Download Simultaneous determination of PM fractions, particle number

Simultaneous determination of PM fractions, particle number and particle size distribution
in high time resolution applying one and the same optical measurement technique
Pletscher, Karsten, Dipl.-Ing. (FH), Palas® GmbH, Karlsruhe, Germany
Weiß, Maximilian, Dr.-Ing., Palas® GmbH, Karlsruhe, Germany
Mölter, Leander, Dipl.-Ing. (FH), Palas® GmbH, Karlsruhe, Germany
Abstract
The Fidas® fine dust monitoring systems comprise an optical aerosol spectrometer, which is
characterized by very good instrument characteristics, and utilize a sophisticated approach to
convert the measured information on particle number and particle size into mass
concentrations. As monitoring of PM requires reliable and quality-controlled performance of
the measuring systems, the described fine dust monitoring systems have a comprehensive
concept for quality assurance and have demonstrated its suitability for regulatory monitoring
of PM10 and PM2.5 in ambient air in an extensive third party approval process. Besides
regulatory monitoring, the Fidas® fine dust monitoring systems can be used in many further
applications like for portable and flexible measurements indoor or at workplaces and even for
measurements in the airspace. Common for all discussed fine dust monitoring systems is the
provision of comprehensive, accurate and reliable information on the fine dust content in the
air, which can help to understand processes and to make informed decisions in order to reduce
the ambient air pollution in future.
1
Gleichzeitige
Bestimmung
von
Feinstaubfraktionen,
Partikelanzahl
und
Partikelgrößenverteilung in hoher zeitlicher Auflösung durch Anwendung von ein und
derselben optischen Messtechnik
Zusammenfassung
Die Fidas® Feinstaubmesssysteme beinhalten ein optisches Aerosolspektrometer, welches sich
durch sehr gute Gerätekenngrößen auszeichnet, und verwenden einen ausgeklügelten Ansatz
zur Umrechnung der gemessenen Information über die Partikelanzahl und –größe in
Massenkonzentrationen. Da die Überwachung von Feinstaub eine verlässliche und
qualitätsgesicherte Leistung der Messsysteme verlangt, verfügen die beschriebenen
Feinstaubmesssysteme über ein umfangreiches Konzept zur Qualitätssicherung und haben ihre
Eignung für die behördliche Überwachung von PM10 und PM2,5 in der Außenluft durch einen
umfangreiche externe Zulassungskampagne demonstriert.
Neben der behördlichen
Überwachung, können Fidas® Feinstaubmesssysteme auch in vielen anderen Anwendungen
wie z.B. für portable und flexible Messungen in Innenräumen oder an Arbeitsstätten oder sogar
für Messungen im Luftraum eingesetzt werden. Gemeinsam ist allen behandelten
Feinstaubmesssystemen die Bereitstellung von umfangreichen, genauen und verlässlichen
Informationen zum Feinstaubgehalt in der Luft, die dabei helfen können Prozesse zu verstehen
und fundierte Entscheidungen zu treffen um zukünftig die Luftverschmutzung zu senken.
2
1. Introduction
Air pollution due to particulate matter (abbreviated PM in the following text) and the related
negative effects (health problems, economical damages) has become one of the major
problems our society is faced with today. According to WHO [1],
- 23 % of all estimated global deaths are linked to the environment,
- 4.3 million deaths per year result to exposure to indoor smoke from cooking fuels,
- 3.7 million deaths every year result to exposure to fine dust.
In order to be able to investigate and to assess the real exposition of the general public in a
comprehensive way and to make well-informed decisions, the performance of precise and
accurate measurements of PM is an essential part of air pollution control – outdoor as well as
indoor. Especially for ambient air monitoring, the majority of applied measuring technologies
only allow measurements at the particle collective and the determination of particle mass
concentrations for one explicit size fraction only. These methods usually neither offer a high
time resolution in the range of minutes nor deliver any information which particle sizes are
present at what concentration (number or mass) for any point of time during the
measurement. This extra information can be very useful in terms of deeper analysis of the
aerosols with respect to health effects or for conclusions /assignments to specific events and
sources.
By implementing a counting optical aerosol spectrometer, the fine dust monitoring system
Fidas® enables to measure simultaneously both particle concentration as well as particle size
distribution. In order to be able to determine particle mass concentrations based on the
aforementioned measurements in a precise and reliable way and to achieve a good correlation
to measured results from gravimetric standard reference methods, the optical aerosol
spectrometer has to fulfil various technical requirements. These requirements comprise for
example an implemented unique calibration curve, the minimizing of border zone effects and
the minimizing of coincidence effects also for high concentration levels. The better these
technical requirements are fulfilled, the better technical characteristics like size resolution and
classifying accuracy are obtained. Very good technical characteristics are the fundamental
basis for a reliable conversion of the number distribution into a mass distribution. Upon this
basis, the accurate determination of particle mass concentrations requires especially
representative information on particle density, particle shape and refractive indexes. This
information is compiled in the so-called evaluation algorithm, which needs to be applied on
the number distribution to obtain the intended particle mass concentrations. For ambient air
monitoring it is of great importance to cover as much as possible different PM scenarios with
only one evaluation algorithm, hence information from many comparison campaigns of the
optical aerosol spectrometer with the gravimetric standard reference method need to be
collected and evaluated. By this, a solid and representative basis for the conversion calculation
is created. Especially for regulatory PM10 and PM2.5-monitoring in ambient air, a high level of
3
confidence in the measured values is mandatory and the calculated particle mass
concentration values have to comply with the strict requirements of demonstrating
equivalence to the gravimetric standard reference method [2; 3]. The suitability for this
measurement task has to be approved explicitly by a third party based a type-approval test
and it is confirmed and maintained in a well-defined certification process.
The measurement technology of the Fidas® sensor allows the simultaneous measurement at
a high time resolution of the relevant PM fractions like e.g. PM10 and PM2.5, the particle
number as well as the particle size distribution. The sensor is implemented in various fine dust
monitoring systems , e.g. in the fine dust monitoring system Fidas® 200, which is approved
and certified for regulatory PM monitoring in line with EN Directive 2008/50/EC [4]. It can be
successfully used in various applications to investigate and evaluate PM conditions in order to
be able to make well-informed decisions.
The following chapter describes the measuring principle and the implemented technology of
the implemented sensor, which is the core of all Fidas® fine dust monitoring systems.
4
2. The measuring principle of the Fidas® sensor
The Fidas® fine dust monitoring systems implement a counting optical aerosol spectrometer
(Fidas® sensor). The measuring principle is scattered light analysis on a single particle, where
the scattered light signal depends on the particle diameter, the refractive index and the
particle shape. Compared to scattered light photometers, which detect the scattered light
signal generated by a particle collective and are thus dependent additionally on the particle
size distribution, optical aerosol spectrometers deliver unambiguous signals, which do not
change
in
response
to
changes
in
the
particle
size
distribution.
With a scattered light analysis on a single particle, the particle size and number can be
determined simultaneously but independently.
The particles individually move through a measurement volume with optical delimitation that
is homogeneously lit with white light. Each individual particle produces a scattered light pulse
that is detected at an angle of 85° to 95°. The particle concentration is detected based on the
number of scattered light pulses per volume. The pulse amplitude is a measure of the particle
characteristic, the particle diameter.
In optical aerosol spectrometers, the particle diameter characteristic is derived from the pulse
amplitude. The particle quantity is derived from the number of pulses, yet independently. The
histogram in figure 1 illustrates the number of particles versus the measured particle size class.
FIGURE 1
The better the classification accuracy and the resolution of a particle measuring device, the
narrower the category - particle size class - can be defined. This device characteristic
determines the quality of the aerosol spectrometer. The quality of the aerosol spectrometer
is determined by the quality of the opto-electronic components used, the optical design and
the signal processing.
Light scattering
The measuring principle of an optical aerosol spectrometer (OAS) is based on the Lorenz–Mie
theory. The particle property, the diameter, is determined using single particles. In the case of
spherical particles, this measuring principle thus corresponds to the "true Mie theory".
Measurements using the particle collective do not correspond to the "true Mie theory".
If light with wavelength λ collides with a spherical particle with diameter d and refraction index
m, it is scattered in various directions. The light scattering at the particle is caused by
5
diffraction, refraction and reflection. The plane of polarization of the incoming light wave is
also influenced. The intensity I of the light scattered by the individual particle depends on the
intensity of the incoming light I0, the angle of polarization φ, the detection angle of the
scattered light θ, the refractive index m, the light wavelength λ and the particle diameter d.
(see figure 2).
I = I0 · f (Φ, Θ, n, λ, d) (1)
The scattering parameter α, which was introduced by Mie,
α
πd
(2)
λ
is used to describe the relation between sphere circumference π x d and wavelength λ.
If α is used in equation (1), the result is equation (3):
I = I0 · f (Φ, Θ, n, α) (3)
FIGURE 2
The introduction of scattering parameter α makes it possible to distinguish between three
scattering ranges:
a) Rayleigh range: α<<1; here the scattered light intensity increases by the sixth power of
the particle diameter. The scattered light is proportional to d6/λ4. That means: If a particle
with half the size of the previous particle is to be measured in the Rayleigh range (lower
detection limit), doubling the amount of light will not suffice. The required amount of light
has to be approx. 64 times stronger than for a particle twice the size. The proportion 1/λ4,
with λ being in the denominator, is responsible for the sky appearing blue.
b) Mie range: 0.1≤α≤10; in this range, the relationship between the scattered light intensity
and the particle size is not uniform for certain optical configurations.
c) Fraunhofer range: α>>1; in this range, there is a square relation between the scatter light
intensity and the particle diameter.
The Mie theory can be used to calculate the calibration curve of optical particle measurement
systems in relation to the diameter for individual spherical particles. Nevertheless it is
necessary to calibrate an optical particle measuring device experimentally, using test aerosols
with a known size and refractive index. Commercial optical measuring systems can be divided
into devices which collect the forward-scattered light and devices which use right-angle
6
scattered light detection. The light sources can be divided into polychromatic incandescent
light and monochromatic laser light.
The influence of the monochromatic and polychromatic light on the calibration curve is
demonstrated in figure 3 and 4. The graphs depict the calibration curves in relation to the
particle diameter for monochromatic laser light and polychromatic white light and the mean
scattering angle of approx. 90°. The curve in figure 3 shows the typical ambiguities in the
respective light wavelength when monochromatic light is used. Furthermore figure 3 shows
that smaller particles can be better detected with short-wave light. White light also contains
short-wave light. The ambiguities mentioned above can be compensated for by using either
white light and a mean scattering angle of 85° and 95° or monochromatic light and wide-angle
collecting optics. In figure 4, the difference in the calibration function between white light and
laser light is illustrated. In the Mie range, i.e. in the range of the laser light wavelength up to
approx. 6 x λ, it is not possible to make unambiguous particle size measurements with any
other optical setup.
FIGURE 3
FIGURE 4
With the white light source and the 90° scattered light detection, a unique calibration curve is
obtained. See figure 4. No ambiguities (such as always are present in the Mie range if
monochromatic light is used) occur between the scattered light pulse and the particle
diameter. Ambiguities that result from the shape of non-spherical particles are reduced if a
white light source is used. In the sensor at hand, a white light LED is used, which is
characterized by a high luminous density, a low energy consumption and a very long life-time
of more than 40,000 hours. Accurate measurements of particle sizes from 180 nm up to 100
µm are possible.
Moreover the unique and unambiguous calibration curve (incl. the Mie range) and the use of
only one amplifier / AD-converter for the entire particle size range allows for the calibration
of the measurement with just one particle size over the entire particle size range. The given
possibility of checking the calibration with just one particle size allows for an easy on-site
quality control of the sensor.
Optical measurement volume delimitation
Optical measurement volume delimitation offers various benefits for particle size and particle
number determination in high particle concentrations.
7
Advantages:
1.Measurements in very high concentrations
2.Homogeneous illumination of the measurement volume
Figure 5 illustrates the influence of the measurement volume size for measurements in high
and low concentrations. In the top and bottom images, the particle count is identical. In the
case of particle measurements in high concentrations, many particles are present in a large
measuring volume simultaneously (coincident), see the image on the bottom left of figure 5.
When coincidence occur during a particle measurement, as shown here, only a single
scattered light pulse is obtained from the multiple particles. The pulse amplitude is too high,
i.e. the quantity detected as "one particle" is much too high and the result for the
concentration measurement is too low.
FIGURE 5
Hence to achieve coincidence-free measurements in very high particle concentrations, a very
small optical measurement volume is required. The described sensor allows coincidence-free
measurements up to Cn = 20,000 particles/cm³.
Handling of border zone errors
Due to the Gaussian intensity distribution of a laser beam, a so-called border zone error occurs
during the particle size measurement. If a particle is in the middle of the laser beam and a
particle with the same size is located at the border of the laser beam, the value measured for
the border particle will be much too small. If a particle is in the middle of the measurement
volume and half of a particle with the same size is outside the measurement volume, the
measured value will also be too small (border zone error).
It has to be pointed out, that in particle size measurements with border zone errors, the values
measured for the particles are too small. The larger the particle to be measured, the larger
the border zone error, i.e. the values measured for the particles are much too small.
Additionally if the aerosol flow is larger than the optical measurement volume, which is always
the case in devices with optical measurement volume delimitation, accurate concentration
determination is only possible if the size of the optical measurement volume is precisely
known.
In order to ensure an accurate concentration determination and to eliminate the
aforementioned border zone errors, the patented T-aperture technology is used in the fine
dust sensor – by this the exact sizes of the measuring volume is known in order to be able to
perform an accurate concentration determination. Furthermore with the T-aperture
technology, no border zone error occurs.
8
T-aperture technology
The Fidas® sensor is based on the patented T-aperture technology. A T-shaped threedimensional measurement volume is created by placing two T-apertures in the optical path
(figure 6). If a particle traverses the center of both volumes, its size is calculated from the pulse
amplitude and the particle number is derived from the number of pulses. If a particle only
moves through the upper volume, the measured value is discarded. This technology makes it
possible to determine the particle sizes and particle numbers in low concentrations with large
measuring volumes and in high concentrations with small measuring volumes without border
zone errors.
FIGURE 6
Digital signal processing, coincidence detection and correction
The Fidas® sensor implements digital signal processing and coincidence detection / correction.
In digital signal processing, each individual pulse is assigned a time stamp. For each pulse, the
time at which the particle entered the measurement volume, the particle's flight time through
the measurement volume, and the pulse amplitude are detected and saved.
Coincidence during a particle measurement occurs if a single scattered light pulse is obtained
from the multiple particles (simultaneous presence of more than one particle in the measuring
volume at a time). In that case, the pulse amplitude is too high, i.e. the quantity detected as
"one particle" is much too high and the result for the concentration measurement is too low.
By using the known measurement volume length and the measured flight time, the coincident
signals and the signals from the border zone of the T-aperture are distinguished from the
correctly measured signals from the center of the T-shaped measurement volume and are
saved.
The coincident fraction of the measuring signals is detected in acc. with the theory of Prof.
B. Sachweh, Dr. H. Umhauer, F. Ebert, H. Büttner and R. Friehmelt (1998): In situ optical
particle counter with improved coincidence error correction for number concentrations up
to 107 particles cm−3. J. Aerosol Sci. 29, 1075–1086 and (if wanted) the measured size
distribution is corrected accordingly. It has to be pointed out, that due to the very small size
of the measured volume in the sensor at hand, coincidence-free measurements are possible
up to a high particle concentration Cn = 20,000 particles/cm³. This means that under
ambient conditions usually there is no need to activate any coincidence-correction.
Besides the elimination of border zone errors and the detection / correction of coincident
signals, the digital signal processing leads to further benefits:
-
Velocity measurement for each particle in the measurement volume
-
High-time resolution with saving of every individual signal
9
Last but not least the counting efficiency for small particles has been significantly improved
due to digital signal processing, which leads – together with optimized optics and high
luminous density of the used LED – to a lower limit for the measurement range of only 180
nm. This lower detection limit is especially relevant for measurement of smaller PM-fraction
such as PM1 and PM2.5.
The aforementioned sensor is the core of all Fidas® fine dust monitoring systems The following chapter now describes the principle of operation of the fine dust monitoring
systems.
3. The principle of operation of the Fidas® fine dust monitoring systems
In all Fidas® fine dust monitoring systems, surrounding air with particles is actively sampled
with the help of a sampling pump.
In order to determine the PM fractions, particle number and particle size distribution in high
time resolution with the aforementioned implemented sensor technology, the following
consecutive steps are followed:
1. Representative sampling of particles of varying sizes. For ambient air monitoring, the
Sigma-2 inlet according to Standard VDI 2119 [5] is used. This passive sampling inlet
allows wind-independent sampling of the aerosol due to settled wind and lowturbulence conditions inside.
2. For ambient air monitoring the sample is conditioned via the Intelligent Aerosol Drying
System (IADS), a heated sampling line, which is controlled as a function of ambient
temperature and relative humidity. The heating procedure is designed in a smart way
in order to dry the particles, avoid condensation of water / fog and likewise prevent
vaporization of semi-volatile particles.
3. After sampling conditioning, the sample enters the sensor and the scattered light
intensity for single particle is measured using the white light LED source and 90°
scattered light detection.
4. Each measured particle (pulse) is getting a time stamp and signal length. With the help
of the T-aperture and the digital signal processing, border zone and coincident signals
are filtered.
5. The particle size (optical diameter) is determined by allocating the scattered light
signal to a particle diameter based on the clear and unique calibration curve, which is
based on PSL. The measuring ranges are from 180 nm up to 100 µm.
6. The measured particle sizes are measured with 256 raw data channels and classified in
size classes in form of a histogram with a resolution of 32 size classes per decade.
10
7. In order to assign representative values for the refractive index, the particle size
distribution is converted from a distribution, based on latex diameter to a distribution
based on an empirically determined refractive index for the environment. This
information is included in the evaluation algorithm.
8. Furthermore the particle size distribution, based on an optical diameter, is converted
into a distribution based on the aerodynamic diameter. This information is included in
the evaluation algorithm.
9. The achieved distribution form is analyzed again and density parameters are assigned
to the different size classes. This information is included in the evaluation algorithm.
10. The separation behavior of individual PM-sampling inlets (impactor heads) is
transformed to the size distribution in order to use the same separation characteristics
as for the gravimetric standard reference methods.
11. In the last step the particle mass is calculated via the size-dependent conversion
function as a function of the distribution form and the PM values are formed.
The determination of the particle number concentration and the particle size distribution is
possible with time resolutions down to the range of seconds. Due to statistical reasons, a
proper formation of PM values can be done down to a time resolution of one minute. With
this time resolution, it is assured, that under ambient conditions enough particles will be
present to be counted and evaluated, especially for the coarser fractions.
The relevant information for the conversion of the size-resolved particle number
concentration into PM values (like refractive index or density parameters) is included in the
evaluation algorithm. The determined size distribution can be considered as a raw data set
and the evaluation algorithm is applied on top of that. For fine dust monitoring in ambient air,
a special evaluation algorithm has been developed in order to make representative
assumptions of the relevant conversion parameters. During the development phase, plenty of
investigations on the optical aerosol characteristics in ambient air for various sites and
conditions have been performed. Basis for all evaluations is the comparison of the fine dust
monitor with standard reference systems. By applying a Monte-Carlo method, the evaluation
algorithm for ambient air has been constantly improved and optimized and has been finally
validated in the type-approval test of the Fidas® 200, performed by TÜV Rheinland between
2012 and 2013.
Subsequent internal evaluations as well as further third-party comparison campaigns
underline, that this evaluation algorithm for ambient air, is capable to handle very different
conditions. This indicates that optical characteristics, densities and form factors of ambient
aerosols seem to be very similar.
While the evaluation algorithm has been fixed for regulatory monitoring of PM 10 and PM2.5 in
ambient air and has been approved and certified by third parties, there is nevertheless the
11
possibility to apply also other algorithms, which can be customized based on available
information of characteristics of a specific aerosol or by comparison measurements with other
samplers or monitors. By this the PM values derived with different parameter settings can also
be evaluated and be compared.
The determined measured values for PM fractions, particle number and particle size
distribution are stored on the internal data logger, can be transmitted via various data
protocols and can be also accessed online via Internet (e.g. on Palas® webserver). Furthermore
full remote monitoring and control of the instrument from any PC on the world via an internet
connection is possible.
All described fine dust monitoring systems are characterized by a low energy-consumption
(small carbon footprint) and a low demand for maintenance thus resulting in low operational
costs. Due to the development of very small and light-weight sensors, a wide range of
applications in the field of fine dust monitoring is given. This will be depicted in chapter 6-8 of
this article.
The following chapter deals with the different measures for quality control of the described
fine dust monitoring systems.
12
4. Quality-controlled operation
Monitoring of fine dust requires accurate and reliable measuring systems. This is especially
the case for 24/7 monitoring of official limit values to demonstrate compliance with
environmental legislation. In order to ensure quality-controlled performance at any time, the
following measures for quality control are implemented in the described fine dust monitoring
systems.
External check and adjustment of the calibration
As already described in chapter 2, the fine dust sensor uses a unique and unambiguous
calibration curve (incl. the Mie range) and only one amplifier / AD-converter for the entire
particle size range. By this it is possible to check and even adjust the calibration with just one
particle size over the entire particle size range. This method is carried out by offering a monodisperse test aerosol with known particle size (MonoDust 1500, d p=1.28 µm) to the system
and check, if the offered test aerosol leads to a clear peak in the expected raw data channel
corresponding to the correct particle size (refer to figure 7). If the obtained peak is outside of
the permissible limits, it is possible to adjust the photomultiplier voltage and shift the
calibration curve in such a way, that the peak corresponds again to the correct size class.
FIGURE 7
The calibration procedure with MonoDust 1500 is easy to handle and can also be performed
onsite at any time with limited time need. It also spares the user from sending back the
instrument to a workshop or to the manufacturer for regular re-calibrations.
During the type-approval test at TÜV Rheinland, the long-term stability of the peak position
for the Fidas® 200 was investigated and at first instance a regular check interval of one month
was granted. Approval testing for an extension of the check interval from one month to three
months is successfully completed.
The external check and adjustment of the calibration with MonoDust 1500 can be performed
for all described fine dust monitoring systems.
13
Online monitoring of the calibration
Especially for the Fidas® 200, which is designed for the regulatory monitoring of PM10 and
PM2.5, a continuous monitoring of the calibration status is a very helpful feature in order to
assess the instrument conditions also from remote places and to detect possible drift effects
in time. This has been realized by a patented method for online monitoring of the optical
amplification.
Due to the shape of the calibration curve, the described fine dust monitoring system has an
optical kink that can be evaluated for online monitoring of the correct amplification of the
entire measurement chain. Figure 8 shows the raw data distribution in 256 channels for a
Fidas® 200 during an ambient air measurement.
FIGURE 8
With a correct optical amplification, there is an accumulation of signals exactly in raw data
channel 100 due to a flat shape of the calibration curve in that area. The position of the peak
at raw data channel 100 can be used to monitor the stability of the calibration curve, as the
position of this peak tends to move away from channel 100 in case of e.g. contamination of
optics or other effects that shift the entire calibration curve. The system continuously
monitors the position of the peak and if it shifts more than 3.5 raw data channels averaged
over a 40 h period, a status signal is generated. The utilized physical effect depends only on
the light spectrum of the white light LED and therefore needs no calibration or adjustment.
However it has to be noted, that the monitoring principle only works, if a certain particle
distribution pattern “Junge-Distribution” can be assumed across a particle size range from
0.38 ̶ 0.50 µm. This is the case in the big majority of ambient air measurements. The online
monitoring of the calibration is a very helpful additional source for assessing the
instrument performance without the need to be present on site (remote control).
The operator can decide whether or not it needs to be serviced on-site, which in practice
leads to possible significant reduction of on-site maintenance visits and therefore cost savings.
In case of drifts or other suspicious behaviour, it is easy to check and adjust the device on-site
with the mono-disperse test aerosol at any time.
The online monitoring of the calibration is implemented in all Fidas® fine dust monitoring
systems, which are designed for 24/7 operation.
14
Further measures for quality control
Beside the possibility to monitor the calibration online, various other operational parameters
are also continuously monitored and recorded. The number and the type of monitored
parameters depends on the type of the used fine dust monitoring system and is most
comprehensive for the instruments designed for 24/7 operation. The monitored parameters
includes for example the nominal flow rate, the performance of the sampling pumps, the
temperature of the sample conditioning system IADS as well as the coincidence status. Due to
assigning a time stamp to every measured particle (digital signal processing) and due to the
exactly known dimensions of the optical measurement volume, it is also possible to calculate
the velocity of the particles in the sensor. This value can be compared to a nominal value
determined during factory calibration and deviations between actual and nominal velocity can
indicate for example a disturbed flow due to leaks.
The instruments designed for 24/7 operation are equipped with two pumps for redundancy.
Under normal conditions both pumps run in parallel. If one pump fails, the second pump is
capable to keep up the flow rate. As the pump performance is monitored, a status signal
indicates a pump failure while the instrument is still operated with the correct flow rate.
Breakdown times due to pump failures are minimized.
Additionally to the external check of the calibration with MonoDust 1500, the fine dust
monitoring systems allow for further easy and fast functional checks, e.g. for instrument
tightness, flow rate and instrument zero.
15
5. Type-approval and certification
As the standard gravimetric reference method for PM measurement is not able to deliver
direct online measured values with a high time resolution and is therefore not suited to
investigate short-term events and to kick off appropriate information to the public or any
counteraction in time, continuous monitoring with automatic PM monitors is of great
importance. At the same time it must be ensured, that the provided measured data for PM
are of ongoing high quality to give the required confidence in the results.
Whereas binding legal requirements on the performance of fine dust monitoring systems for
indoor and workplace measurements are still missing, there is comprehensive legislation
available in the area of ambient air monitoring. For example the EU Directive 2008/50/EC on
Air Quality and Clean Air for Europe [4] requires in its annex VI, that all measuring system used
for regulatory monitoring of limit values need to be either the reference method or an
equivalent method.
As the reference method for PM is a gravimetric method [3], all automatic PM monitors have
to demonstrate equivalence with the reference method. This demonstration of equivalence is
carried out in a type-approval test by a third-party test house. Upon commissioning of the
instrument manufacturer, the test house performs various test under laboratory and field
conditions incl. a complete equivalence test according to the European Guide on
Demonstration of Equivalence (GDE) [2]. The minimum requirements and the test procedures
for this type-approval test are currently described in various VDI and EN Standards but will be
compiled in near future in the Standard EN 16450 (currently available as EN TS 16450 [6]).
In Germany - in case of successful testing in laboratory and field - the test house submits a test
report to the relevant committee “FG Prüfberichte” for assessment. In case of a positive
assessment, the committee recommends to the responsible LAI committee the publication of
the tested system as a type-approved monitor in the German Federal Gazette.
Since 2009, the type approval is part of a complete certification scheme for automated
measuring systems for both stack monitoring as well as ambient air monitoring, which is
described in the Standard EN 15267-1 [7]. This certification scheme consists of the following
sequential steps:
-
Type approval test
Initial assessment of quality management system of manufacturer
Certification
Product surveillance after certification
This means, that beyond the successful passing of a type approval test, the quality
management system of the manufacturer has to fulfil specific requirements especially in terms
of a consistent manufacturing process and a controlled handling of modifications to the
certified system. The compliance of the implemented QMS of the manufacturer with the
requirements stated in Standard EN 15267-2 [8] is verified during an initial audit before the
first certification and then monitored on an annual basis in surveillance audits. Only if both
16
the type approval test and the initial audit are approved by the relevant committee, a
certificate is issued by the German Federal Environment Agency (UBA) and the test house and
is issued in the German national register under [9].
The German approval / certification serves also as a basis for the British MCERTS/DefraApproval [10], which nevertheless requires some additional testing and specific evaluation.
In 2012 the fine dust monitoring system Fidas® 200 S (designed for ambient air monitoring
with outdoor installation) has been submitted to TÜV Rheinland, Cologne, Germany for typeapproval testing for the relevant components PM10 and PM2.5. The tests have been carried out
with two identical and complete measuring systems and started with a laboratory test in order
to check compliance with general requirements on construction as well as with performance
requirements such as e.g. detection limit or dependency of measured values on ambient
temperature. After successful passing of the laboratory tests, the field test started. The field
test consists mainly of an equivalence testing in line with the GDE [2]. This means, that
comparison campaigns between the gravimetric standard reference method [3] and the
candidate systems have been carried out – four comparison campaigns at in total three
different sites with different PM characteristic (urban background, traffic, rural area) and over
a total time of approx. one year to cover all seasons. The total number of valid data pairs (24 h
basis) has been greater than 220. The comparison measurements are evaluated with respect
to the “in-between-uncertainty” between both candidate systems as well as to determine the
calibration function between reference and candidate via orthogonal regression. Furthermore
the expanded measurement uncertainty has been determined for the full data set, the single
campaigns and for data sets split ≥ 30 µg/m³ for PM10 and ≥18 µg/m³ for PM2.5. The obtained
expanded measurement uncertainties fulfilled the requirement of 25 % for all comparisons
already for the raw data set for PM10 and for PM2.5 after slope and offset correction. So the
equivalence with the reference method could be clearly demonstrated. Figure 9 shows the
comparison between the two candidate systems for PM10, Figure 10 shows the comparison
between the reference method and the candidate for PM2.5, Figure 11 shows the comparison
between the reference method and the candidate for PM10.
FIGURE 9
FIGURE 10
FIGURE 11
During the field test, further test points such as availability, maintenance interval, long-term stability of zero and span (the latter checked with MonoDust 1500) are checked.
The test report on the type approval test for the Fidas® 200 S has been assessed positively by
the relevant German committee in late 2013 and the declaration of suitability has been
published in the German Federal Gazette (Bundesanzeiger) in spring 2014.
17
Based on the successful type approval and based also on confirmed compliance of the quality
management system with EN 15267-2, the respective EN 15267 certificate was published in
2014 and is available on [9].
After the initial certification, the process of ongoing surveillance of the certified product and
the manufacturing process has started in 2014. Since then several modifications on the
instrument have been documented, assessed and approved finally by the relevant committee
in Germany. These modifications comprise among others the approval of the instrument
versions Fidas® 200 (for installation at temperature-controlled sites like cabinets) and Fidas®
200 E (with external sensor unit).
The concept of the certification scheme of EN 15267 allows for an ongoing, focused and
controlled further development of the type-approved and certified fine dust monitor.
In 2016 the system has additionally been granted by the British MCERTS/Defra-approval [10],
which is another independent confirmation of the compliant performance of the Fidas® 200
for regulatory monitoring of PM10 and PM2.5.
18
6. Application „Regulatory PM-monitoring“
Regulatory monitoring of fine dust PM in ambient air is required by legislation (e.g. European
Directive 2008/50/EC) and his carried out in monitoring networks since many years. Due to
the limitations of the gravimetric reference method with respect to time resolution (24 h) and
contemporary availability of measured data (usually available after 1-2 weeks after sampling),
the operation of automatic measuring system is necessary to be able to deliver measurement
results with high time resolution and without delays. As described in the chapter 5, the precondition for using automatic measuring systems for official monitoring task, is the successful
passing of a type-approval test and an up-to-date certification. As the Fidas® 200 has achieved
both the German TUV/UBA-approval as well as the British MCERTS/Defra-approval, the
system fulfils the necessary requirements to be implemented in monitoring networks all over
Europe and beyond. Due to the availability of three different versions of the fine dust monitor
(refer to Figure 11), the system can be installed in new measuring cabinets (Fidas® 200) or
completely outdoor (Fidas® 200 S, due to weatherproof IP65-cabinet) as well as be integrated
in existing infrastructures with spatial limitations (Fidas® 200 E, due to external sensor unit).
FIGURE 12
The use of an optical aerosol spectrometer for regulatory monitoring of PM 10 and PM2.5 in
monitoring networks offers several advantages. First and foremost the user does not only get
simultaneous information on PM10 and PM2.5 in high time resolution, but also other size
fraction like PM1, PM4 and TSP as well as the particle number concentration and the particle
size distribution. The following figure 13 until figure 15 illustrate exemplarily the high amount
of information, which can be retrieved when using the described fine dust monitoring system.
Figure 13 shows the course of time for various PM fractions at a traffic hot spot site (15min
time resolution). Figure 14 shows selected particle size distributions for number
concentrations out of the same measurement period and figure 15 shows the same particle
size distributions, this time for mass concentrations.
FIGURE 13
FIGURE 14
FIGURE 15
This additional information can be very helpful for the investigation of specific time periods
(like days of exceedances) or events, as for example changes in the size distribution might
19
indicate a change in the aerosol composition and thus a change in the contribution of different
sources. Hence the obtained information can also be helpful for source appointment.
Beyond this high amount of extra information on the aerosol, the described fine dust
monitoring systems have low operational costs due to low power consumption, very limited
maintenance work, almost no consumables, no radioactive material to handle and the
capability for full remote monitoring (incl. calibration status) and control. Necessary QA
measures can be done easily and in short time on site and the external check and adjustment
of the calibration with the mono-disperse test aerosol offers a tool for on-site stability check
and spares the user from regular sending back of the systems to a workshop for recalibration.
20
7. Application „Portable measurements“
Beyond the field of stationary regulatory monitoring of PM10 and PM2.5 in ambient air, there
is also a wide range of applications for short-term measurements of the PM content, especially
in the fields of workplace measurement / industrial hygiene as well as for indoor air. Usually
this type of measurements requires a highly flexible and portable PM monitor, which can be
operated battery-supplied and can be easily carried around.
The regular sensor system, which is implemented for example in the Fidas® 200 systems, is
already quite compact in size and weight (approx. 1.4 kg), but still is too large and too heavy
to be incorporated in real portable monitor. Hence a new compact and light-weight sensor
unit has been developed by scaling down the well-tried regular sensor systems both in size
and weight but still applying exactly the same measurement technology and using the same
evaluation algorithm. This lead to a compact sensor system with a weight of only around 200
g and so small that it can be easily integrated in a case smaller than shoebox. Based on this, a
new portable fine dust monitoring system for indoor and workplace applications – Fidas® Frog
– has been developed.
FIGURE 16
This new portable device applies exactly the same measurement technology like the regular
Fidas® sensor systems and thus offers the same advantages like scattered light analysis at
single particles (no dependency on changes in particle size distribution), a unique calibration
curve (unambiguous signals over the entire range, calibration with one particle size possible),
a very small measurement volume (coincident-free measurements up to 20,000 P/cm³), Taperture technology (no border zone errors) as well as digital signal processing. It also offers
the simultaneous determination of PM1, PM2.5, PM4, PM10, TSP, particle number and particle
size distribution (32 size channels per decade) in a measured size range from 180 nm up to 40
µm (two ranges in total) at high time resolution. The implementation of the aforementioned
technology together with the use of the evaluation algorithm for ambient air (= conversion of
size distribution information into mass concentrations resp. PM fractions), which has been
approved versus the gravimetric reference method in many comparison campaigns, allows
PM measurements at a high confidence level.
There is nevertheless the possibility to apply also other algorithms, which can be customized
based on available information of characteristics of a specific aerosol (e.g. at workplaces) or
by comparison measurements with other samplers or monitors. By this the PM values derived
with different parameter settings can also be evaluated and be compared.
21
This system is compact in size (100 x 240 x 150 mm) and weight (2.1 kg) and can either be
operated with battery (up to 8h operational time) or with mains voltage. It is controlled by a
detachable tablet PC, which is connected to the measuring unit via WLAN. This allows a
remote control of the measurement and thus measurements at difficult to access sites or sites
with unpleasant or even unhealthy conditions. The set-up of a measuring network with several
of the described monitors and the central control via an external PC is also possible with that
concept. For quality assurance, it is also possible to check and adjust the calibration on site by
offering the monodisperse test aerosol MonoDust 1500.
This portable fine dust monitor can be used in a very flexible way and can be used for
measurements ranging from single spot-check measurements to long-term monitoring.
Applications range from indoor measurements (e.g. in public buildings, schools…), workplace
exposure measurements (HSE-management), measurement in cabins (car, public transport…)
up to (area-wide) monitoring of buildings / production facilities or processes.
22
8. Application „Measurements in airspace“
With the compact sensor, as described in chapter 7, a small and light-weight sensor system is
available, which even can be integrated in a carrier system like a flight robot. By this a
complete new dimension for PM measurements is opened up. Figure 17 shows the Fidas® Fly
200, which is the integration of the compact sensor into the flight robot HORUS of the
company Airclip, Dresden, Germany. However the compact sensor can basically be integrated
in other carrier systems (by land, sea and in the air), too.
FIGURE 17
The HORUS flight robot is designed as a flying clip on frame and can carry weights up to 6.5
kg. This means, that besides the compact fine dust sensor it could also carry further sensor
systems (e.g. for gases). Equipped with the compact sensor, it can fly between 20-30 min per
battery charge and the time need for battery changes on the ground is only two minutes. The
system can be flown completely manually but also even allows an independent performance
of way-point-flights under reproducible conditions. Because of redundant design, operation
safety is maximized.
Being able to perform simultaneous measurements for PM1, PM2.5, PM4, PM10, TSP, particle
number and particle size distribution (32 size channels per decade) in a measured size range
from 180 nm up to 40 µm (two ranges in total) at high time resolution also up in the air offers
various additional possible applications, such as:
•
PM monitoring esp. for line and area sources (e. g. roadside, open mining, quarries)
•
PM monitoring at difficult accessible sites (e.g. in or outlets of ventilation systems e.
g. for industrial processes)
•
Screening campaigns e. g. at construction sites, demolition projects…
•
Characterization of stack emissions, esp. when entering the atmosphere
•
Indoor air quality esp. in large buildings
•
R&D projects e.g. verification of emission
factors for diffusive sources or delivery of information for dispersion modelling
The capability to measure and monitor this wide range of parameters simultaneously and
with a high time resolution with a well-approved technique even up in the air space has a big
potential to contribute significantly to the future gain of knowledge for aerosol research.
23
9. Summary
Air pollution due to PM is a very serious matter and the related negative effects (health
problems, economical damages) have become one of the major problems our society is faced
with today. In order to be able to investigate and to assess the real exposition of the general
public in a comprehensive way and to make well-informed decisions, the performance of
precise and accurate measurements of PM is an essential part of air pollution control –
outdoor as well as indoor. For this a measurement method is needed, which is able to measure
simultaneously the particle number and the particle size and to provide the particle mass
concentrations online at a high time resolution. Thus optical aerosol spectrometers are well
suited for this task, if they fulfil certain technical requirements. Besides a high ability of
resolution and a good classifying performance, this requires for the basic measurement a
unique calibration curve as well as a method, which allows measurements without border
zone errors and without coincidence. The Fidas® sensors detect the scattered light in a 90°
angle on single particle. By using a polychromatic white light LED as source, a unique
calibration curve is obtained over the entire size range (starting from 180 nm up to 40 µm /
100 µm) with no ambiguous measured signals. Every single particle produces a scattered light
pulse, that is counted and whose height is a measure for the particle diameter. Due to the
very small size of the measurement volume, single particle analysis without coincidence is
ensured also for high concentrations up to 20,000 P/cm³. The patented T-aperture technology
together with digital signal processing ensures measured values without border zone error
effects as well as detection (and if desired correction) of coincident values. The described
sensor is the core part of all Fidas® fine dust monitoring systems. A representative sample of
particles is drawn into the system via a sampling line (conditioned for ambient air
measurements to prevent the measurement of water droplets as particles) and the particle
number and particle size distribution is determined in the sensor. In order to convert the
available information into mass concentrations, an evaluation algorithm is applied. This
algorithm contains representative information on parameters such as refractive index or
density and has been developed especially for ambient air monitoring out of measured results
from many different campaigns around the world. An independent confirmation of the
suitability of this evaluation algorithm for regulatory PM10 and PM2.5 monitoring has been
achieved by the successful passing of the type-approval test and the following approval and
certification in Germany and the UK. Monitoring of PM with confidence requires reliable and
quality-controlled performance of the measuring systems, especially if used for 24/7
regulatory monitoring. The described fine dust monitoring systems have a comprehensive
concept for quality assurances including unique features like an external check of the
calibration with a monodisperse test aerosol and the patented online monitoring of the
calibration status (only for 24/7 versions) and a wide range of status monitored status
parameters.
Due to binding legislative requirements for the use of an automatic fine dust monitoring
systems as an equivalent method to the gravimetric standard reference method, the Fidas®
200 systems, designed for regulatory monitoring of PM 10 and PM2.5 in ambient air, have
undergone an extensive third party approval, consisting of a type-approval test and an up-todate certification procedure. Due to the overall positive test results, the system has been
24
granted the status of an equivalent and type-approved PM monitor and respective certificates
in Germany (TUV/UBA) and the UK (MCERTS/Defra) have been issued. Thus the Fidas® 200
system can be used for official monitoring purposes in line with the European Directive
2008/50/EC.
A sensor system that provides simultaneous information on PM10 and PM2.5 in high time
resolution, but also other likewise information on size fraction like PM1, PM4 and TSP as well
as the particle number concentration and the particle size distribution, can be used in very
different applications. The possible applications range from stationary monitoring in line with
legislative requirements (type-approved and certified Fidas® 200 systems) over portable and
flexible measurements indoor or at workplaces (Fidas® Frog) up to measurements even in the
airspace (Fidas® Fly 200). Common for all these fine dust monitoring systems is the use of
sophisticated measurement technique, which provides comprehensive, accurate and reliable
information on the fine dust content in the air, which allows on the one hand the fulfilment
of official monitoring tasks and on the other hand provides a lot of additional information
which is very useful to investigate closer the character of the measured aerosol and to draw
conclusions on possible sources or processes. With this knowledge, it helps significantly to
understand processes and to make informed decisions in order to reduce the ambient air
pollution in future.
25
10. Literature
[1]
Link WHO http://www.who.int/gho/phe/en/
[2]
Guide To The Demonstration Of Equivalence Of Ambient Air Monitoring Methods,
Januar 2010
[3]
EN 12341: Ambient air - Standard gravimetric measurement method for the
determination of the PM₁₀ or PM2.5 mass concentration of suspended particulate
matter; German version EN 12341:2014
[4]
Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008
on ambient air quality and cleaner air for Europe
[5]
VDI 2119: Ambient air measurements - Sampling of atmospheric particles > 2.5 µm on
an acceptor surface using the Sigma-2 passive sampler - Characterization by optical
microscopy and calculation of number settling rate and mass concentration; June 2013
[6]
CEN TS 16450: Ambient air - Automated measuring systems for the measurement of
the concentration of particulate matter (PM10; PM2.5); German version
CEN/TS 16450:2013
[7]
EN 15267-1: Air quality - Certification of automated measuring systems - Part 1:
General principles; German version EN 15267-1:2009
[8]
EN 15267-2: Air quality - Certification of automated measuring systems - Part 2: Initial
assessment of the AMS manufacturer´s quality management system and post
certification surveillance for the manufacturing process; German version EN 152672:2009
[9]
German register of automated measuring systems, certified according to EN 15267
www.qal1.de
[10]
UK register of MCERTS certified products – MCERTS for UK Particulate Matter (Defra
Approval)
http://www.csagroupuk.org/wpcontent/uploads/2016/04/MCERTSCertifiedProductsDEFRACAMS.pdf
[11]
TUV Rheinland test report No. 936/21227195/A of 9th March, 2015: Report on
supplementary testing of the Fidas® 200 S respectively Fidas® 200 measuring system
manufactured by PALAS GmbH for the components suspended particulate matter
PM10 and PM2.5, English version
26
Overview tables / figures
Figure 1: Histogram
Figure 2: Principle of incoming scattered light
d - Particle diameter
λ - Light wavelength
m – Refractive index
Θ,Φ - Scattering angle
Figure 3: Scattered light intensity for different wavelengths of monochromatic light (scattering angle
85°-95°,m=1.59+0i)
27
Figure 4: Scattered light intensity of a white light source in comparison to a light source with
monochromatic light (scattering angle each with 85° - 95°, m = 1.59 + 0i)
Figure 5: Measurement volume sizes
Figure 6: T-aperture technology
28
Figure 7: Calibration screen with MonoDust 1500 peak at raw data channel 141.39
Figure 8: Raw data distribution with signal accumulation in channel 100
Figure 9: Results of parallel measurements with the devices SN 0111 / SN 0112, measured component
PM10, all test sites [11]
29
Figure 10: Reference device vs. candidate, SN 0112, PM2.5, all test sites, raw data [11]
Figure 11: Reference device vs. candidate, SN 0112, PM10, all test sites, raw data [11]
Figure 12: Approved versions of Fidas® 200 for regulatory monitoring
30
Figure 13: Course of time for various PM-fractions at the traffic hot spot site
Figure 14: Size distribution for number concentrations
Figure 15: Size distribution for mass concentrations
31
Figure 16: Portable fine dust monitoring system Fidas® Frog
Figure 17: Fidas® Fly 200
32