in any work
on measuring carbonate system parameters detailed meta-data accompanies any
reported data and that all the relevant parameters are determined, so that like with like
can be compared in the future.
Definition of pH and pH scales
The activity of a species i, is defined as the difference between the chemical potential
of the species in the sample solution and its chemical potential in a reference state,
referred to as standard state:
µi - µi◦ = RT ln (ai) = RT ln (ci γi)
(1)
where µi and µi◦ are the chemical potentials (J/mol) of species i in the actual and
standard states, respectively, ai is the activity of species i, R is the gas constant (in
J/(K·mol), T is temperature (in K), ci is concentration on an appropriate concentration
scale and γi is the activity coefficient. The activity coefficient is by definition unity in
the standard state (γi1 in pure water), but is less than one in seawater.
The pH is defined from the activity of the hydrogen ion:
pH = –10log (aH+)
(2)
As a solution with zero ionic strength (corresponding to the standard state) cannot be
prepared, and because single ion activity coefficients cannot be determined, it is not
possible to measure pH as defined in equation 2. Therefore an operational definition
based on potentiometric measurements and an activity coefficient convention has
been introduced, and uses buffers with assigned pH values which are close to the best
estimates of –10log (aH+). This scale is known as the NBS pH scale.
For seawater measurements, the low ionic strength NBS buffers cause significant
changes in the liquid junction potential between calibration and sample measurements
when using an electrode system. Unless the change is carefully characterized for each
electrode system, the error introduced is larger than the desired accuracy of 0.010.001 pH units (Wedborg et al., 1999).
The situation has been greatly improved by the introduction of pH buffers based on
synthetic seawater, which have a composition close to that of the sample, thereby
reducing the liquid junction potential changes between calibration and sample
measurement.
The seawater pH scales are based upon the adoption of seawater as the standard state
(thus setting γi1 in seawater), with concentration and activity being identical (see
equation 1). Three different seawater pH scales have been defined, based on differing
ways to define the hydrogen concentrations. The free hydrogen ion scale (pH(F)) uses
the concentration of free hydrogen ions to define the hydrogen ion activity (Bates and
Culberson, 1977):
aH+(F) = [H+]
(3)
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pH(F) = –10log (aH+ (F))
(4)
As a proportion of acid added to seawater is bound to sulphate and fluoride ions, the
concentration of free hydrogen ions cannot be determined analytically. As fluoride
forms a minor component of seawater, a fluoride-free synthetic seawater was adopted
by Hansson (1973). This approach provides the total hydrogen scale (pH(T)):
aH+(T) = [H+] + [HSO-4] = [H+] {1+ KHSO4-[SO4]tot}
(5)
where
KHSO4 = [HSO4-]/([H+][SO42-])
pH(T) = – 10log (aH+ (T))
(6)
Dickson and Riley (1979) and Dickson and Millero (1987) proposed inclusion of
fluoride in the buffer, and this yielded the seawater hydrogen ion concentration scale
(pH(SWS)):
aH+(SWS) = [H+] + [HSO-4] + [HF]
= [H+] {1+ KHSO4-[SO4]tot + KHF[F]tot}
(7)
where
KHF = [HF]/([H+][F-])
pH(SWS) = –log10 aH+ (SWS)
(8)
The pH (T) scale is used commonly and the recommended scale for our monitoring
activities (, 1994). An important advantage in the use of this scale is that problems
associated with the uncertainties in the stability constants for HF are avoided and the
preparation of appropriate buffer solution is simplified.
Seawater buffers
Hansson (1973) selected Tris (2-amino-2-hydroxymethylpropoane-1,3-diol) as buffer
compound, and (AMP) 2-aminopyridine has been added to this (DOE, 1994 - SOP 6
see Appendix) to check electrode system response. The DOE report (Dickson and
Goyet, 1994) has provided a procedure for preparation of the Tris and AMP buffer
solution at salinity 35.
Direct determination of pH in seawater
Determination of pH using potentiometric methods.
The potentiometric method for the determination of pH is based upon the
measurement of the cell electromotive force (emf) in a system comprising a hydrogen
48
selective electrode and a reference electrode (is described below following DoE 1994
SOP 6). The pH determination is based upon the Nernst equation:
Buffer: Estandard = EC – RT ln(10) · pHstandard
F
(9)
Where Estandard is the emf measured by the pH cell in the standard buffer solution, Ec is
an arbitrary constant and F is the Faraday constant.
Sample: Esample = EC – RT ln(10) · pHsample
(10)
F
Where Esample is the emf measured by the pH cell in the sample solution
Combining equations 9 and 10 gives:
pHsample = pHstandard + (Estandard - Esample) · F
RT ln(10)
(11)
It is assumed that Ec is constant for the standard and sample solution measurements.
However, changes in the liquid junction potential between different solutions will
result in changes to Ec. The use of seawater buffers will minimize this error.
Covington and Whitfield (1988) considered this in depth for the International Union
of Pure and Applied Chemistry.
pH cell
The highest quality pH measurements are obtained using separate glass and reference
electrodes. The combination glass / reference electrode may typically be the most
convenient to use, however it suffers more strongly from liquid junction potential
errors. The best reference electrode arrangement includes an open free-flow liquid
junction (Butler et al., 1985). Following on from the work of Whitfield PML
developed a fully automated system, which employs an open free-flow liquid junction
between the reference electrode and the sample cell to reduce the liquid junction
potential errors. A number of these units were built for the Environment Agency in
the mid 1990s and proved to give data which fully meet the expected improvement
from using free-flow liquid junction to give precisions of the order of 0.003
(Whitfield et al., 1995). The EA (Andrew Wither) have made instruments available to
NOC and PML for evaluation in this project.
Figure 5.1 below shows a schematic of the (EA Whitfield et al) potentiometric pH
system currently in use at the NOC.)
49
Potentiometric pH system
Embedded
PC/ Data
acquisition
Sample/ Buffer out
Container with
2.5M KCl
electrolyte
V
Platinum
Resistance
Thermometer
(PRT)
Electronics box
12V DC Power
supply
Peltier
controlled
reference
block (20ºC):
Ag/AgCl
electrode
pH cell / Glass
pH electrodes
V
Capillary
liquid
junction
Valve/Pump
housing
(V= solenoid valves,
P= peristaltic pump)
P
Tris buffer
V
Seawater
sample
Figure 5.1 Potentiometric pH system for pH determination in marine waters.
Components
Voltmeter
It is important to use a voltmeter with a high input impedance (or resistance to
alternating current) because of the high inner resistance of the glass electrode. The
emf of the glass / reference electrode cell can be measured with a pH meter, as these
instruments fulfill this requirement. The sensitivity in pH measurements is ±0.002 pH
units, in case a pH meter with a sensitivity of ±0.1 mV is used for emf measurements.
The sensitivity can be improved to better than ± 0.001 pH units by using a high
sensitivity voltmeter with a high input impedance (>1013 Ω) (DOE, 1994).
Calibration and measurements
The pH of water is operationally defined and therefore it is not possible to define an
absolute value for the accuracy of pH measurements. The accuracy can only be
defined as the reproducibility of the system against an agreed standard, such as
measurement of pH derived from the determination of other components of the
carbonate system. The quality of the pH determination is determined by the accuracy
and precision of the preparation of the Tris seawater buffer. A precision of 0.01 pH
units is possible with commercially available reference half cells without additional
actions. If free flow liquid junctions are used, the precision can be improved to 0.002
pH unit (Covington and Whitfield, 1988).
The sample and buffer pH and the Nernstian responses of the electrodes depend on
temperature. Consequently, the temperature should be known (using a thermometer
50
accurate to ± 0.05 °C) or controlled to within 0.1 °C during the measurement. The
calibration of the potentiometric cell should occur at the same temperature as the
sample measurements. Regular calibration using Tris seawater buffers needs to be
undertaken. Test of the EA system at NOC show that a precision of 0.003 is
achievable.
Spectrophotometric methods
Spectrophotometric pH measurements provide an alternative to potentiometric
measurements and offer the possibility of achieving precision better than 0.001. The
spectrophotometric approaches use multi-wavelength combinations to eliminate the
need to know the exact total concentration of the indicators used. (They are described
below following DoE 1994 SOP 7)
The most appropriate indicators for the determination of pH in seawater, include
sulfonephthalein indicators such as m-cresol purple, thymol blue and cresol red. These
indicator dyes can exist in three forms:
I2- + H+ ↔ HI-
KHI- =
[HI-]
[H+] [I2-]
(12)
KH2I =
[H2I]
[H+] [HI-]
(13)
and
HI- + H+ ↔ H2I
with a total indicator concentrations of
Itot = [I2-] + [HI-] + [H2I]
(14)
or
Itot = [I2-]· (1+ KHI- · [H+] + KHI- · KH2I [H+]2 )
(15)
At seawater pH, H2I is negligible and the reaction in equation 12 is dominant resulting
in:
pH(T) = 10log [KHI-(T)] + 10log ( [I2-] / [HI])
(16)
The principle of this approach is that the different forms of the indicator have
different absorption spectra. Thus the information contained in the composite
spectrum can be used to estimate [I2–] / [HI–]. The determined absorbance in a cell
with a path length, l, at an individual wavelength, λ, is given by the Beer-Lambert law
as:
A
   (HI  )[HI  ]    (I 2 )[I 2 ]  B  e
l
(17)
51
where Bλ corresponds to the sample background absorbance and e is an error term due
to instrumental noise. Provided that the values of the extinction coefficients: ελ(HI–)
and ελ (HI2–) have been measured as a function of wavelength, absorbance
measurements made at two or more wavelengths can be used to estimate the ratio [I2–]
/ [HI–].
In the case that only two wavelengths are used, and provided that the background can
be eliminated effectively by a subtractive procedure, equation 17 can be rearranged to
(assuming no instrumental error):
A1 A2  1 (HI )  2 (HI  )
[I2 ]

[HI ] 1 (I 2 )  2 (HI  )  ( A1 A2 ) 2 (I 2 )  2 (HI  )
(18)
the numbers 1 and 2 refer to the wavelengths chosen. The optimum precision is
obtained when using the two wavelengths at the maximum absorbance of the base (I2–
) and acid (HI–) forms. The various terms of ε indicate the extinction coefficients of I2and HI- at wavelengths 1 and 2, respectively.
The accuracy of pH determinations using spectrophotometric methods is constrained
by the accuracies of the stability constant KHI- and the molar absorptivity ratios in
equation 18. The indicator dyes have been calibrated using Tris buffers according to
Dickson (1993). The published accuracy for the Tris buffers is ±0.002 pH units. For
meta-cresol purple and thymol blue, the calibrations have been reported by Clayton
and Byrne (1993) and Zhang and Byrne (1996), respectively. For thymol blue, a
pH(T) relationship with temperature and salinity has been defined for the temperature
range 278<T<308 and 30<S<40 by Zhang and Byrne (1996).
Measurement procedures
The absorbance ratio (A1/A2) can be determined using a spectrophotometry in a
manual or an automated flow system. A stock solution of the indicator needs to be
made up. Typically a 0.2 mol/L solution of the sodium indicator salt is prepared in
deionised water or seawater. Addition of the indicator dye to the sample solution will
perturb the pH. Therefore, the pH of the indicator stock solution needs to be as close
to the sample pH as possible, and a correction of the pH perturbation due to the
indicator addition needs to be undertaken. The pH of the dye needs to be checked on a
regular basis, by the determination of absorbances at appropriate wavelengths using a
short path-length cell (0.5 mm or less).
Instrumentation
The majority of published results are for discretely collected water samples from
depth profiles. The measurement of absorbance values at several wavelengths can be
undertaken using a scanning spectrophotometer (McElligott et al., 1998), or a charge
coupled device (CCD) spectrophotometer. The bench top scanning
spectrophotometers with photomultiplier detectors have high quality optics, are
expensive and often not suitable for shipboard operations because of vibrations. The
successful application of miniaturized CCD spectrophotometers (e.g. Ocean Optics
USB2000) has been reported recently for pH measurements using an automated pH
system (e.g. Friis et al., 2004, Bellerby et al., Tapp et al. 2000). The
spectrophotometers with CCD detectors have the advantage of a small size,
ruggedness, low cost and a rapid whole spectrum scan.
52
In the manual approach for spectrophotometric pH measurements, the indicator dye is
directly added to the seawater in the optical cell. Optimal accuracy is obtained with
absorbance readings in the range 0.2-1, by using a path length of 10 cm, and a volume
of 30 ml, maintained at a regulated temperature. The use of a long path length cell is
also preferred as it reduces the required indicator concentrations and hence indicator
pH correction.
The determination of pH in seawater requires corrections for the perturbations made
by added indicator (1), and the difference between the sample measurement
temperature (2) and pressure (3) from the in situ conditions (Wedborg et al., 1999).
Correction (1) can be undertaken by doubling the indicator concentrations in seawater
solutions at different pH values. Linear regression of a plot of the absorbance ratio
versus indicator concentration can be used for the correction. Alternatively, a
theoretical titration of a sample with indicator can be used to construct an error
diagram, and the correction can be derived from this (Wedborg et al., 1999).
Correction (2) can be undertaken using empirical equations as proposed by Millero
(1979 and 1995). The accuracy of the correction is estimated as 0.003 pH unit.
Alternatively, pH can be converted to in situ pH using the pH-temperature
relationship calculated from concomitant DIC and pH values (Bellerby et al., 2002,
Hunter, 1998). Correction (3) can be undertaken using an empirical correction
proposed by Millero (1979).
Automated flow systems make use of syringe or peristaltic pumps for seawater and
indicator transfer. The mixing of the indicator and seawater can occur in a mixing coil
placed before the flowcell (Friis et al., 2004) or in the optical cell itself (Bellerby et
al., 2002). The flow is stopped when the absorbance readings are taken. Figure 5.2
shows an example of a CCD spectrophotometric pH system as described by Friis et al.
(2004). The system allows for autonomous pH measurements of discrete samples. The
systems described by Bellerby et al. (2002) and Tapp et al. (2000) are for continuous
surface water measurements. Knowledge of the temperature of the sample during
analysis is crucial, and high quality thermistors are required. Whilst some workers
undertake sample thermostating at 22ºC (Friis et al, 2004) or 25ºC (Tapp et al., 2000),
others work without temperature control and perform corrections utilising high quality
temperature data (Bellerby et al., 2002). The latter approach has major advantages
when working in low temperature environments, where heating of the sample would
cause bubble formation, and hence problems with the spectrophotometric
measurements, and also out-gassing of CO2. The temperature corrections will involve
small errors, as the pKHI- for Thymol Blue and other indicators are well known and
their temperature dependence is similar to that of the pKs for the carbonic acid system
(Zhang and Byrne, 1996).
53
Figure 5.2 Autonomous spectrophotometric pH system (Friis et al., 2004).
Indirect determination of pH
Measurement of pCO2 in surface seawater
At constant alkalinity, the pH of a seawater and the corresponding pCO2 are tightly
linked and inversely correlated. Measurements of pCO2 coupled with measurements
of alkalinity (or estimation of alkalinity see Lee et al., 2006) provide an effective
proxy for the determination of changes in the pH of seawaters.
The UK has a high level of capacity for the determination of surface seawater pCO 2.
Instrumentation has been installed on all NERC’s Research Vessels and the PML
vessel the Plymouth Quest and Bangor’s Prince Madog. UEA and NOC operate
systems on voluntary observing ships which provide consistent data sets recording
annual changes (Schuster and Watson, in review), this is also achieved by the use of
the Prince Madog in the POL Coastal Observatory, and by the use of the Plymouth
Quest for the PML E1 and L4 time series. The system on the RRS James Clark Ross
provides data from repeat sampling lines throughout the British Antarctic Sector. The
other research ships provide some repeat sampling lines (e.g. Ellet Line, AMT), but
mainly data from inconsistent oceanic routes which are better used for validation of
model estimates than as part of a direct monitoring programme.
Basic underway pCO2 system description
The basic design of most systems in use round the world is similar to the design
published by Cooper et al (1998). The latter was developed by Watson’s group at
PML with proof of concept funding from Defra. While different groups (e.g. UEA,
PML) have developed these systems independently, all those considered here run
according to these basic principles.
The pCO2 measurement system has two main components:-
54
1) The control of sample gas flow into the non-dispersive infrared gas analyser,
marked Li-Cor Detector in Figure 5.3 below.
2) The seawater equilibration system. The seawater is drawn from the ship’s water
supply at a 5 m intake depth. A bridge mounted GPS system logs position. The
seawater xCO2 is measured at ambient pressure and temperature. Seawater flow rates
are monitored and maintained at 2 – 5 l/min and gas flow rates are maintained
between 80-130 ml/min. The gas from headspace equilibrated with the seawater and
from the standards is circulated in rotation in a continuous loop. Two NOAA
calibrated gas standards of CO2 in air with values of ~ 250 ppm and ~ 450 ppm are
used as calibration gases. The data output is at ~ 30 sec intervals. The Licor measures
dry molar xCO2.
Figre 5.3 A schematic of the CO2 system gas flow paths and equilibrator design
reproduced from (Cooper et al., 1998)
pCO2 (fCO2) data correction
The data correction sequence described below is based on current practice and the
recommendations being used in the EU Framework 6 project CarboOcean in 2007
(Are Olsen Bjerknes Centre, Norway, pers comm). Comparison of procedures used by
different groups in CarboOcean in 2006 found differences of >6 µatm pCO2 when
groups carried out calculations on the same data sets. This illustrates that calculation
of in situ pCO2 from the measured values of xCO2 is complex and any reporting must
contain meta-data that fully describes the step and procedures used. The detail of the
data correction sequence is described below in the order it is executed. The aim is1) To remove those data points where the gas flows and water flows are out side
proscribed limits.
2) To remove the first data points in each run of the standards as the gas flowing
through the CO2 analyser is often contaminated with gas from the previous cycle run,
55
3) interpolate the standards using a Matlab function over the sampled time to generate
a standard value at each sample time,
4) Calculate the offset for each data point from the interpolated standards by linear
regression at each measurement
xCO2_corr = a * (xCO2_uncorr) + b
where a is the slope of the line and b is the offset,
5) Calculate the partial pressure, pCO2 from the mole fraction, xCO2 in the
equilibrator, dried prior to analysis (xCO2dry) using
pCO2 = xCO2dry (p_eq - pH2O)
where xCO2 is the mole fraction, p_eq (atm) is the pressure at the equilibrator and
pH2O (atm) is the water vapour pressure (Körtzinger, 1999). The saturation vapour
pressure of water is calculated from the equation
ln pH2O = 24.4543 – 6745.09/T – 4.8489ln T/100 – 0.000544S
(Weiss and Price, 1980). The salinity, S, is taken as 35. For evaluation of the vapour
pressure of seawater, the error if salinity is ignored altogether is ~ 2%, and even
smaller (~ 0.05%) if salinity is in error by 1 salinity unit. As pCO2 varies with
temperature, a correction is required to compensate for the difference between the
temperature of the sampled seawater, and the temperature by the time it reaches the
equilibrator. The CarboOcean team uses the Takahashi et al. (1993) equation
pCO2 at SST = pCO2_eq*exp(0.0423(SST - Teq))
where Teq is the temperature at the equilibrator and SST is the sea surface
temperature. Registration of the temperature difference between seawater and the
equilibrator to better than 0.2°C is essential for accurate pCO2 measurements.
6.) The final stage in the data correction is the conversion from partial pressure to
fugacity. This allows for a more accurate interpretation of the marine CO2 system
allowing for the non-ideal behaviour of CO2
fCO2 = pCO2 * exp(p* (B+2δ/RT))
where fCO2 and pCO2 are in atmospheres, total pressure p is in Pa (1 atm =101325
Pa), B is in m3 mol-1, R the gas constant is 8.314 J K-1 mol-1 and δ the cross virial
coefficient is (57.7 -0.118T)*10-6 m3 mol-1, with
B = (-1636.75 + 12.0408T – 0.0327957T2 + 0.00003.16528T3)10-6
from Weiss (1974).
Measurement of alkalinity (TA) and total CO2 (TCO2)
Alkalinity is determined using a carefully controlled acid titration. This enables
alkalinity to be reliably determined to an accuracy and precision of better than 0.1%.
TCO2 can be measured by acidifying the sample of seawater, to extract the CO2 as the
gas and measuring its amount produced by gas chromatography, infrared
spectroscopy, conductivity titration determination and computing TCO2 as the
difference between the first and second end point or collecting the CO2 in a solution
and titrating it coulometrically. Only the coulometeric method currently offers the
ability to obtain precision better than 0.1 %.
At present the best available instrumentation for making these measurements is a semi
automated system the “VINDTA” built by L. Mintrop, Marianda Co. Germany
(http://www.marianda.com/prod02.htm). The VINDTA system provides a combined
system for parallel measurements of alkalinity and total CO2 from the same sample
bottle. In the UK VINDTA Measurements can be made at NOC and UEA.
56
Alkalinity (TA) Analyses
Alkalinity (TA) is determined by potentiometric titration, in a similar method to that
first described by Dyrssen (1965) and modified by subsequent workers (e.g. Edmond,
1971; Bradshaw et al., 1981; Brewer et al. 1986, Dickson and Goyet, 1994). The
VINDTA titration systems consists of a water-jacketed, Perspex titration cell that is
closed with a perspex top, containing separate glass and calomel reference electrodes,
a thermister (readable to 0.1°C), and a capillary tube that supplies acid from a burette.
A pH meter (Metrohm model Titrino), readable to 0.1 mV, monitors the titration and a
motor driven piston burette (reproducible to + 0.001 ml), delivers acid to the cell.
Both cell and seawater samples are maintained at known temperatures using a
thermostated circulator.
A precise amount of seawater is dispensed into the cell, and titrated with hydrochloric
acid past the carbonic acid endpoint. The acid titrant, approximately 0.10-0.12N
hydrochloric acid (HCl), is prepared in a solution of standard sodium chloride (NaCl)
of approximately 0.6 M, its ionic strength adjusted to that of seawater. The acid titrant
is calibrated against solutions of Na2CO3 (Goyet and Hacker, 1992). Titration data
past the carbonic acid end point (~4.5 pH) are used to calculate TA. In the VINDTA
the system is computer-controlled. For the analysis the cell is automatically rinsed
with seawater to remove any acidified seawater from the previous titration. The
thermostated seawater sample is pumped into the cell avoiding any bubble formation.
The titration takes place with variable volume addition with the computer using a
simple linear relationship to adjust how much acid should be added to get a pre-fixed
e.m.f. increment. The location of the endpoint is demanding: e.g. to achieve a
precision of close to 1 µmol/kg in TA from the titration of 100 ml of sample with 0.1
molar HCl, the endpoint has to be located to within ±1 µl (while volume increments
are about 100- 200 µl per step). The location therefore has to be done by mathematical
means. Total alkalinity is computed from the titrant volume and e.m.f. data using a
least-squares procedure based either on a non-linear curve fitting approach or on a
modified Gran approach.
The accuracy of the measurement is maintained by routine analyses of CRM samples
(see below).
Measurement of TCO2
Total CO2
A known amount of seawater (~20 ml) is acidified with H3PO4 (10%) in a glass
stripping chamber and the resulting CO2 gas is purged with an inert gas (N2). The CO2
gas is dried in a condenser at 1-5 ºC (peltier system) and is determined by absorbing
the CO2 in an absorbent solution. The coulometric cell contains mono-ethanolamine
and a colorimetric pH indicator in the cathode cell with a platinum cathode, and a
silver anode in the anode solution. The hydroxyethylcarbamic acid formed is titrated
coulometrically with hydroxide ions generated by the coulometer circuitry. The final
titration point is determined spectrophotometrically by maintaining the transmittance
of the solution at a constant value.
1.- Absorption of CO2 by the cathodic solution (Cathode reaction)
CO2 + HOCH2CH2NH2 = HOCH2CH2NHCOOH
monoethanolamine hydroxyethylcarbamic acid
The acid causes the blue color indicator to fade and the % transmission to increase,
and titration current is automatically activated
57
2.- Electrochemical generation of OH- (Cathode reaction)
2 H2O + 2 e- = H2 (g) + 2 OH3.- Neutralization of absorbed CO2 reaction product by electrochemically generated
OHHOCH2CH2NHCOOH + OH- = HOCH2CH2NHCOO- + H2O
4.- Anode reaction
Agº = Ag+ + eThe titration current is measured continuously and integrated to a selected unit on the
digital display. The Faraday’s law applies. For every faraday of electricity utilised,
one gram equivalent weight of CO2 is titrated (Johnson et al., 1993). Since the
electrical calibration of the coulometer is not perfectly accurate, accuracy of the
measurement is maintained by routine analyses of CRM samples.
Certified Reference Materials for determination of alkalinity and Total CO2
The accuracy of TCO2 data is established by routine analysis of seawater certified
reference materials (CRM standards are supplied by Andrew Dickson, Scripps
Institution of Oceanography). CRM's have a certified TCO2 concentration determined
by an extraction/manometric technique established by C.D. Keeling’s laboratory,
Scripps Institution of Oceanography and now continued by A.G. Dickson.
Limitations of indirect measurements
Calculated pH
The accepted precision and accuracy for the potentio-metric determination of TA is 2
and 4 µmol/kg, respectively (Millero et al., 1993). Measurements of TCO2 by
coulometry can be determined with a precision and accuracy of 1 and 2 µmol/kg,
respectively (Johnson et al., 1993) and the measurement of fCO2, typically using an
infrared gas analyser, has a precision of 0.5 µatm, and accuracy of 2 µatm
(Wanninkhof and Thoning, 1993). Given these limitations, the estimated probable
error in calculating pH from any two of these parameters is given below (Millero,
1995; Zeebe and Wolf-Gladrow, 2001):
pH 0.0025 from fCO2 & DIC
pH 0.0026 from fCO2 & TA
pH 0.0062 from TA & DIC.
If electrode based pH measurements are coupled with measurements of fCO2, and
TCO2 or alkalinity, the system can be “over determined”, thus increasing the
reliability of the findings. These measurement systems and expertise on them are
available in the UK.
pH quality checks
pH accuracy is difficult to determine because no aqueous standards exist. Generally,
the precision of spectrophotometric pH is considered to be 0.001 (Clayton and Byrne,
1993). However the calculations and constants required for the determination are still
being improved. Following on from large scale hydrographic programmes in which
carbonate system parameters were measured, it has been possible to assess the
consistency and precision of the different data sets (McElligott et al., 1998: Lamb et
al., 2002). Lamb et al (2002) reviewed the precision of the various pH measurements
58
made during the Pacific Ocean CO2 surveys. All pH analyses were done at 25 C, and
no conversion was made to in situ temperatures. The limited number of crossovers
available for the Lamb et al (2002) study suggests that the spectrophotometric pH
measurements were precise and consistent between cruises. However they also noted
that DelValls and Dickson (1998) had suggested, that the pH values initially assigned
to the ‘tris’ buffers used to characterize the indicator, m-cresol purple should be
increased by 0.0047. Lamb et al., (2002) considered that this revision should be
applied to the measurements of pH made by colorimetric methods in the Pacific and
such an adjust was consistent with earlier evaluations by McElligott et al. (1998) and
Lee et al. (2000). A recent poster by Dickson has retracted the need for this
correction. This is an example of why it important that for any reported measurements
or calculations of pH to be accompanied by meta-data that defines how the values
were arrived at.
Variations from use of different formulations
In “over determining” the system it is critical that all calculations are done using
consistent and documented procedures. There are several different formulations of K1
and K2 (above) and also several formulations for the other dissociation constants of
interest, on various pH and concentration scales. Lewis and Wallace (1998)
considered that many of these differences are slight, but their importance is in direct
proportion to the desired precision of the calculated values. Lewis and Wallace
(1998) evaluated several programs that performed calculations relating the seawater
CO2 system parameters. These programs differed in the values of the constants used
and in what contributions to the alkalinity were considered. Lewis and Wallace
inspected the code and found reasons for the differences in different corrections for
KSO4, pressure, OH, non-ideality of CO2, equilibrium constants etc.
To get an idea of the differences, they ran three programs with the following inputs:
TA = 2300 umol/kg-SW; TCO2 = 2000 umol/kg-SW; no phosphate or silicate (two of
the programs didn't have an option to include these); and temperature (degrees
Celsius), salinity (on the Practical Salinity Scale), and pressure (in decibars) equal to
20, 35, and 0, respectively. The results were as follows:
Program
1
2
3
pCO2
(uatm)
336
351
330
pH
(umol/kg)
8.229
8.088
8.247
pH Scale
(umol/kg)
free
seawater
NBS
HCO3-
CO32-
1767
1772
1782
223
216
208
The pH values calculated appear to be significantly different in comparison to
expected changes in pH that will result from projected changes in concentrations of
CO2 in the atmosphere. This difference is largely due to the calculation of pH on
different pH scales and clearly demonstrates the need for clear definition of which
scale has been used when reporting measurement or calculation of pH. The values for
pCO2, though, should be the same regardless of pH scale, as should the values for the
concentrations of HCO3- and CO32-. It can thus be seen how different programs, with
no coding errors, can yield very different results. Because of this, Lewis and Wallace
(1998) decided to provide a single program that encompassed a wide variety of
choices for CO2 system constants, pH scales, etc. in order to facilitate the assessment
59
of the CO2 system calculations to such choices. This code will be used for all
calculations reported in the proposed programme of work.
The pH values calculated at different pH scales are significantly different in
comparison to expected changes in pH that will result from projected changes in
concentrations of CO2 in the atmosphere. These differences clearly demonstrate the
need for unambiguous definition of which scale has been used when reporting
measured or calculated pH values.
Future Developments
Introduction
To effectively monitor and understand the role of pH in the marine system robust data
is required. As is the case with any spatially and temporarily variant parameter, this
data must be gathered with greater temporal and spatial resolution than any
observable feature, or must integrate the determinand in space and time such that the
Nyquist criterion (Martz, 2003) are satisfied (e.g. the sampling frequency is over
twice that of the integration period, or fastest observable feature).
Traditionally chemical data has been obtained by sample collection and analysis in a
laboratory. This method delivers low resolution sampling (in space and time).
Therefore any conclusions drawn from this data assume that each sample is a
representative integral, or that there are no features present at higher spatial or
temporal frequencies. These assumptions may not be true and therefore, improved
measurement strategies are required. The most likely solution to this impasse is the
development of systems that automate this sampling and measurement, either onboard
a suitable vessel, or by immersed measurement in situ.
Promising underway (Bellerby, 2002; Friis, 2004) and in situ (Martz, 2003; Byrne,
2004] chemical sensor and analytical systems exist, some of which are available
commercially (e.g., Sea Bird Electronics Inc., YSI Inc., In Situ Inc., Analytical
Sensors Inc.). However, to compete with sample collection and laboratory analysis
these systems must provide data with sufficient resolution to be scientifically
significant. For pH accuracy in the order of 0.002 pH and resolution in the order of
0.001 pH is preferred (Bellerby, 2002; Friis, 2004). Analytical systems based on
spectrophotometric methods and dissolved reagents (Hopkins, 2000; Bellerby, 2002;
Friis, 2004) are able to achieve very high performance and in the near term promise to
be the most likely to deliver wide scale capability for the determination of pH.
Electrochemical (electrode) (Dickson, 1993) and optical (optode) (Klimant, 1997)
sensors have been developed for the measurement of pH and are attractive because of
their simplicity and robustness. One approach is to combine these technologies with
self calibrating systems (e.g. fluidics supplying a standard, and blank in the form of
buffered solutions). This complicates these analytical systems, removing one of the
key advantages of this technology. But the system developed by Whitfield et al (1995)
for the UK EA although not widely known, has demonstrated the potential to deliver
data at the precision required by a monitoring programme but the stability of this
system in long term used needs to be assessed.
Spectrophotometric, electrochemical and optical techniques for the determination of
pH are reviewed here briefly together with a discussion of future trends. A discussion
of the relative merits of sampling and in situ systems is included for completeness.
60
Sample collection or in situ measurements?
Periodic collection of water samples can provide spatial coverage along a ship’s track,
but may miss episodic events. To resolve short term variations for extended periods
autonomous in situ sensors are required (Buffle, 2000).
Water sampling risks contamination, or change in the sample. Particular problems are
associated with changes in temperature and pressure. For pH measurements, gas
exchange (e.g. CO2) and biological activity (Martz 2003) are known problems. In situ
instruments require little or no intervention upon deployment, but have to be
extremely reliable. By definition, in situ techniques allow for the species of interest to
be measured quantitatively in its surrounding environment, inducing minimum
perturbations to the system. However, in situ operation places additional demands on
the design and attributes of the measurement system. For example low power
consumption (autonomous and long term measurements), pressure resistance (or
compensated) to allow for deep sea measurements, small size to enable system
integration with other instruments, robust measurement protocols and high reliability
are all required. In situ instruments require careful calibration of the instrument before
and after the deployment. Newly developed sensors require comparison to
independent measurements during the initial deployments. Existing pH measurement
technology have a cross-sensitivity to temperature and salinity, these additional
parameters have to be measured.
Electrochemical systems
Although popular for their low cost and commercial availability (Sea Bird Electronics
Inc., YSI Inc., In Situ Inc., Analytical Sensors Inc., Idronaut S.r.l), potentiometric pH
electrodes suffer from irreproducible junction potentials and junction potential drift
encountered both in seawater and freshwater. Even when calibrated frequently, large
systematic errors can arise from differences between standard and sample junction
potential. These issues are well documented and the scientific community is well
aware of them (Maberly, 1996; Brezinski, 1983; Dickson 1993; Davidson 1985;
Byrne, 1989; Granato, 1999; Buffle, 2000). However, implementing interlaboratory
standard protocols is difficult and no protocol is unanimously accepted and used by
the scientific community (Gardner, 1990). Among the off the shelf solutions, the
closest to our requirements is the Idronaut pH electrode which claims a dynamic range
of 0-14 pH with an accuracy of 0.01 pH, a resolution of 0.001 pH and a response time
of 3 s. There is no mention of the drift associated with it or the lifetime of the
electrode nor its potential to undertake long term in situ deployments.
Electrodes have generally been a neglected area of technology. However the study of
Covington and Whitfield (1988) suggested that by developing the use of appropriate
combinations of electrode couples and junctions, more precise instrumentation could
be developed. This was done by PML for the UK EA but has not been made widely
known. Whitfield et al (1995) delivered a system, which achieved precision, required
by a monitoring programme but the stability of this system in long term used needs to
be assessed. The skill base to take this forward is still available from David Pearce at
CEFAS who was part of the PML development team and John Wood of Ruthern Ltd
who built the EA systems. David Pearce (pers comm) has suggested that a system
using a sodium reference electrode could provide a robust system for use in marine
waters. The systems built for the EA used a flowing liquid junction, which enabled
61
them to be used in both saline and fresh waters, but adds complication to the
mechanical arrangements needed.
Spectrophotometric (absorbance) systems
Spectrophotometric methods for measurement of pH have a history of successful
development of high precision bench-top instruments if used by skilled operators.
Precisions of ± 0.001 down to ± 0.0004 pH have been reported (Clayton and Byrne,
1993; McElligott, 1998; DelValls, 1999; Tapp, 2000; Bellerby, 2002; Martz, 2003;
Friis, 2004). Typical spectrophotometric pH systems are composed of a light source at
single or multiple wavelengths, an absorption cell and a photodetector. Seawater is
mixed with a colour indicator (thymol blue) and the light absorption of the solution
measured. When corrected for salinity and temperature effects the absorption is
directly related to pH concentration following Beer – Lambert’s law (Zhang, 1996).
Underway system are usually thermostated and the temperature of the solution is
measured for further correction (Zhang, 1996; Friis, 2004).
The biggest challenges are thorough mixing of indicator and sample, the removal of
gas bubbles in the solution that interfere with the absorption measurement, and the
offset in the pH value inferred by the addition of the indicator solution which must be
quantified. High precision in the delivery of the indicator volume and reproducible
mixing throughout the optical path and in the absorption cell must be achieved
(Bellerby, 2002; Martz 2003; Friis, 2004).
Existing in situ systems (Hopkins, 2000; Martz, 2003; Byrne, 2004) exist and perform
to the required precision, but their size, power requirements and cost related to the
technology used (expensive Teflon AF 2400 liquid core waveguide) prevent them
from being used on a wider range of applications.
Recent technology improvements in light sources and photodetectors allow for the
design of smaller systems, opening a path to in situ measurements using small
instruments. Light emitting diodes (LEDs) now provide an energy efficient means of
producing monochromatic light acting as a concentrated small emitter. Their low cost,
broad range of wavelengths and wide availability make them ideal for miniature
analytical devices (Dasgupta, 2003). Liquid core waveguides (LCW) are now
commercially available and can be used as an absorption cell (Waterby, 1996; Byrne,
2002), thus, minimising the volume of reagent and water sample needed.
Because of the new technologies available, miniaturisation of spectrophotometers for
precise in situ pH measurements can be undertaken. The technological challenge will
lie in the mixing of the indicator with the seawater sample and in retaining the high
sensitivity and precision available with bench top underway systems while reducing
the size of all the elements. Control and readout electronics, liquid core waveguide
technology and fluidic systems can be borrowed from an existing in house system (Fe
/ Mn analyser (Statham, 2005) developed at the NOC. The Fe / Mn analyser is housed
in a 200 mm long, 90 mm diameter pressure balanced housing and uses a quartz LCW
associated to a LED and photodiode. Mixing of the reagent and sample happens in
situ and a set of small solenoid pumps (Lee Products Inc.) and valves (BioChem Inc.)
complete the fluidic system. This proven concept can be used as a working base for
the development of future in situ pH sensors, thus reducing the development time
needed. The recent introduction on the market of small inexpensive LCW (Polymicro
Technologies LLC) and T connectors (Upchurch Inc.) allow for reproducible optical
connections using optical fibres and promise potential improvement in the above
system.
62
The NOC and the University of Southampton is just beginning a large four year
project (www.soton.ac.uk/rmst) that will deliver ultra miniaturised (lab-on-a-chip)
chemical sensor technology (Ruggedised MicroSystem technology (RMST)). This
will provide miniaturised electronic, fluidic and optical systems as a toolkit for the
development of robust, miniature and low cost sensing systems. The construction of
pH sensors using this technology is feasible and will allow the future use of pH
sensors in situ systems. Potentially these devices could be produced economically in
large numbers and promise to make a significant impact on the temporal and spatial
resolution at which measurements are made.
Optical indicator based sensors (Optodes)
The “optode” type sensor has proved very successful in its application to
measurement of dissolved oxygen in seawater. The concept can be applied to the
measurement of pH, but a device with the resolution needed for monitoring changes
in pH is some way off yet. Overcoming the problems associated with the pH optodes
is a long term goal of research being carried out in Southampton and else where. This
is an active research area and further funds are currently being sought through
European and national sources. This is a promising technology but it is likely that in
the immediate future marine pH optodes will only be used in applications, which do
not require high resolution.
A number of optical sensors have been developed for the determination of pH
(Draxler, 1995; Kosh, 1998; Weidgans, 2004). Typically these employ an indicator
material that has one or more of its optical properties modulated by pH. Examples
include fluorescence lifetime (Wolbeis, 1986; Leiner, 1991; Kosh, 1998; Weidgans,
2004) and absorption (Monici, 1987; Dasgupta, 2003). To create a sensor (or optode)
this sensing element is combined with a light source, a detector and electronics to
control and interpret the response of the device. Optodes have revolutionised the
detection of dissolved oxygen. To date pH optodes have not provided the performance
required in the oceanographic setting.
The indicator is usually composed of a dye immobilised in a proton permeable
polymer matrix. The choice of the indicator is dependent on the pH range to measure
as the dynamic range of pH optodes is limited to pKa ± 1.5. However, this small range
results in a high signal change with pH and therefore in a high sensor resolution of
0.005 pH (Kosh, 1998). Other criteria that influence the choice of indicator include
the quantum yield, photostability, fluorescence lifetime, excitation and emission
wavelengths and response time. Polymer combinations can also be optimised to
reduce cross sensitivity towards ionic strength. There are number of challenges
remaining with this technology including interferences from ionic strength sensitivity,
the temperature dependence and the photobleaching of the sensing element which
reduces the accuracy and long term stability of the sensor (Weidgans, 2004).
Temperature dependency can be characterised during calibration and compensated
for. The ionic strength (IS) cross sensitivity requires calibrating the pH sensor at an IS
close to the measurement conditions and limits the use of optodes to applications with
a fairly constant IS (unless measured). The IS sensitivity is highly dependent on the
membrane and indicator used and can range from 0.02 pH to 0.3 pH for an IS change
of 100 mM (i.e. a salinity change of 35‰ to 40‰). A sensor displaying a 0.02 pH
cross sensitivity to IS is considered as presenting a low ionic strength sensitivity
(Schroder, 2006). Photobleaching is not a reversible effect and will limit the lifetime
63
of the sensing element as well as inducing an artefact signal and must be therefore
monitored. Measurement schemes using low levels of excitation and based on the
measurement of the fluorescence lifetime rather than fluorescence intensity could
extend the sensor life in the field and are under investigations.
PreSens (Precision Sensing GmbH) supplies commercial pH indicator elements and
complete optode systems based on the fluorescence decay variation with pH. The
electronics they supply measure the phase shift between the excitation and the
emission signal to determine fluorescence lifetime (Klimant, 1997). The system is
available as a dipping probe, a set of pH foils or a flow through system and presents a
dynamic range of 5-9 pH with a resolution of 0.005 pH. Temperature and IS crosssensitivities are calibrated. The response time (t90) is less than 1 minute and the
typical drift of the system is 0.1 pH per week in 2 minutes mode. Exposing the sensor
to a pH higher than 9 will permanently damage it.
Recent work (Schroder, 2006) using two novel indicators and matrixes displays an
apparent pKa of 8.4 which is well matched with seawater pH. For a given type of
membrane, the cross sensitivity towards IS was found to be almost negligible and
presented a low temperature dependency, opening the possibility for measurements in
seawater without IS corrections or in medium where the IS is rapidly changing.
However, this promising sensor suffers from a slow response time (2 – 4 min) and a
poor precision (0.02 pH) at this early stage of development.
New light source technology could revive the use of pH dependent photoinduced
electron transfer indicators (Draxler, 1995) which have a faster response time (t90 <
10 s), a long storage time with reduced need for recalibration before use, and long
operation periods with limited maintenance.
Possible development
Future monitoring of ocean pH is based where possible on direct measurement of pH.
Within the proposed Defra monitoring plan for the UK (2008-2010) this can be done
using an electrode based system developed for the Environment Agency. This system
provides a reliable way of making measurements with a precision, which meets the
target of 0.003 pH units and with the use of appropriate calibration protocols
measurements will be consistent to a similar degree of accuracy to the precision.
These measurements will be crossed checked against determination of the pH done
through calculations based on the measurements of other carbonate system
parameters. The skill based from the 1990s developments for the EA is still available
this work. This would enable a sodium electrode based system to be produced and
tested within 6 months of a contract being let. The cost of this would be about £23k
for the manufacture of the first instrument (Ruthern Instruments Ltd) with an input of
an extra 15 days of time from David Pearce at CEFAS for software and calibration
development. The cost of repeat units would be about £9k.
In the longer-term measurement of pH may be done using colorimetric measurements.
These methods are under development for use in monitoring. They offer a higher
precision and the likely hood of being more reliable in routine use. Development
could be speeded up. Develop an in situ spectrophotometric system using a liquid
core waveguide together with infrastructure from the existing Fe/Mn programme at
NOC. An operating underway system could be produced for testing and evaluation in
the field for £100k within 1 year of contract being let. Production of system for
routine use and easy manufacture could be done in parallel a similar cost.
An automated TCO2 analyser at PML for autonomous deployment
64
Any two of the four variables: total alkalinity (TA), pH, pCO2 and TCO2 can be used
to calculate the other constituents of the marine carbonate system. However, there is
a global shortage of data for any of these parameters for studies of the impact of
anthropogenic CO2 on the marine ecosystem and future climate. Ships of opportunity
offer a means of acquiring data cost efficiently, but the sensors must be able to
function in an autonomous (or nearly autonomous) mode for long periods: typically
weeks to months. The quantification of pCO2 using an infrared gas analyser is
particularly suitable for unsupervised operation (see Section 5b) but pH, TA and
TCO2 are more difficult to automate to a similar standard for various reasons.
Recently, Hales et al. (2004) and Bandstra et al. (2006) have published methods that
described rapid, automated measurement of TCO2 from acidified seawater using a
gas permeable membrane to ‘strip’ the evolved CO2 from seawater into a carrier gas
stream leading to an infrared gas analyser. This method is therefore very compatible
with pCO2 measurement.
Research at PML is underway to investigate whether the gas-stripping methodology
of Bandstra et al., can be adapted to run in parallel with an autonomous pCO2
apparatus. Factors that are being addressed are minimising the consumption of
reagents and the frequency of measurement so that the system can operate unattended
for periods of weeks to months. This work is at a preliminary stage but raises the
possibility of quasi-autonomous characterisation of the carbonate system to a level
that enables pH to be calculated.
5b Development of an observing plan
Requirements
The purpose of the planned monitoring programme is to provide reference data, or a
“baseline”, against which data collected in the future can be compared for the
detection of long term changes. The relatively large natural changes throughout a year
require that these intra-annual changes are well documented. The data must be
representative of UK waters and quantify differences due to biological,
biogeochemical and hydrodynamic processes. The study should consider marine
waters ranging from ocean waters to regions of fresh water influence (Simpson,
1997). Improved data on the scale of riverine inputs of material affecting the
carbonate system around coast of the UK will be a key factor in improving the output
of numerical models but must be proved elsewhere. Additionally, the monitoring
programme should provide a data set that will assist the development and validation
of numerical models. Such models will enable better understanding of ocean
acidification and likely future rates of change.
The work must improve our understanding of how the pH of seawater is determined
both by (i) local scale biogeochemical and hydrodynamic forcing and (ii) by larger
scale flows and mixing from estuaries and to oceans. This will allow consideration of
wider scale phenomena such as the North Atlantic Oscillation (Met Office, 2007)
which can produce significant inter-annual changes in the composition of ocean
waters (Schuster and Watson, in review) and in UK shelf sea waters (Kelly-Gerreyn et
al., In Press).
65
The likely annual rate of change in the pH of seawater due to uptake of anthropogenic
CO2 from the atmosphere is small and close to the detection limit of current methods.
Thus, measurements of the highest accuracy and precision are required. Measurement
of three or more inorganic carbon parameters allows over determination of the
carbonate system, such that the results by direct measurement and calculation from
other parameters can be compared to assess the accuracy and precision of current
methodologies, and that recalculations can be performed in future, if necessary due
improvements in knowledge.
These criteria require that

measurements be made with sufficient frequency over the year;

measurements be made with sufficient geographical spread;

measurements be made at defined times and regular locations;

measurements of pH and other carbonate system parameters be made
along side measurements of additional physical, chemical, and biological properties;

measurements be made using the best available technologies and
methodologies;

measurements be made of sufficient carbonate system parameters.
These requirements and the costs of these measurements imply that we need to align
such a monitoring programme with existing collection of inorganic carbon data, as
well as relevant biological, chemical and physical parameters and expand the current
activities where necessary in time and space or with more ancillary data
Monitoring around the UK
(see http://www.bodc.ac.uk/projects/uk/merman/project_overview/)
66
Figure 5.4 MAP of the current OSPAR review regions in UK waters.
The currently designated OSPAR regions for the determination of conditions in
marine waters are designed for the study of contaminant inputs from land and so are
located close to the coast (Figure 5.4). For the study of CO2 up-take it is necessary to
consider offshore waters which will influence UK territorial waters, as those offshore
waters are the source waters for North West European shelf seas which are flushed by
them on a time scale of the order of few years (Prandle, 1984). The North Atlantic
Ocean is major sink for atmospheric CO2 (Schuster and Watson, in review), and hence
a baseline monitoring must consider these waters to identify how changes in this
“mixing end member” influence UK waters.
67
Figure 5.5 MAP of UK CSEMP (formerly NMMP) sites
The frequency of monitoring in the CSEMP (Figure 5.5) is too low to resolve the
intra-annual changes in pH resulting for biogeochemical changes in the water and the
grid is not fine enough to resolve small scale water mass changes that are significant
in coastal seas. The “Smart Buoy” based monitoring programme operated by CEFAS
does provide a platform that gives both the required ancillary data and high temporal
resolution. However, at present, high precision methods for determining pH are not
sufficiently advanced for use on a buoy system.
Proposed working areas
The criteria listed above for a monitoring system can be met using five existing UK
research programmes. Most of the required measurements are already being made in
four of these projects and advantage can be taken of elements of existing data sets.
They provide the required progression through some of the major environments in UK
waters and knowledge of the changes taking place in ocean waters that act as the
source waters for UK seas.

POL Coastal Observatory, Liverpool Bay.

FRS, Aberdeen, Stonehaven sampling site

Western Channel Observatory (L4, E1, data buoys, Quest transects)

NOC Pride of Bilbao (VOS) data collection

UEA Transatlantic (VOS) data collection
POL Coastal Observatory, Liverpool Bay: provides detailed coverage of a region of
strong fresh water influence in a nutrient enriched system. Waters are sampled over a
68
grid of stations 8 times per year using the RV Prince Madog, which carries a
PML/Dartcom autonomous pCO2 system. Ecosystem modelling in this area can be
carried out on much higher resolution grid than any other region of UK waters.
Waters in the coastal observatory area are either well mixed or stratified due to fresh
water influences.
FRS Aberdeen Stonehaven sampling site: is generally sampled weekly for
hydrographic, biological and chemical parameters. Given sufficient resources, it
would extend the monitoring into northern waters.
PML Western Channel Observatory: looks at waters which are transitional between
regions of strong fresh water influence and ocean influence that are deep enough to be
thermally stratified in summer. Hydrographic, biological and chemical sampling takes
place at and between two stations L4 (50o 15.00’N 004o 13.02’W) which is sampled
weekly depending on weather, and E1(50o 02.00’N 004o 22.00’W) sampled
fortnightly. The research vessel, RV Plymouth Quest visits these stations regularly and
is already instrumented for pCO2:- providing detailed information between Plymouth
and the seasonally stratified waters at the E1 site, 22 miles offshore. This will allow
observation of the balance of production and respiration processes above and below
the stratified layer to be determined.
NOC Pride of Bilbao (VOS) data collection: provides high time resolution data (each
position along the track is re-sampled at an interval between 4 hours and 3 days)
which allows detailed recording of changes in pH in relation to biogeochemical
activity in a wide range of environments, from nutrient enriched harbours to the deep
temperate North Atlantic Ocean. It provides key information on the differences in
behaviour between shelf seawaters and the ocean waters that are the source for those
shelf sea waters. The areas of sampling “cross over” with both the Western Channel
Observatory and the UEA Transatlantic (VOS) data collection.
UEA Transatlantic (VOS) data collection: provides data from the longest running UK
data set of marine carbon data. Data are collected from the southern UK shelf through
to the open mid-latitude North Atlantic on a monthly basis, giving a unique
opportunity to study the inorganic carbon chemistry of source waters for the UK shelf,
and their seasonal to inter-annual variability (Cooper et al., 1998; Schuster and
Watson, in review). This data set is part of an international VOS network for pCO2
with European and US partners. The network allows quantification of the North
Atlantic CO2 sink and contributes to quantification of the terrestrial northern
hemisphere CO2 sink by providing an important constraint for atmospheric inversion
models. Operational funding for the UK VOS line ends in late 2008.
Present Status of UK capability for making high precision chemical
measurements of the carbonate system in marine waters:
Carbonate system
pCO2 is the only parameter that is currently measured, automatically and at high
resolution with confidence. These systems are presently operational in each of the 4
survey areas, on the MV Santa Maria, MV Pride of Bilbao, RV Prince Madog and RV
Plymouth Quest.
69
pH can now be measured to a degree of precision required for this work ~0.002 pH
unit using a flowing liquid junction electrode system (developed for the EA).
However experience with this system in terms of maintaining that precision is lacking.
This is currently being evaluated by NOC. Measurements of the required precision
can be achieved by a colorimetric method. A resolution of 0.001 pH unit has been
achieved with a laboratory-based automated systems. An automated system capable of
autonomous operation at sea is being developed by NOC and should be operational by
the end of 2007.
TA and TCO2 can only be measured to the required degree of accuracy and precision
using discreet samples. The facilities for doing this are available at UEA and NOC.
These are key measurements, and an international accepted certified reference
material will be used to control the accuracy of these analyses.
Other biogeochemical system determinands
Salinity data is available from all areas and calibration is based on well-established
procedures and traced back to use of IAPSO standard sea water.
Temperature data are commonly available but for carbonate system work data must
be accurate and precise to better than 0.1oK. A protocol for the required traceability of
temperature is being developed at NOC by Charlene Bargeron in consultation with
Andrew Dickson as part of the currently on going revision of DoE CDIAC (DOE
(1994) Handbook of methods for the analysis of the various parameters of the carbon
dioxide system in sea water. (Version 2), A. G. Dickson & C. Goyet, eds.
ORNL/CDIAC-74.)
Nutrients are currently measured throughout the year in four of the programmes by
NOC and PML and at Stonehaven by FRS, Aberdeen.
5c Quality control
Requirements:

pH is in practice defined (from the activity of the hydrogen ion) against
four different scales and it can also be calculated from other components using
different equations. Any reporting of pH values must therefore be accompanied by
full and traceable details of the procedures used to derive the reported value.

The programme provides reference points for the detection of long term
changes against a background of natural variation. This requires that all the methods
used are “Traceable” and that all results are reported with the “meta-data” to ensure
that this is possible.

The methodologies and calculation used by different laboratories must be
traceable between the laboratories. Best practice should be applied following the
internationally agreed recommendations (currently CDIAC (DOE (1994) Handbook
of methods for the analysis of the various parameters of the carbon dioxide system in
sea water. (Version 2), A. G. Dickson & C. Goyet, eds. ORNL/CDIAC-74. which is
under revision ).

Measurements made by individual laboratories should use the certified
reference solution supplied by A. Dickson at Scripps for the determination of total
70
alkalinity and total CO2. In addition an inter-comparison of results of measurements
made on common samples should also be carried out.

Measurements of carbonates system parameters will be made along side
measurements of hydrodynamic and biogeochemical conditions these needed to be
reported with the appropriate meta-data that allows there quality and reliability to be
assessed. Best practice should be applied in collection and archiving of the data.
Quality control procedures should follow those the WOCE Hydrographic Program
WHP Operations and Methods Manual
Standardisation:
Equipment

All the analytical and sampling equipment used in this work will be
traceable through the meta data provided alongside the reported data
Methods

The analytical methods used will follow current best practice as described
in the DoE CDIAC Handbook
Calculations

When reporting data derived from calculations, statements will accompany
data of which carbonate system parameterisation have been used. Data for pH will be
reported on the total hydrogen scale, with hydrogen ion concentrations reported in
mol kg-1. An appropriate programme for thermodynamic calculations of the CO2
system is CO2SYS, which is available from the server of the Carbon Dioxide
Information Analysis Centre (http://cdiac.esd.ornl.gov/oceans/co2rprt.html). For
calculations the CO2SYS programme should be used with the HSO4- constant from
Dickson (1993) and the carbonic acid constants from Mehrbach et al. (1973) as
refitted by Dickson and Millero (1987).
Data Management
Note on MERMAN from BODC webpage
MERMAN (Marine Environment Monitoring and Assessment National database) is a
new national database created by IBM while BODC acts as the Data Manager for
MERMAN. It is designed to hold and provide access to data collected under the Clean
Safe Seas Environmental Monitoring Programme (CSEMP) — formerly the National
Marine Monitoring Programme (NMMP). MERMAN is funded by the Department for
Environment, Food and Rural Affairs (Defra), the Department of Agriculture
Northern Ireland (DARD), the Environment and Heritage Service Northern Ireland
(EHS) and the Scottish Executive Environmental and Rural Affairs Department
(SEERAD). CSEMP itself provides a coordinated approach to environmental
monitoring in the UK's coastal and estuarine areas. The programme fulfils the UK's
commitment to European directives including its mandatory monitoring requirements
under the Oslo and Paris Convention (OSPAR) Joint Assessment Monitoring
Programme (JAMP).
The general aims of CSEMP are to

Detect long-term spatial and temporal trends in physical, biological and
chemical variables at selected estuarine and coastal sites

Support consistent standards in national and international monitoring
programmes for marine environmental quality

Establish appropriate protective regulatory measures

Coordinate and optimise marine monitoring in the UK
71

Provide a high quality chemical and biological data set from the UK’s marine
environment
Approximately 80 stations, see map above are monitored around the UK coastline.
Contaminants are measured in waters, sediments and biota to assess their distribution
and fate in the environment. Biological effects are also measured to determine the
response of organisms to contaminants. Data are quality assured using internal and
external programmes. The principal output from the coordinated monitoring is an
annual submission of quality assured data to the International Council for the
Exploration of the Seas (ICES).
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75
Appendix 1
ME2109 original Objectives and Approaches (revised 2007)
The objective of this work is to produce a report by which will describe a costed two
year trial programme of work for sustained data collection, data stewardship, analysis
and synthesis in order to assess the impact of acidification on UK marine waters. This
report and its contents is Milestone 1, the single milestone in this proposal.
This programme will feed into the work of the Marine Assessment and Reporting
Group, MARG. The results will enable MARG to carry out its functions with respect
to the impact of increased levels of atmospheric CO2 on the marine ecosystem. These
functions are:1.
to define the monitoring programme to meet national and international
policy, legislative and operational needs;
2.
to provide overall direction to the monitoring programme ensuring that the
most efficient use is made of resources;
3.
to approve and integrate periodic assessments;
4.
to identify emerging understanding to support policy and governance;
5.
to ensure policy drivers (marine objectives) are met;
6.
to introduce changes in the monitoring programme in the light of
improvements in measurement; technologies and scientific understanding.
To this end the proposed project is structured so that it develops an observational
program based on a sound understanding of information already available, such as
existing in situ observations and results from numerical models. The report will
therefore contain:1.
A review of existing data sets in which pH has been measured directly or can
be calculated from other carbonate system measurements.
2.
A formal validation of existing model performance against data provided by
the review of existing datasets
3.
A study ranking the biogeochemical and physical processes likely to determine
the acidity of seawater in different shelf regions.
4.
A desk study of where UK marine waters are likely to be most vulnerable to
changes in acidity.
5. Development of an observational programme will be based on:5a A critical review of existing and developing technologies for measuring
components of the carbonate system which allow an accurate assessment of seawater
acidity (pH). The review will include considerations such as availability, ease of
deployment and operation, purchase and running costs, as well as maintenance
requirements
5b A review of the UK capacity for the collection of appropriate samples as part of
existing observational programmes and more continuous observations such as may be
based on Smart Buoy or Ferry Box technologies.
76
6
The Monitoring Plan which will be developed through the production of a
“strawman”. The “strawman” will be the basis for discussions with a number (5-7) of
other stakeholder representatives both within and outside the existing UK MMP
supplied by Defra. Feedback from these stakeholders will facilitate the production of
the final Monitoring Plan to be delivered at the end of this project.
7.
(c)
Approaches and research plan
1. A review of existing data sets in which pH has been measured directly or can
be calculated from other carbonate system measurements.
This work will clearly establish what relevant information is available that could be
used to establish a baseline against which changes might be identified, and to conduct
a formal validation of existing model performance (Objective 3). The geographical
area considered for data assembly will be that of the POLCOMS (Proudman
Oceanographic Laboratory Coastal-Ocean Modelling System) 40-65°N, 20°W-15°E.
1.
A review of data available in the published literature and elsewhere and
data that may become available at some time in the future from other sources such as
the EU-FP6-project CARBOOCEAN, will be carried out.
2.
Data sets under the ownership of project participants will be made available
to project partners (by end September) to validate existing model systems. The largest
is that of underway pCO2 collected on the UEA trans-Atlantic (Portsmouth Caribbean) Ship of Opportunity. This has been running continuously since April 2002
with earlier measurements in 1994 and 1995. Additionally there are data from the
PML monitoring stations E1 and L4 in the English Channel, data collected by the
CASIX systems on NERC research ships, and from NOC on the Portsmouth – Bilbao
Ferry box line since September 2005. The best approach for estimating pH from data
sets with a single inorganic carbon parameter (eg. pCO2) will be investigated.
3.
A preliminary MCCIP report card will be produced on the basis of the
existing data.
2. Conduct a formal validation of existing model performance against data
mined for this project.
The numerical ecosystem model ERSEM-POLCOMS has reached a level of maturity
where it can potentially contribute successfully to the extrapolation of information
into the future and estimation of conditions in areas where in situ data has not been
collected. However the appropriate use of models depends on their accuracy and an
understanding of why and when models and data deviate. Under ME2107 effort is
underway to validate models; however this project will deliver some new data sets not
currently available. Hence we will:1.
Conduct a formal model validation exercise incorporating newly available
data and quantify model accuracy and error, as an input to objective 3.
3. A study ranking the biogeochemical and physical processes likely to determine
the pH of seawater in different shelf regions.
77
At a particular location the acidity of seawater will vary through the year because:- (i)
seasonal changes in sea surface temperature and mixing affect surface water pH, (ii)
processes of biological growth and decay influence CO2 in the water, (iii) of external
inputs such as rain and rivers, and (iv) the acidity of the sea water is also determined
by reaction with mineral phases.
1.
Based on the existing literature, a ranking will be made of the processes that
are most likely to determine the acidity in different regions of the UK marine
environment at different times of year. These will be compared with results from the
existing ERSEM model systems.
4. A desk study of where our shelf seas are most vulnerable to pH changes.
We will use modelling and laboratory results from ME2107, current literature and the
output from task 3 to estimate the vulnerability of UK Shelf Seas to acidification.
The ME2107 partners will (by March 2007) “create a
UK modelling capacity for exploring the effects of
high CO2 (including lowered pH) on the marine
ecosystem of UK shelf waters and make an initial
exploration of marine ecosystem response to elevated
CO2.” This work is progressing to plan. Blackford &
Gilbert (2006) assesses the annual pH range, its
drivers and future predictions for the southern part of
the North Sea (south of 56°N). Work is near
complete in extending the model domain to the whole
of the UK shelf waters (see figure) which will enable us to analyse, spatially and
temporally, the model output to determine the degree of acidification at 700 and 1000
ppm atmospheric CO2.
In addition to the work planned under ME2107 we will:1.
Compare for each part of the model domain the in situ pH range for each
scenario and evaluate the deviation between ranges from each scenario.
2.
Quantify the distribution of vulnerability in UK waters as a function of in
situ pH range and deviation.
3.
Given the known sensitivities of specific biogeochemical processes to pH
changes we will briefly and qualitatively assess the potential disruption to the shelf
ecosystem.
5. A critical review of existing and developing technologies for measuring the
variability and change of pH.
This review is necessary so that the most appropriate methods for measurement are
used during the monitoring programme. It will cover:-
78
10.
The accuracy and precision of methods will be assessed with respect to
those required to detect change at the present rate of atmospheric CO2 increase and
the corresponding CO2 uptake into the coastal seas.
11.
Existing measurement methods currently in use in the UK and abroad will
be reviewed in the light of whether they might be appropriately introduced into the
programme. The review will include considerations such as availability, ease of
deployment and operation, purchase and running costs, as well as maintenance
requirements.
12.
The introduction of errors from the sampling and measurement process will
be assessed.
13.
The reliability of determining pH on the basis of measurements of pCO2,
salinity and temperature will be assessed.
14.
A key development in marine monitoring is the development of automated
monitoring methods and the use of chemical sensors. These developments will be
reviewed from the point of view of possible applications, and the timescale for
implementation in a UK coastal monitoring program.
15.
The components of the carbonate system to be measured in a monitoring
programme will be clearly defined.
16.
Quality control procedures will be defined.
17.
Procedures for data reporting and the assembly of meta-data will be
defined.
18.
As part of the development of the strawman a review will be carried out of
the UK capacity for the collection of appropriate samples as part of existing
observational programmes and more continuous observations such as may be based
on Smart Buoy or Ferry Box technologies. The best ways forward for combining this
capacity and measurement technologies will be recommended.
6. Strawman of monitoring proposals.
A monitoring programme for carbonate system parameters that covers all relevant UK
waters cannot be carried out cost effectively without the active co-operation of a range
of bodies already contributing to existing UK monitoring programmes both formally
through the UK MMP and informally through the MECN (Marine Environmental
Change Network).
A plan for sample collection and measurements will be evolved through the course of
this contract in consultation with UKMMAS, MCCIP, SAHFOS, MECN groups and
including discussion with Cefas and FRS with respect to the use of continuous
observation platforms i.e. Smartbuoys. A strawman of monitoring proposals will be
circulated to a limited number (5-7) of stakeholders supplied by Defra, for comment.
The plans will be evolved and a final plan will be selected on the basis of findings
from activities 1-5 and the input from the projects stakeholders.
7. Workshop
PML is hosting a workshop on modelling the response of ecosystems to high CO2
which will involve many of the worlds leading acidification researchers. This will
79
provide an opportunity to obtain international expert advise on the most suitable
monitoring methods and programme.
8. The monitoring plan with recommendations to the Project Board for a costed
two year trial programme of work for sustained data collection, data
stewardship, analysis and synthesis.
Notes
Casix (http:// pml.ac.uk/casix/)
The Centre for observation of Air-Sea Interactions and fluxes CASIX is an inter UK
institution partnership. UK Natural Environment Research Council funds CASIX as a
NERC “Centre of Excellence”. Its prime purpose is exploiting Earth Observation
(EO) data to determine air-sea fluxes of CO2.
Model Systems
The model system (proposed here and in use in ME2107) is a coupling involving
three well established model codes covering the carbonate system (HALTAFALL;
Ingri et al, 1967), the marine ecosystem (ERSEM; Baretta, 1995; Blackford, 2004)
and a 3D hydrodynamic simulation of the UK shelf system (POLCOMS; Holt and
James, 2001). The physics have been comprehensively validated (Holt et al., 2005).
This is the only model available that combines UK shelf wide physics with ecosystem
dynamics and carbonate chemistry. This model system has already been used to give
estimates of southern North Sea acidification and the relationship of pH to
environmental and biological processes affecting CO2 such as photosynthesis and
respiration, riverine boundary conditions and atmospheric concentrations (Blackford
& Gilbert, 2006). The model system is fully described in Blackford & Gilbert 2006,
and references therein.
References
Baretta, J.W., Ebenhöh W. and Ruardij P., 1995. The European regional Seas
Ecosystem Model, a complex marine ecosystem model. Netherlands Journal of Sea
Research 33, 233-246
Blackford, J.C., Allen, J.I., Gilbert, F.G., 2004. Ecosystem dynamics at six contrasting
sites: a generic modelling study. Journal of Marine Systems 52, 191-215.
Blackford, J.C., Gilbert, F.J., 2006. pH variability and CO2 induced acidification in
the North Sea. Journal of Marine Systems. Available online
doi:10.1016/j.jmarsys.2006.03.016.
Holt, J.T., James, I.D., 2001. An s-coordinate model of the North West European
Continental Shelf. Part 1 Model description and density structure. J. Geophys. Res.
106(C7): 14015-14034.
Holt, J.T., Allen J.I., Proctor, R and Gilbert, F., 2005. Error quantificaction of a high
resolution coupled hydrodynamic-ecosystem coastal-ocean model: Part 1 model
overview and assessment of the hydrodynamics. Journal of Marine Systems 57, 157188.
Ingri, N., Kakolowicz, W., Sillén, L.G., Warnqvist, B., 1967. High-speed computers
as a supplement to graphical methods-V. HALTAFALL, a general program for
calculating the composition of equilibrium mixtures. Talanta 14, 1261.
80
MECN (http://www.mba.ac.uk/MECN/)
The Marine Environmental Change Network (MECN) is a collaboration between
organisations in England, Scotland, Wales, Isle of Man and Northern Ireland
collecting long-term time series information for marine waters.
81
Appendix 2
Consultation document circulated to MARG for comments
Defra contract ME2109: Developing a trial Monitoring Strategy for pH in UK Marine
Waters:
A proposed “baseline” study
The “baseline study”
We propose a two-year programme of carbonate system measurements that will
provide “baseline” data on the current levels of acid in UK waters. This work will
only look at the current level of acidity in marine water and the eco-system processes
that determine acidity. It will not look at effects of pH on the eco-system. “Baseline”
means a set of measurements that are carried out using the most appropriate
available technology and recording both the data and methods used so that any
measurements made at a future time can be referenced back to these measurements
with a knowable degree of accurately and precision.
What determines the acidity of marine waters
Chemical equilibration between components of the carbonate system principally: pH
(acidity), pCO2 (partial pressure of carbon dioxide), TA (total alkalinity) and TCO2
(total concentration of carbonate species) determines the acidity of seawater. There is
no evidence that reactions with silica containing compounds have any measurable
influence. The equilibrium constants describing the relationships are known and allow
the calculation of any two components when data is known for the other two.
However, there is still scientific interest in the limits (accuracy and precision) to
which this is true and for this monitoring programme we propose to “over determine”
the system, where possible measuring three or more components so that the
determined pH will be cross checked by calculating pH from the other carbonate
constituents.
Purpose of monitoring:

To provide reference points for the detection of long term changes against a
background of natural variation.

To provide a data set which will assist the development and validation of
numerical models that will enable better understanding of ocean acidification and
likely future rates of change.
Challenges

Atmospheric concentrations of CO2 are rising; as the sea is in equilibrium with
the air its CO2 concentration is increasing and so, through chemical equilibria, the pH
of the sea is falling.

On average the annual change is small >0.003 pH units and similar to the
precision of our measurement capability.

In the sea, change has the potential to be easily masked by the annual cycle in
pH resulting from biological production and decay and changes in geographical
sources of waters mixed at a given location at a given time.

Available data confirms the above statements but is of too poor a quality due
either to the frequency of the sampling, precision of the methods used and/or the
82
detailing of the methods used, to provide a “baseline” against which future changes
over the next 20 years will be measurable.

pH is in practice defined (from the activity of the hydrogen ion) against four
different scales and it can also be calculated from other components using different
equations. Any reporting of pH values must therefore be accompanied by full and
traceable details of the procedures used to derive the reported value.
Requirements

Measurements made using the best available technologies and methodologies.

Measurements documented in such away that derived values of pH can be
recalculated, if required, using future improvements in the accuracy of the chemical
equilibria of the carbonate system in sea water.

Measurements be made with sufficient frequency that the annual range of pH
in UK waters is defined and, for the first time, meeting the criteria above.

Measurements made with sufficient geographical spread that both dynamic
changes resulting from hydrographic and biogeochemical processes can be realised.

Measurements made of sufficient carbonate system parameters that the system
is “over defined” and the results of direct measurement and calculation of the
components can be compared to assess the accuracy and precision of current
methodologies, and so that recalculations can be performed in future, if necessary, due
improvements in knowledge.

Measurements be made at defined times and regular locations so that they are
appropriate for use in the development and validation of prognostic models.

Measurements of carbonate system parameters be made along side
measurements of the hydrodynamic and biogeochemical state so that both direct
understanding and prognostic modelling expertise is improved by the availability of
the new data sets.

The methodologies and calculation used by different laboratories must be
traceable between the laboratories. Best practice should be applied following the
internationally agreed recommendations (currently CDIAC (DOE, 1994) Handbook of
methods for the analysis of the various parameters of the carbon dioxide system in sea
water. (Version 2), A. G. Dickson & C. Goyet, eds. ORNL/CDIAC-74. which is under
revision ).

Measurements made by individual laboratories should use the certified
reference solution supplied by A. Dickson at Scripps for the determination of total
alkalinity and total CO2. In addition an inter-comparison of results of measurements
made on common samples should also be carried out.
Proposed Programme: Stage 1: the initial two year baseline study
Where to monitor
Previous observations and modelling work undertaken to inform this proposal suggest
that UK marine waters may all be equally affected by increased CO2 levels while in
estuarine regions control of pH will be dominated by biogeochemical forcing well
into the future. On this basis we propose a baseline study which will be based around
four key existing UK non-estuarine programmes. The use of existing platforms and
sampling infrastructure enables cost effective collection of new samples and data.
Some of the required measurements are already being made in these projects and
advantage can be taken of elements of existing data sets. The four programmes will
83
provide data with the required spatial and temporal coverage to determine baseline
conditions and allow the robustness of UK waters to be assessed.
The four programmes are, in order of distance from shore:

POL Coastal Observatory, Liverpool Bay, RV Prince Madog

Western Channel Observatory (L4, E1, data buoys off Plymouth, RV
Plymouth Quest transects)

NOC Pride of Bilbao (VOS) data collection

UEA Transatlantic (VOS) data collection
(VOS is Voluntary Observing Ship)
POL Coastal Observatory, Liverpool Bay: provides detailed coverage of a region of
strong fresh water influence in a nutrient enriched system. Waters are sampled over a
grid of stations 8 times per year using the RV Prince Madog. The Prince Madog
carries a CarbonOps autonomous pCO2 system. Ecosystem modelling in this area can
be carried out on much higher resolution grid than any other region of UK waters.
PML Western Channel Observatory: builds on nearly a century of hydrographic,
biological and chemical sampling off Plymouth at L4 (50o 15.00’N, 004o 13.02’W)
which is sampled weekly depending on weather, and E1 (50o 02.00’N, 004o 22.00’W)
sampled fortnightly. The installation of additional moored data buoys within the
Oceans 2025 Programme will improve the resolution of this data. The RV Plymouth
Quest visits these stations regularly and is already instrumented for pCO2 with the
autonomous CarbonOps system - providing detailed information between Plymouth
and the seasonally stratified waters at the E1 site, 22 miles offshore. This will allow
observation of the balance of production and respiration processes above and below
the stratified layer to be determined.
NOC Pride of Bilbao (VOS) data collection: provides high time resolution data (each
position along the track is resampled at an interval between 4 hours and 3 days) this
will allow detailed recording of changes in pH in relation to biogeochemical activity
in a wide range of environments from nutrient enriched harbours to the deep
temperate latitude Atlantic Ocean. It provides key information on the differences in
behaviour between shelf seawaters and the ocean waters that are the source for those
shelf sea waters
UEA Transatlantic (VOS) data collection: provides data from the longest running UK
data set of marine carbon data. It is critical that we can assess the variability in the
ocean uptake of CO2 and the extent and implications of the consequent year to year
fluctuations in pH.
Present Status:
Carbonate system
pCO2 is the only parameter that is currently measurable automatically and at
sufficiently high resolution with confidence. Autonomous pCO2 analytical systems
are presently operational in each of the 4 survey areas, on the MV Santa Maria, MV
Pride of Bilbao, RV Prince Madog and RV Plymouth Quest.
84
pH can now be measured to the degree of precision required for this work ~0.002 pH
using a flowing liquid junction electrode system (developed at PML for the EA).
However, experience with this system in terms of maintaining that precision is
lacking. This is currently being evaluated by NOC. Measurements of the required
precision can achieved with a colorimetric method and pH indicator solutions. A
resolution of 0.001pH has been achieved with a laboratory-based automated system.
An automated system capable of autonomous operation is being developed by NOC
and should be operational by the end of 2007.
TA and TCO2 can only be measured to the required degree of accuracy and precision
on discreet samples. We propose to over determine the system by collection and
measurement of samples in each of the 4 survey area. These are a key measurement as
an internationally accepted certified reference material will be used to control the
accuracy of these analyses.
Other biogeochemical system determinands. By basing the work on 4 existing
monitoring programmes we can support this work within an existing frame work that
is already collecting high quality data.
Salinity data is available from all areas and calibration is based on well-established
procedures and traced back to use of IAPSO standard sea water.
Temperature data are commonly available but for carbonate system work data must
be accurate and precise to better than 0.1oK. A protocol for the required traceability of
temperature is being developed at NOC by Charlene Bargeron in consultation with
Andrew Dickson as part of the currently on going revision of DoE CDIAC (DOE
(1994) Handbook of methods for the analysis of the various parameters of the carbon
dioxide system in sea water. (Version 2), A. G. Dickson & C. Goyet, eds.
ORNL/CDIAC-74.)
Nutrients are currently measured throughout the year in each of the four programmes
by NOC and PML.
What to measure
All the systems will provide records of hydrographic conditions and changes in
concentrations of nutrients tracking the inputs of land based influences, over turn of
the water and the intensity of the annual production/respiration cycle in the different
areas. All areas will provide data from autonomous pCO2 systems and water samples
will be collected for the determination of total alkalinity and total CO2 so that the
carbonate system can be over determined in each area.
POL Coastal Observatory, Liverpool Bay:

8 times per year using the RV Prince Madog. Autonomous continuous pCO2
will be made on the survey cruises.

Waters samples for the determination of TCO2 and TA will be collected on
each survey and sent to NOC for analysis along with nutrient samples collected at
each station.
Western Channel Observatory (L4, E1, data buoys, RV Plymouth Quest transects)

Weekly section to L4.
85

Continuous pCO2 surface records.

Samples collected twice-monthly through the water column at E1 and L4 (5
depths*24 = 120 samples a year) measured at NOC or UEA

pH electrode system measurements made with the exEA system (options exist
of building a second system for use on RV Plymouth Quest or shipping the existing
system to and fro between PML and NOC for use on RV PQ and PoB).
NOC Pride of Bilbao (VOS) data collection

Continuous observation of pCO2 330 days per year.

20 samples for TA and TCO2 collected on each of 8 manned crossings per
year = 160 samples measured at NOC.

pH electrode system measurements made with flowing liquid junction capable
of resolving pH to 0.002 will be implemented to run continuously along side the pCO2
system.

pH measured by automated colorimetry will be implemented in 2007.
UEA Transatlantic (VOS) data collection

Continuous observation of pCO2 >120 days per year.

20 samples for TA and TCO2 will be collected on each of 13 crossings per
year = 260 samples measured at UEA. [NB, collection of discrete samples was
subsequently deemed to be impractical with untrained ships staff.]
Standardisation:
Equipment
All the analytical and sampling equipment used in this work will be traceable through
the meta data provided alongside the reported data.
Methods
The analytical methods used will follow current best practice as described in the DoE
CDIAC Handbook.
Calculations
When reporting data derived from calculations, data will be accompanied by
statements of which carbonate system parameterisation have been used.
86
Appendix 3
ME2109 Summary of Feedback to Consultation Exercise
(with consortium responses to comments in italics)
CEFAS
Broadly supportive (with others) of the need for this work and the proposed approach
and recognises the need for some technical development and equipment resourcing
before embarking on study.
Raises the issue of whether the study has the ‘power’ to resolve anthropogenic signals
from natural variability.
Under consideration.
Raises (with others) the absence of sampling in the North Sea or further north in more
pristine waters, e.g. off Scotland.
We concur, with the provisos below:
The Dutch already have extensive data for the North Sea and propose to
continue this work at intervals. There could be potential for International
collaboration/exchange of data.
There is a need to sample pristine, northern waters if only to corroborate the
model results which imply that this is not necessary. However, each additional
sampling scheme and the associated analyses will increase costs proportionately.
We are developing linkages with Dave Pearce at CEFAS
Advises formation UKMMAS ad-hoc working group to better integrate National
schemes.
We agree.
Environment Agency
Broadly supportive of these proposals and keen to integrate (and possibly enhance)
EA data collection in fresh and transitional waters that might contribute to this work.
We have some doubts over the precision and accuracy of pH measurements in
estuaries and rivers.
Marine Strategy Branch, SEERAD
Key issue is the absence of sampling other than in the Channel, and Liverpool Bay
We have made contact with FRS Aberdeen and SAMS at regarding Oban
additional sampling opportunities.
Also notes that data handling, archiving and availability were not dealt with.
To be addressed in the revised monitoring proposal.
87
Defra
Perceived need to broaden sampling scheme geographically and into estuaries.
We accept that wider sampling should be undertaken, possibly in Scottish waters; this
will have a cost implication.
The complexity and extreme variability of estuaries (very high respiration and
primary production with wide salinity range and variable river composition) means
that it is presently difficult to generate sufficiently high quality data in estuaries.
Defra
Definition of carbonate system?
Covered in the review report
EDU
Concern over water depths/volumes and ‘age’ of water
Only significant for ocean depths, not an issue for shallow, coastal (shelf)
waters
Marine Data Information Partnership IACMST
On behalf of MDIP and MEDAG, provision should be made to ensure the data are
lodged with the appropriate MDIP DAC. Also we could recommend that a clear
statement on policy with regard to data access and use is put in place at the beginning.
In hand.
88
Appendix 4
Written responses to Strawman consultation exercise
Via Defra
The proposed monitoring sites have taken advantage of current monitoring
programmes which as stated in the specification is welcomed. However the sites
chosen are all located in the South and Western sides of the UK. One of the aims of
the pilot study is to give us baseline data for the UK, we fully understand that it is not
possible to gather data across the whole of the UK Shelf, however as the programme
currently stands, there will be gaps in the North and to the East. Would it be
possible to include measurements in the East (North Sea) and to the North (off of
Scotland)? Members of the UKMMAS community do have monitoring programmes
in these areas, some of which may be able to develop a network of pH CO2
measurements. It is understood that this may increase the costs, but if the
programmes are already running then these costs should be minimal. It may be
necessary to carry out an intercalibration exercise, training and competence regimes
to ensure that the data is intercomparible across the UK, however the inclusion of
different sites around the UK would provide a better overall UK baseline.
There was also some concern that no information on estuaries would be collected. It
is known that the data will be variable but if we do not have baseline data from any
estuaries then we will not be able to see future changes in trends (if there are any). In
the proposal it also states that the strategy will "provide records of hydrographic
conditions and changes in concentrations of nutrients tracking the inputs of land based
influences......" it is not clear how this will be achieved if there is no monitoring in
estuaries or closer inshore.
Environment Agency
We would certianly support the proposal in priciple as we see long term high
frequency data as key to understanding changes we detect in programmes such as
CSEMP and the Water Framework Directive. We consider the long term effects of
acidification as a potential pressure on the marine environment which could have an
effect on our assessments of ecological status. With this in mind we support this
initiative.
The Environment Agency takes a limited number of pH samples in marine waters
mainly in connection with existing European drivers such as the Dangerous
Substances Directives, Shellfish Waters Directive, Bathing Waters Directive and the
Titanium Dioxide Directive. We would of course be happy to supply an extra data
from these programmes if it was seen to be of use.
Marine Data Information Partnership
A comment on behalf of MDIP and MEDAG, is that that provision should be made to
ensure the data are lodged with the appropriate MDIP DAC.
89
Also we could recommend that a clear statement on policy with regard to data access
and use is put in place at the beginning (perhaps by using/adapting the data clause
adopted by DTI for their North Sea work).
EDU
Not clear from the proposal what depths of water will be monitored other than a
general statement of 5 depth measurements at WCA. Depth is important since
different "ages" of water may have different acidities. Any overall assessment of
acidity will have to factor in the different volumes of the various bodies of water.
Marine Strategy Branch, SEERAD
Many thanks for letting us see this, through Jamie, and for the opportunity to
comment. I have discussed with FRS and offer these comments based on very helpful
observations by them.
The program as drafted does not at all fully cover UK waters nor does it make full use
of existing monitoring programmes, including those at FRS. Given the existence of
CEFAS, DOVE, FRS, NAFC, SAMS and MILLPORT this seems odd. It concentrates
its proposed efforts in the English Channel and western approaches with one other site
in Liverpool Bay, so certainly will yield very poor spatial resolution given the
diversity of waters around the UK, and the North Sea is not covered at all.
We recognise that, as is stated, any trends in pH are likely to be similar in all offshore
UK waters whereas trends in inshore/estuarine areas are driven much more by
biogeochemical fluxes from land/sea interactions. However, particularly for Scotland
with its semi-enclosed Clyde Sea and varied basins and lochs of the west coast, many
economically important shelf areas, apart from the channel and inshore Liverpool
Bay, will not be monitored in this proposal. Climate changes will result in increased
rainfall etc and so the land-sea interactions will change, perhaps also for pH in coastal
waters. Surely gathering monitoring data around all UK waters will be a step towards
understanding not only coastal regions but their interactions with offshore regions too.
On a political and scientific note I think this not a good start since MARG's remit
is about integrating UK monitoring efforts. This is an opportunity to initiate a new
“standard” set of measurements and a joint programme for monitoring with fruitful
exchange of expertise and information. There are also no costings in this document
associated with the proposal so it is not easy to judge feasibility of including other
sites and areas, but even some other measurement sites and measurements would add
much to the study.
We fully accept and agree that this pilot project should not aim to look at effects of
acidification on the ecosystem. However, such a narrow approach would very much
reduce the effectiveness of the work, especially as an ecosystem approach is central to
government policy. As much of the monitoring will be done in association with other
monitoring for many other ecosystem elements (e.g. at L4 off Plymouth) it makes
90
sense to ensure; (1) that there is best use made of sites where other kinds of data are
regularly monitored, and (2) that at least metadata for such associated measurements
are collated and stored alongside the pH monitoring results. Else, when it comes to
estimating or modelling the effects of any measured trends, the programme will not be
able to interpret variability or easily relate its pH data to any other environmental or
biological trends. For example it is already known that pH is altered by the presence
of phytoplankton blooms.
While we applaud the technically comprehensive approach to measurements, we
would also expect to see a more comprehensive description of how the data and
results will be stored, analysed and shared.
CEFAS
Comments on Consultation Document – A proposed “baseline” study.
1.
It is understood that the ‘consultation document’ is an early output from the
ME2109 project. Reviewing the proposal would be facilitated if the final report of the
project had been available. This may explain some of the choices made, from any
options considered, in meeting the original specification for the project. In particular
understanding how the ‘baseline’ work may fit into an overall national strategy and
when that would be trialled.
2.
The proposal seems to be focussed on, first, getting the technical side of
making these ‘difficult’ measurements sorted and, second, to get some information
about environmental variability in coastal waters.
3.
Given the challenges, and given our experience of perhaps simpler
measurements [semi] autonomously, there is a risk that the proposed two-year study
may not deliver sufficient information. Before embarking on any practical trial it
would be prudent to analyse the power required of the programme to detect change
(using the methods developed by Nicholson and Fryer) and covering estuaries as well
as coastal waters.
4.
We recognise the need to start somewhere and it seems that the NOC Pride of
Bilbao operation offers the most effective test-bed of those proposed. This is because
it seems likely to provide the most frequent sampling, opportunity to give the
equipment a good level of support and because it visits several different water types.
Will the 8 surveys, per annum, from Prince Madog provide sufficient temporal
resolution.
5.
The ‘over-determination’ and stringent data and meta-data regime are to be
commended as essential if precise and, importantly, accurate measurements are to be
made. Given what is already known about environmental variability and our
measurement capability it appears that we may be looking for a needle in a haystack.
6.
The rationale for the range of supporting measurements is less clear. It seems
undoubted that the seas are becoming more acidic from consideration of basic physics
and chemistry (and the well established model studies) so the real question is the ‘so
what?’ one. However, the proposal excludes the effects of pH on the ecosystem.
7.
While it is clearly sensible to make the best use of existing observation
programmes, the proposal seems to miss an opportunity (in the context of the
UKMMAS) to make use of other agencies observation programmes in order to
91
transfer knowledge and to broaden the geographic scope. The links to the AQC and
Protocols Groups in UKMMAS is not made.
8.
The absence of a programme in the North Sea, or in Scottish waters, seems
odd given that aspects of the North Sea situation were flagged in the original
specification and that ‘risk’ is deemed to be the same for all UK waters (can this be
right?).
9.
There is no information about the proposed costs of the programme to judge
likely value for money.
10.
The area of work is new and has significant associated technical challenges.
The risk of failure is reduced by the technical competence of the PML, NOC, POL,
UEA teams but would benefit from greater consideration of the overall national and
ecological context. A suitably constituted UKMMAS ad-hoc working group would
be a sensible means of taking the work forward.
92
Appendix 5
ME2109 Factors limiting additional UK sampling capability for pH
The review presented here has determined that, with the present UK capability, pH
cannot be measured directly with sufficient precision and accuracy to detect
anthropogenic signals within the natural seasonal variability. There is instrument
development at PML and NOC (and possibly elsewhere) that may allow direct pH
measurement in due course either by potentiometric means, with borderline accuracy,
or using spectrophotometry with better precision/accuracy. Development and
establishment of confidence in these direct approaches is dependent on the parallel
carbonate observations outlined below.
The only practical and sufficiently accurate approach to determining pH that is
currently available is to calculate it from two out of three of the other carbonate
species: dissolved carbon dioxide gas (pCO2), total alkalinity (TA) or dissolved
carbonate (TCO2). We argue that for confidence, we need to over determine the
carbonate parameters so that there is scope for internal cross-checking.
There is a reasonable UK capability to measure pCO2 - a number of ships, including
most of the UK Research Vessel fleet, are instrumented with sufficiently high quality
apparatus.
The limiting factor is the UK capability to measure TA and TCO2 on discrete samples
with sufficient precision/accuracy. This capability presently only resides at NOC.
We are not aware of other UK capability at present. Investing in the necessary
hardware, standards and training to provide additional analytical capability would
likely cost £150K in the first year and £50K/year to operate thereafter (staff and
consumables).
The proposed monitoring scheme for pH in UK waters takes advantage of three
existing sampling schemes: 1) Plymouth’s Western Channel Observatory – L4 & E1
sampling, 2) the Southampton, Pride of Bilbao (VOS) continuous programme and 3)
the POL Liverpool Bay Observatory with routine sampling from Prince Madog. It is
proposed that discrete samples for TA & TCO2 be taken on each of these schemes in
addition to pCO2 measurements. Even at a modest sampling frequency, the discrete
samples generated by these three sampling schemes would almost exceed the NOC
capacity to measure TA & TCO2.
A fourth sampling scheme at Aberdeen (FRS, Stonehaven Station) has been identified
where routine sampling for biological and chemical parameters has been in place for
almost 10 years and the collection of additional samples for TA and TCO2 is practical.
This sampling site is attractive because it extends the UK coverage further to the
north. Aberdeen presently does not have the pCO2 or direct pH capability to parallel
this sampling so pH calculation would be dependent on just two carbonate parameters
measured on discrete samples at NOC. However, more fundamentally at the
moment, the extra samples involved exceed the analytical capability at NOC.
93
There will almost certainly be other UK marine sampling schemes underway with the
potential to collect samples cost-effectively. From the literature and personal
experience, we are not aware of any additional schemes where systematic carbonate
or pH work is undertaken – certainly not to the necessary precision and accuracy.
These schemes could collect more samples but, as identified previously, the capacity
to analyse them is currently limited.
In the longer term, the potential exists through technological development e.g. direct
spectrophotometric measurement of pH at high precision and improved potentiometric
measurement systems, which would enable the spatial and temporal scale of the
monitoring of pH in UK waters to be increased without the analytical overhead
associated with the precision measurement of TA & TCO2 on discrete samples.
94
Appendix 6 Outline costings for a UK trial monitoring scheme as proposed
Year 1
Year 2
Task 1 total
£63,105
£17,641
£14,962
£13,041
£108,749
£63,105
£64,433
£15,421
£12,667
£155,625
£108,749
£155,625
Task 2 total
£24,660
£24,660
£24,660
£24,660
£24,660
£24,660
Task 3 total
£40,000
£40,000
£40,000
£40,000
£40,000
£40,000
Task 4 total
£38,685
£10,800
£49,485
£3,851
£6,400
£10,251
£49,485
£10,251
Task 5 total
£11,343
unknown
£11,343
£11,854
unknown
£11,854
£11,343
£11,854
£234,237
£242,390
TASK 1 Two year programme to establish “baseline” condition in UK water
types.
NOC
PML
PML
FRS
PIF 1
PIF 3
Analyses plus PoB & POL sampling
WCO sampling†
Project management
Aberdeen sampling*
TASK 2 NOC North Atlantic Data Base
UEA
RV Santa Maria programme (no sampling)
TASK 3: UK acid seas monitoring, data archive, management and web interface
BODC
Data Management (0.5 person)
TASK 4: Reliable direct measurements of the pH of seawater
PIF pH
PML
CEFAS
pH sensor production
pH sensor development
TASK 5: Design of a statistically-robust, long-term programme
PIF 5
PML
CEFAS
Data assessment/modelling
Statistical evaluation
* note 1: there is still a sample capacity issue at NOC over FRS involvement.
†note
2: underway pCO2 for RVs Madog and Quest covered by NERC in kind contribution (£300K Carbon-Ops)
until Feb 2009; costs included here for 10 months continuation.
95
Grand
Total