Download Quantifying the contribution to atmospheric iodine from

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Quantifying the contribution of marine organic gases to atmospheric
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iodine: Supporting Material - further method details
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Further details of ambient VOIC analysis
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Both GC/MS instruments incorporated a 50 m x 0.32 mm internal diameter SGE BPX5
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capillary column, and detection by electron impact ionisation quadrupole mass
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spectrometry. Calibration of the GC/MS instruments for ambient VOIC analysis was
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achieved based upon a permeation oven based technique described fully in Wevill and
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Carpenter [2004]. Permeation tubes containing pure liquid halocarbons (>97%, Aldrich)
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maintained in a thermostated oven with a constant nitrogen flow rate were used to deliver
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a constant calibration gas output containing dilute levels of halocarbons. Sequential 10 μl
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volumes of permeation gas were injected into a stream of nitrogen (BOC, CP grade) in
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order to generate a linear calibration of each halocarbon at pptv levels. Full permeation
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oven calibrations were performed at the beginning and end of each cruise, whilst day-to
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day variability in instrument sensitivity was accounted for using an in-house prepared gas
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standard, containing pptv-levels of all VOICs studied.
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During the MAP cruise the air sample inlet was located on the foredeck to the port side of
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the ship and during RHaMBLe the inlet was sited on the port side. During both cruises
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ambient air was pumped through a PFA Teflon line (50 m length for the MAP cruise, 20
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m length for RHaMBLe, both ½ i.d) using a diaphragm pump at a rate of ~30 l min-1,
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and clean air was sub-sampled diverted upstream of the diaphragm pump via a metal
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bellows pump (Senior Aerospace Limited) and delivered to a cold trap at -30 °C within
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the thermal desorption unit to pre-concentrate volatile components prior to GC/MS
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analysis.
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Sea-air flux calculations
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Sea-air fluxes were calculated as a function of the concentration difference across the sea-
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air boundary (ΔC, mol cm-3) and the total gas transfer velocity k (cm h-1, which
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incorporates both the waterside and airside transfer velocities - kw and ka, respectively),
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according to Equation (1). We used the Nightingale et al. [2000] parameterization for the
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waterside transfer velocity, (kw = {0.222u2 + 0.333u}{SC/660}-1/2) and the McGillis et al.
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[2000] expression to include the airside resistance (k = kw {1 – γa}) (where γa is the
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airside gradient function), with minute averaged wind speeds u (in m s-1), and the
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dimensionless Henry’s law coefficients (H) from Moore et al. [1995]. The Khalil et al.
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[1999] approximation was used to derive the temperature dependent Schmidt numbers
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(Sc) for each gas (Equation 2), where T is seawater temperature and M is the relative
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molecular mass. Sea-air fluxes were also calculated according to the Liss and Merlivat
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[1986] and Wanninkhof [1992]
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parameterizations resulted in mean average CH3I, CH2ICl and CH2I2 fluxes for both
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cruises which differed by ±25-30%.
parameterizations for kw, and the three different kw
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F = kΔC
(1)
Sc = 335.6M1/2 (1-0.065T + 0.002043T2 - (2.6x10-5)T3)
(2)
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Further details of the Ocean Mixed Layer Model
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The ocean mixed layer (OML) model contains explicit parameterisations of the
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mechanisms which drive mixing within the surface ocean. Mixing in surface waters
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occurs as a response to two broad categories of forcing: wind-driven forced convection
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(caused by surface and internal waves, shear instabilities, secondary circulations - e.g.
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Langmuir cells - and various interactions between them) and density instability-driven
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free convection. Buoyancy instabilities driving pure free convection may be a result of
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surface cooling or evaporation. Free convection is usually combined with some degree of
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forced convection. A model aiming to accurately reproduce the behaviour of physical
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properties in oceanic surface waters must therefore explicitly describe or parameterise the
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above mechanisms adequately. The OML model is based on Large et al. [1994] and uses
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a parameterisation for the depth-resolved eddy diffusivity profile based on criteria
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consistent with the conservation equations in their primitive form. The model uses a non-
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local eddy diffusivity (K) profile parameterisation (KPP) after Troen & Mahrt [1986]
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with local parameterisations for the three mixing processes in the interior added by Large
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et al. [1994], as well as adaptations of the rules for matching boundary layer properties at
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the ocean interior, and of the boundary conditions at the surface and bottom.
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The model was validated for physical property prediction by running with a standard set
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of initialisations and external forcings, and comparing predicted physical properties such
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as the mixed layer depth with those obtained by six other turbulent vertical mixing
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models reported in Martin [1985, 1986] under the same synthetic forcing conditions.
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Having validated the model skill in reproducing physical properties, it was modified and
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extended to integrate the vertical production, destruction and transport of dissolved
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gaseous CH2I2 and CH2ICl.
The depth-dependent biological production terms were
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equivalent in structure to a typical Chl a profile (based on in-situ measured depth profiles
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of Chl a for MAP) maximising at the bottom of the predicted mixed layer, and were
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simply scaled to approximately reproduce the observed CH2ICl and CH2I2 concentrations
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at 2 or 6 m depth. Maximum production rates (at the Chl a maximum) were ~6 pmol dm-
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3
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photolysis rates were calculated using wavelength-resolved solar irradiance from
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NASA’s
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(http://snowdog.larc.nasa.gov/jin/rtset.html) combined with absorption cross-sections and
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quantum yields for photolysis of dihalomethanes in seawater from Jones and Carpenter
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[2006]. Sea–air exchange losses are parameterised using the wind speed dependent gas
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transfer velocities used in sea-air flux calculations (described above). Initialising the
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model with vertical salinity, temperature and velocity profiles (temperature and salinity
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from in-situ profile measurements during MAP and from archived BODC data from the
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same region and time of year for RHaMBLe; initial velocity profiles were based on
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acoustic doppler current profiler (ADCP) measurements from previous studies in similar
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regions) and forcing it with in-situ ship-board meteorological observations (~10 m above
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sea level), the physical properties and VOIC vertical profile evolution were predicted.
d-1 and ~1.5 pmol dm-3 d-1 for CH2I2 and CH2ICl, respectively. Depth-dependent
Coupled
Ocean
Atmosphere
Radiative
Transfer
(COART)
model
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Further details of the MISTRA 1D Atmospheric Model
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The one-dimensional Lagrangian MISTRA model, which contains detailed treatment of
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the thermodynamics and microphysics and explicit gas and aqueous phase chemical
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mechanisms, was used to simulate iodine sources to the MBL. The gas-phase chemical
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mechanism used in the MISTRA model was updated to the latest IUPAC
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recommendation (June 2006), with a revised DMS mechanism mainly based upon Barnes
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et al. [2006]. The chemistry of hydrocarbons and halogenated hydrocarbons was revised
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by including more explicit treatment of the intermediates, based on the Master Chemical
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Mechanism protocol [Saunders et al., 2003]. Besides chlorine and bromine inorganic
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chemistry, the model includes a detailed iodine mechanism, similar to the one described
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in Pechtl et al. [2006], with some minor revisions and updates.
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The model is initialised with ozone mixing ratios typical of clean sub-tropical MBL air
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that has travelled for at least 3 days in the boundary layer from the mid-latitude to the
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sub-tropical Atlantic Ocean (30-40 ppbv, Read et al., [2008]). The model was initially
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run for 1.5 days to spin-off the meteorology and the microphysics, after which the
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chemistry was reinitialized and the model run for 3 days.
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Although the total organic iodine flux derived from our measurements is similar to that of
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the Vogt et al. [1999] base case fluxes used in the model (which were based on coastal
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studies), the relative abundance of each gas is different, with lower CH2I2 and higher
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CH3I fluxes from the measurements; the net effect being much slower release of reactive
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iodine using the measured fluxes.
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Sensitivity tests on MISTRA model
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Sensitivity tests were carried out on the iodine mechanism in the model, in an attempt to
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resolve the discrepancy between the modeled and measured concentrations of IO
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(average measured IO was ~1.4 pptv, compared to the maximum modelled value of ~0.3
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pptv). Rate coefficients and/or products and branching ratios of selected reactions were
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altered, including: mass accommodation coefficients of HOI and HIO3, photolysis rates
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of OIO, I2O2 and CH2I2, products of the decomposition reaction of I2O2, products of the
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OH+IBr, I+INO3 reactions and of the IO self-reaction, formation of HIO3 and I2O3,
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photolysis of iodinated oxygenates formed by peroxy radicals self- and cross-reactions
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and addition of a speculative HIO3 loss term. However, none of these reactions impacted
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the modelled concentrations of IO by more than a few percent, except when HIO3
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formation was set to zero or when a loss term for HIO3 (with a pseudo first-order rate
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coefficient of 1.0x10-3 s-1) was added to the mechanism, but even in this case the model
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still underestimated the observed IO mixing ratios by up to a factor of 3. Even doubling
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the measured VOIC fluxes did not provide an iodine source sufficient to produce the
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observed IO concentrations (doubling the upwelling VOIC fluxes resulted in IO
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concentrations of ~0.6 pptv).
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The model was also run with an iodine mechanism similar to that used by Gomez-Martin
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et al. [2009], which resulted in higher modeled IO (~0.6 pptv compared to ~0.3 pptv with
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the initial mechanism). Although a direct comparison between the two models is not
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straightforward, we believe that the main reasons for the differences are (i) the Gomez-
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Martin et al. [2009] mechanism does not include the IO+CH3O2 reaction, (ii) the rate
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coefficient of the OIO+OH reaction used by Gomez-Martin et al. [2009] is 6x10-12 cm-3
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molecule-1 s-1 and the product is HOI, while in our mechanism the rate coefficient is
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5x10-10 cm-3 molecule-1 s-1 (at 298 K, Plane et al. [2006]) and the product is HIO3, and
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(iii) mass accommodation coefficients for inorganic iodine species are a factor of 2 (HI)
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and up to a factor of 50 (HOI) lower than those used in the present work. The differences
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in the gas-phase mechanism may or may not be significant (the reaction IO+CH3O2 is, in
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fact, extremely uncertain), although the model is very sensitive to HIO3 formation and
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loss. The ultimate fate of iodine is accumulation in particles in the form of iodide and
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iodate [Pechtl et al., 2006] and this typically occurs by uptake of inorganic iodine (mostly
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INOx, HIO3 and IxOy); therefore all processes that reduce the formation rate of these
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species and/or their uptake onto particles (such as OIO+OH and the mass accommodation
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coefficients) will result in higher concentrations of inorganic iodine in the gas-phase.
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Supporting references
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Barnes, I., J. Hjorth and N. Mihalopoulos (2006), Dimethyl sulfide and dimethyl
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sulfoxide and their oxidation in the atmosphere, Chemical Reviews, 106, 940-975.
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Gomez-Martin, J. C., S. H. Ashworth, A. S. Mahajan and J. M. C. Plane (2009),
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Photochemistry of OIO: laboratory study and atmospheric implications, Geophys.
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Res. Lett., 36, L09802.
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Jones, C. E. and L. J. Carpenter (2006), Solar photolysis of CH2I2, CH2ICI, and
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CH2IBr in water, saltwater, and seawater, Environ. Sci. Technol., 40(4), 1372,
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doi:10.1021/es058022e.
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Khalil, M. A. K., R. M. Moore, D. B. Harper, J. M. Lobert, D. J. Erickson, V.
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Koropalov, W. T. Sturges and W. C. Keene (1999), Natural emissions of chlorine-
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containing gases: Reactive Chlorine Emissions Inventory, J. Geophys. Res. - Atmos.,
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104(D7), 8333-8346.
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Large, W. G., J. C. McWilliams and S. C. Doney (1994), Oceanic vertical mixing: a
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review and a model with a non-local boundary layer parameterization, Rev. of
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Geophys., 32, 363-403.
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Liss, P. S. and L. Merlivat (1986), Air-sea gas exchange rates: Introduction and
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synthesis, in The Role of Air-Sea Exchange in Geochemical Cycling, edited by P.
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Buat-Menard, pp. 113 – 127 D. Reidel, Norwell, Mass.
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Martin, P. J. (1985), Simulation of the mixed layer at OWS N and P with several
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models, J. Geophys. Res., 90, 903-916.
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Martin, P. J. (1986), Testing and comparison of several mixed-layer models, U. S.
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Moore, R. M., C. E. Geen and V. K. Tait (1995), Determination of Henry Law
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Constants for a suite of naturally-occurring halogenated methanes in seawater,
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Mössinger, J. C., D. E. Shallcross and R. A. Cox (1998), UV-VIS absorption cross-
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sections and atmospheric lifetimes of CH2Br2, CH2I2 and CH2BrI, J. Chem. Soc.
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Faraday Trans., 94, 1391-1396.
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Nightingale, P. D., G. Malin, C. S. Law, A. J. Watson, P. S. Liss, M. I. Liddicoat, J.
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parameterizations using novel conservative and volatile tracers, Global Biogeochem.
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Pechtl, S., E. R. Lovejoy, J. B. Burkholder and R. von Glasow (2006), Modeling the
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possible role of iodine oxides in atmospheric new particle formation, Atmos. Chem.
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Phys., 6, 505-523.
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Plane, J. M. C., D. M. Joseph, B. J. Allan, S. H. Ashworth and J. S. Francisco (2006),
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An experimental and theoretical study of the reactions OIO + NO and OH + OH, J.
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Phys. Chem. A, 110(1), 93-100 doi: 10.1021/jp055364y.
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Rattigan, O. V., D. E. Shallcross and R. A. Cox (1997), UV absorption cross-sections
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and atmospheric photolysis rates of CF3I, CH3I, C2H5I and CH2ICl, J. Chem. Soc.
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Faraday Trans., 93, 2839-2846.
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Read, K. A., et al. (2008), Extensive halogen-mediated ozone destruction over the
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tropical Atlantic Ocean, Nature, 453, 1232-1235 doi:10.1038/nature07035.
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Saunders, S. M., M. E. Jenkin, R. G. Derwent and M. J. Pilling (2003), Protocol for
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the development of the Master Chemical Mechanism, MCM v3 (Part A): tropospheric
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degradation of non-aromatic volatile organic compounds, Atmos. Chem. Phys., 3,
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Vogt, R., R. Sander, R. von Glasow and P. J. Crutzen (1999), Iodine chemistry and its
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role in halogen activation and ozone loss in the marine boundary layer: A model
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Troen, I. and L. Mahrt (1986), A simple model of the atmospheric boundary layer -
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sensitivity to surface evaporation, Boundary-Layer Meteorology, 37, 1-2, 129-148.
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Wanninkhof, R. (1992), Relationship between wind-speed and gas-exchange over the
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ocean, J. Geophys. Res. - Oceans, 97(C5), 7373-7382.
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reactive volatile halogenated organic compounds in the atmosphere, The Analyst, 129,
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634-638.
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Supporting Material Figure S1. Rate of iodine atom release from VOICs within the
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atmospheric boundary layer, based upon the open ocean sea-air fluxes (ocean) and
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upwelling fluxes (upwell). Total VOICs corresponds to iodine atom production from all
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measured VOICS (CH3I, C2H5I, 1-C3H7I, CH2IBr, CH2ICl and CH2I2).
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