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Correction notice Nature Geosci. 6, 108–111 (2013) Atmospheric iodine levels influenced by sea surface emissions of inorganic iodine Lucy J. Carpenter, Samantha M. MacDonald, Marvin D. Shaw, Ravi Kumar, Russell W. Saunders, Rajendran Parthipan, Julie Wilson & John M. C. Plane In the version of this Supplementary Information originally posted online on 13 January 2013, equation (21) and the units for [O3] in the text after equation (19) were incorrect. These errors were corrected on 27 March 2013. In the version of this Supplementary Information originally posted online on 13 January 2013, and in the revised Supplementary Information posted online on 27 March 2013, equations (20) and (21) contained errors. These errors were corrected on 25 April 2013.
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Supplementary Information
Atmospheric iodine levels influenced by sea surface emissions of inorganic
iodine
Lucy J. Carpenter1*
, Samantha M. MacDonald2, Marvin D. Shaw
1, Ravi Kumar
2, Russell W.
Saunders2, Rajendran Parthipan
1,3 , Julie Wilson
4,1, and John. M. C. Plane
2*
1Department of Chemistry, University of York, Heslington, York YO10 5DD, UK
2School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK
3Now at College of Chemical Sciences, Institute of Chemistry, Ceylon, Sri Lanka
4Department of Mathematics, University of York, Heslington, York YO10 5DD, UK
*Correspondence to: lucy.carpenter@york.ac.uk and j.m.c.plane@leeds.ac.uk
Methods
Gaseous I2 measurements by spectrophotometry
Ozone was produced from dry hydrocarbon-free air by its exposure to a commercial ozone
generator (185 nm excitation, UVP) and monitored using a model 49i ozone analyzer (Thermo
Scientific). Ozonolysis of iodide solutions was carried out using a 2 L reaction vessel with a 49
cm2 surface area of solution (Fig. S1). Flow rates over the solution were maintained at 0.2 L
min-1
. 20 mL iodide solutions (10-6
- 10-5
M KI: AnalaR®
, BDH, ≥ 99.0 %) were prepared either
from phosphate (Fluka, 99.8 %) - buffered HPLC water at pH 8 or by spiking seawater collected
from the coastal waters of Cargese, Corsica (42.14º N, 8.60º E, filtered through 0.45µm filter
paper (Whatman) and stored in amberised glass bottles at 2- 8 °C) with iodide (0.2 mL, 1 x 10-3
M). Iodide solutions were administered into the reaction vessel using a gas tight syringe
(Samco) via a Luer lock tap.
Control experiments were conducted to establish whether ozone was deposited to the
experimental system in the absence of solution, to HPLC water or to the phosphate buffer
solution in the absence of iodide. The uptake of ozone in all control experiments was small (<
5% of the rate over buffered iodide solutions). The aerodynamic resistance (Γa) in the
experimental system was determined using very concentrated iodide solutions (0.02 M) at which
the surface resistance Γs is zero (1). Γs and hence k1 (Eq. (1) and (2) in main text) was
determined from observed pseudo first order ozone uptake rates at 70 ppbv O3(g) over buffered
iodide solutions (10-6
- 10-5
M [I-]) (1). Exponential curves fitted the observed data with r
2
values of 0.99 and the measured ozone uptake rates were very similar to the results of Garland et
al. (2) such that vd was fitted well using equations (3) and (4) with λ= k1[I-] and k1= 2.0 × 10
9
M-1
s-1
at 293 K – this value was used in the interfacial model (Table S1). Although there is
substantial evidence that iodide accumulates at the air/water interface compared to the bulk (3),
the derived rate constant k1 integrates such effects.
Prior to measurement of iodine emissions, ozone was passed through the system until a constant
concentration (± 2.5%) was observed. Moisture was removed from the gas stream using two
SUPPLEMENTARY INFORMATIONDOI: 10.1038/NGEO1687
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spiral condensers in series held at 0 and -10°C. Total gaseous iodine losses within the
experimental system were typically 40 ± 2.2% and these were accounted for.
Emissions of gaseous I2 were measured by trapping the gas flow in n-hexane at ≤ -50 °C for 60
minutes followed by spectrophotometric detection at 522 nm (4) using a dual beamed Perkin
Elmer Lambda 25 UV/VIS spectrophotometer fitted with 10 mm quartz sample and reference
cells. I2 calibration curves were constructed using standard solutions prepared from ground
molecular iodine (puriss, p.a., Riedel-de Haen) dissolved in hexane (HPLC grade, Fischer UK)
within the concentration range 0.5 – 5 x 10-3
M. Trapping efficiencies and losses of gaseous
iodine were determined using a commercial I2 permeation tube (Kin-Tek™, USA) maintained at
60°C.
Gaseous HOI measurements by spectrophotometry
The experimental set up used was similar to that shown in Fig. S1 except no condenser was
used. Experiments were carried out by supplying dry ozone (150 ppbv, 100 sccm) to the 2 L
reaction vessel containing 1 L of phosphate buffer solution (PBS; 0.1 mol L-1
, pH 8, Fluka
Analytical) and KI (10 – 50 x 10-6
M; Fisher Scientific). Evolved gaseous HOI was collected
within a blacked-out midget bubbler (25 mL, Supelco) containing phenol red (PR;
phenolsulfonphthalein) solution (15 mL, 50 x 10-6
M; Sigma Aldrich) and PBS (at pH 7) at 5
°C. Experiments were conducted with (∼ 120 rpm) and without mixing of KI PBS to
investigate the effect bulk mixing had on gaseous HOI evolution (Fig. 1, lower panel).
The assay for the determination of evolved gaseous HOI was based upon the selective
iodination of aqueous PR to iodophenol blue (IPB; 3, 3, 5, 5 - tetraiodophenolsulfonphthalein)
(5), adapted from a previous HOI in-situ incubation study of seaweeds (6). IPB was measured
spectrophotometrically at 591 nm with ε = 47.4 mmol L-1
cm-1
(5) (Fig. S2). Since PR is in
excess and four halogenations of PR are required to form the final tetrahalo product, the molar
HOI: IPB ratio is 4. The ratio of PR loss to IPB generated was observed to be 1, confirming
that the tetrahalo- compound is formed from rapid halogenation of PR and is the only stable
product.
The selectivity of the IPB method for HOI over I2 was confirmed by the complete absence of
IPB after passing gaseous I2 (40 ppbv) for 150 min through PR solution, as shown by Figure S2.
The trapping efficiency of gaseous HOI by the PR trap utilized was investigated by using two
midget gas bubblers (25 mL, Supelco), each containing 20 mL of PR (50 x 10-6
M, pH 7, 0.1 M
PBS), in series under the experimental conditions. Gaseous HOI was produced by introducing
gaseous ozone (150 ppbv, 100 sccm) to a 20 x 10-6
M KI, 0.1 M PBS, pH 7 (1 L) solution
magnetically stirred at 60 – 120 rpm. Gaseous HOI enrichment was carried out in triplicate for
2.5 hrs and determined spectrophotometrically from IPB produced in each trap. IPB was not
detected within the second trap suggesting a HOI trapping efficiency of 100%.
Gaseous I2 and HOI measurements by iodine oxide particle (IOP) detection
Iodide solutions (10-7
– 10-3
M) were prepared using KI (Alfa Aesar, 99%) and deionised water
and the pH of the solution before and after the experiment recorded (Jenway 350 pH meter). 85
mL of iodide solution was added to a 133 mL cylindrical reaction vessel with a surface area of
54.7 cm2
(Fig. S3). Ozone was produced from O2 by exposure to a mercury UVP pen-ray lamp
(185 nm excitation) and was measured using a model 49c ozone analyzer (Thermo Scientific).
The ozone concentration was varied by changing the distance of the pen-ray lamp from the
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photolysis cell. The ozone flow rate was maintained at 0.01 L min-1
with an additional 0.04 L
min-1
N2 flow giving a flow rate through the reaction vessel of 0.05 L min-1
. The gas flow
containing emitted I2 was then introduced to the iodine oxide particle generation cell where an
additional 0.04 L min-1
O3/O2 flow and 0.51 L min-1
N2 flow were added. I2 was detected by
conversion into iodine oxide particles (IOPs); this was achieved by photolysis using a small
tungsten lamp to produce I atoms and subsequent reaction with O3. The IOPs were then detected
using an electrical mobility spectrometer (EMS VIE10: Tapcon GmbH), consisting of a nano-
differential mobility analyser (nano-DMA) and a Faraday cup electrometer (FCE).
Detection of HOI was achieved by replacing the tungsten lamp with a Xenon lamp, in order to
increase the relative fraction of light output in the UV. Using a blue glass band-pass filter
(Schott UG-1, transmittance window 270-420 nm, and >670 nm), selective photolysis of HOI
was achieved. This technique is thus specific for an iodine-containing species that absorbs
strongly in the near-UV region, i.e., the technique is designed to discriminate specifically
against I2. In the experiments which involved just buffered I- solutions, HOI is therefore the only
possible candidate. A yellow glass long-pass filter (Schott GG495, transmittance > 480 nm)
was employed to selectively photolyse I2. The photolysis rates of HOI and I2 through each of the
filters were determined by convoluting the transmitted spectral intensity of the Xenon lamp
(measured using a grating spectrometer and CCD detector) with the respective molecular
absorption cross section (7). The IOP masses measured with each of the filters were then used to
determine the ratio [HOI]/[I2] from the following expression (note there is negligible photolysis
of HOI through the yellow filter):
€
HOI[ ]I2[ ]
=Mb * 2J
y (I2 )M y −2Jb (I2 )
Jb (HOI)
where Mb and My
are the IOP masses observed using the blue and yellow filters, respectively;
Jy(I2) is the photolysis rate of I2 through the yellow filter, and Jb(I2) and Jb(HOI) are the
photolysis rates for I2 and HOI through the blue filter.
To calibrate the iodine detection system a cell containing iodine crystals was added to the
experimental set-up in place of the solution cell, and maintained at 273 K using an ice bath. N2
was flowed over the cell at varying flow rates from 5 mL min-1
to 25 mL min-1
and a linear trend
in IOP mass observed. The vapour pressure of I2 at 273 K was taken from Baxter and Grose (8).
The concentration of I2 entrained in the N2 flow was further diluted by the addition of the O3/O2
and N2 flows in the IOP generation cell. By comparing the concentration of I2 flowing through
the IOP generation cell, and the amount of I2 in the IOPs measured (assuming a composition of
I2O5 as deduced by Saunders and Plane (9), and a bulk I2O5 density of 5.0 g cm-3
), a percentage
efficiency for the system was calculated for each flow rate. The average efficiency was
4.2 × 10-2
%, and this conversion factor was used to convert the measured IOP mass into I2
number density in order to estimate the I2 flux from the solution. The conversion efficiency is a
function of the photolysis rate of I2 (or HOI) and the residence time of the gas flow in the
photolysis cell. Both of these parameters can of course be changed to increase the efficiency.
However, it is important that the IOP mass determined from the measured particle size
distribution is a linear function of the I2 in the flow, and the particles do not grow to have
diameters in excess of 40 nm (the cut-off of the Tapcon EMS instrument). These constraints
were met by photolysing only a small fraction of the I2. The linearity of the detection system
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4
was checked using flows of known I2 concentration, as described above. Finally, even though
the conversion efficiency was kept low, the IOP method has an excellent detection limit of ~ 2
ppt for I2.
Modelling iodine emissions: The sea-surface model
Model set up and initiation The commercial modeling program FACSIMILE (MCPA Software Ltd, UK) was used to solve
the chemical reactions involved in interfacial aqueous iodine dynamics, including iodide
oxidation, iodine disproportionation, and iodine reduction. The complete reaction scheme is
shown in Table S1. Separate experiments were performed to verify the model could
successfully simulate iodine disproportionation at different pH levels (Fig. S4).
Iodine production is initiated by the gas phase flux of O3 into the interfacial layer, FO3
FO3 = vD [O3(g)] (7)
The change in O3 concentration in the interfacial layer, [O3int], is thus:
d[O3int] /dt = A/VvD [O3(g)] (8)
where A/V is equal to the inverse of the depth of the interfacial layer, defined as the
reactodiffusive length δ.
Rapid production of I2 follows the reaction of deposited O3 from the atmosphere with iodide at
the sea surface (Eq. 1 and 2 in main text). We note that Sakamoto et al. (10), also measured IO
production, inferred as a by-product of an IOOO- intermediate formed in reaction (1), at about
1% of the gaseous I2 formed. Because of the minor importance of this species compared to I2
and HOI, and the lack of kinetic data, we ignore its production in this study.
The interfacial layer was treated as a box, assuming no horizontal advection (i.e. assuming horizontal gradients to be small) but mixing vertically with bulk mixed layer water at a fixed
interfacial layer turnover time (for laboratory studies) or by a wind speed-dependent expression
for transfer velocity for ambient conditions (11) calculated from the water-side resistance to
HOI and I2 transfer. Calculation of mass transfer velocities for both laboratory and environmental conditions are described below.
Concentrations of [I-], [H+] and [OH-] were fixed for each model run. For all computations, all iodine species except IO3
- and HIO2 (neither of which effect the gaseous iodine evolution) were
in steady state after a few seconds. For modelling surface seawater iodine emissions, we
simulated an open ocean scenario with pseudo first order rate constants for “O3 + DOM”
interfacial reactions of 100 s-1
(12) and “I2/HOI + DOM” of 5 x 10-5
s-1
(13) and a coastal ocean
scenario with the same reactions at 500 s-1
(12) and 7 x 10-3
s-1
(13), respectively. Thus we
assume that organic reactions dominate chemical control of O3 deposition in coastal waters (12,
14, 15) and are competitive with the iodide reaction in open ocean waters. Total ozone
deposition velocities vD were calculated from equations (3) and (4) in the main text (where λ
includes “O3 + DOM” reactions), using aerodynamic resistances typical for the marine boundary
layer (12).
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Mass flux calculations According to the two-resistance model for air-water partitioning (16, 17), the mass flux F (mol
m-2
s-1
) of a trace gas species is described by:
F = Kt (cw – cg/H) (9)
l/ Kt= 1/Kw+ 1/HKa (10)
where cw and cg are the respective water and gaseous concentrations, H is the dimensionless gas-
over-liquid form of the Henry’s law constant and Kt, Kw and Ka are the total, liquid and air mass
transfer coefficients (m s-1
), respectively. The inverse of Kw and Ka are the respective water-side
and air-side resistances.
For water-soluble molecules such as HOI, the rate of mass transfer is dominated by the air-side
resistance thus Kt = HKa. For sparingly soluble molecules such as I2, the rate of mass transfer is
dominated by the water-side resistance, though the air-side resistance can reduce the total mass
transfer by several percent.
Calculating mass transfer coefficients for laboratory conditions To calculate Ka for our laboratory experiments, we use an empirical formulation relevant for
laminar flow conditions/indoor environments (18) based upon the dimensionless Sherwood
number, Sh:
Ka = (ShDa)/L (11)
Here Ka is in m h-1
, L is the characteristic length (m) calculated from the square root of the
source area, and Da is the diffusivity in air (m2 h
-1).
The Sherwood number is function of the temperature-dependant Schmidt number of the gas in
question in air (Sca, dimensionless), and the Reynold’s number Re:
Sh = 0.664 Sca1/3Re
1/2 (12)
Sca = µ/(ρDa) (13)
Re = (Luρ)/µ (14)
Where u is the air velocity (m h-1
), ρ is the density of air (g m-3
) and µ is the viscosity of air (g
m-1
h-1
). Thus, at 20 oC, we calculate Ka= 4.2 x 10
-4 m s
-1 for HOI and 3.7 x 10
-4 m s
-1for I2.
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For calculating the liquid mass transfer coefficients Kw from still water, we again use the
empirical approach of (18) where:
Kw = 2.99 Dw (15)
In this case Kw and Dw (the liquid phase diffusion coefficient) are in m h-1
and m2 h, respectively.
Total mass transfer coefficients Kt for laboratory conditions were calculated from Eq. 10 using
temperature - dependant Henry’s law constants for HOI (19) and I2 (20).
Calculating mass transfer coefficients for environmental conditions Various empirical parameterisations for the air side mass transfer coefficient Ka applicable to the
environment and for experiments in wind tunnels have been derived as a function of wind speed,
u, friction velocity, u*, the Schmidt number of the gas, and the drag coefficient, CD. For
calculating Ka (m s-1
) for environmental conditions, we use the parameterization suggested by
Johnson (21):
Ka = (16)
where κ is the von Karman constant (commonly taken to be 0.4 in seawater) and u* and CD are
calculated from equations given in (21) for a wind speed of 7 m s-1
.
We used the Nightingale et al. (11) parameterization for the waterside transfer velocity. Total
mass transfer coefficients Kt for environmental conditions were calculated assuming a seawater
temperature of 15 oC, air temperature of 20
oC and 10 m wind speed of 7 m s
-1.
The change in aqueous phase concentration due to volatilisation (assuming cg/H to be
negligible) is thus:
dcw/dt = Kt/ δ .cw (17)
where δ is the interfacial layer thickness, defined here as the reacto-diffusive length for O3.
Multiple linear regression model for defining marine emissions of HOI and I2
In order to derive algorithms for marine emissions of HOI and I2, the relationships between each
covariate, iodide concentration (I-), ozone concentration (O3) and wind speed (ws), and the
response variable in question (as computed from the sea-surface model) were investigated.
Ozone levels have a simple multiplicative effect on the response in both cases, i.e. increasing the
ozone level by a factor, k, increases both HOI and I2 emissions by the same factor. Therefore,
models were initially developed for a constant ozone level and later multiplied by the
appropriate factor. Ozone and wind speed were considered separately for fixed values of the
other covariate in each case. Although both responses show clear association with iodide and
wind speed individually, none of the relationships are linear and the covariates were therefore
transformed before fitting a linear regression model. The functions applied to each covariate to
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7
achieve a linear relationship with the response are shown in Table S2, together with the
correlation in each case.
A multiple linear regression model of the form
€
ˆ y = β0 +β1x1 +β2x2 +β3x1x2 (18)
where
€
x1 and
€
x2 denote the transformed covariates and the
€
βi , for i = 0,…,3 are coefficients to
be determined, was fitted to allow for interaction between the variables. For both responses, the
intercept was not significant in the regression and models were subsequently fitted without the
€
β0 term.
In the case of I2, the coefficient of
€
(I − )1.3was also not significant, but the coefficients of ln(ws)
and the interaction term,
€
ln(ws)*(I − )1.3, were both highly significant (p < 0.0001). The F-
statistic for the comparison of the two models, with and without the
€
(I − )1.3term, showed that the
reduced model is preferable. The final model to predict I2 flux is therefore
€
FluxI2 = O3(g)[ ]∗ I−(aq)[ ]1.3∗ 1.74×109 −6.54×108 ∗ ln(ws)( )
(19)
where the flux is in nmol m-2
d-1
, [O3] is in nmol mol-1
, [I-] in mol dm
-3 and wind speed in m s
-1
This fit resulted in a correlation of 0.9991 between the calculated and predicted I2 values.
For HOI, the coefficients of both covariates and the interaction term were all found to be highly
significant, leading to the following model with a correlation of 0.9986 between calculated and
predicted values:
€
FluxHOI = O3(g)[ ]∗ 4.15×105 ∗I−(aq)[ ]ws
−20.6ws
−2.36×104 ∗ I−(aq)[ ]
(20)
The least significant term in this model is that involving the wind speed alone, although the p-
value of 1.02 x 10-08
for the corresponding coefficient shows that this is highly significant. With
a p-value < 2.2 x 10-16
, the partial F-test also shows that this variable is significant in the model.
However, fitting a model without this variable leads to a simpler model that still has a
correlation of 0.9945 between calculated and predicted values:
€
FluxHOI = O3(g)[ ]∗ I −(aq)[ ] * 3.56×105
ws− 2.16×104
. (21)
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8
Tropospheric HAlogen chemistry MOdel (THAMO)
THAMO is a 1-D chemistry transport model with 200 stacked boxes at a vertical resolution of 5
m (total height 1 km). Wind speed measurements collected at three heights (4, 10 and 30 m)
were used to construct an eddy diffusion coefficient (Kz) profile (22, 23). Aircraft measurements
of temperature in the BL at Cape Verde show a strong temperature inversion about 1 km from
the surface (22), indicating that the BL at Cape Verde is decoupled from the free troposphere.
Hence, the Kz profile is assumed to increase up to a height of 30 m (where it peaks at 3 x 104
cm2 s
-1), after which it decreases at a constant rate to a value of 2 cm
2 s
-1 at the top of the BL.
The model treats iodine, bromine, O3, NOx and HOx chemistry using over 210 reactions. The
chemical scheme is from Saiz-Lopez et al. (25), updated by Mahajan et al. (22). The model is
constrained with typical measured values of the following species: [NOx] = 25 ppt; [CO] = 110
ppb; [DMS] = 30 ppt; [CH4] = 1820 ppb; [ethane] = 925 ppt; [CH3CHO] = 970 ppt; [HCHO] =
500 ppt; [isoprene] = 10 ppt; [propane] = 60 ppt; [propene] = 20 ppt (24, 26, 27, 28, 29) . The
average background aerosol surface area used is 1.0 × 10-6
cm2 cm
-3, an average value measured
at Cape Verde by Allan et al. (30). The modelled HOx concentrations are in sensible accord with
measured values at Cape Verde (31).
The sea-air flux of HOI and I2 from the interfacial model were computed for an average wind
speed of 7 m s-1
, temperature of 296 K, O3 mixing ratio of 30 ppbv and a sea-surface iodide
concentration of 100 x 10-9
M. Under these conditions the model predicted surface
concentrations of [HOI] = 5.7 x 10-9
M and [I2] = 6.6 x 10-12
M, with sea-air transfer velocities of
KtHOI
= 4.9 × 10-5
and KtI2
1.9 × 10-3
cm s-1
. The sea-air fluxes of HOI and I2 were calculated both
as purely evasive terms and also as equilibrated with their surface atmospheric concentrations.
Figure S5 illustrates the height-time profiles for IO, OIO, I2 and HOI over a diurnal cycle, where
the iodine source comprises the measured iodocarbon flux (22, 24) together with the fluxes of
HOI and I2 calculated (taking account of the partial pressure difference across the interface).
This figure illustrates a number of features: IO is only present at significant levels during
daytime, and extends from the surface up to about 300 m (~50% of the near surface mixing
ratio); OIO is only present at significant levels during the night because of its rapid photolysis
during daytime (32) although the mixing ratio is always relatively low; daytime levels of I2 are
very low because of rapid photolysis (33), and after an initial increase following sunset the I2
mixing ratio decreases during the night because of its reaction with NO3; HOI is present during
the day because of HOx chemistry, and increases towards sunset as its photolysis rate decreases;
HOI is present at low levels during the night because any HOI which evades from the ocean
reacts quite rapidly on sea-salt aerosol.
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26. L. J. Carpenter et al. Seasonal characteristics of tropical marine boundary layer air measured at the
Cape Verde Atmospheric Observatory. J. Atmos. Chem. 67, 87–140 (2010).
27. J. D. Lee et al. Year-round measurements of nitrogen oxides and ozone in the tropical North
Atlantic marine boundary layer. J. Geophys. Res. 114, 1–14 (2009).
28. A. S. Mahajan et al. DOAS observations of formaldehyde and its impact on the HO(x) balance in
the tropical Atlantic marine boundary layer. J. Atmos. Chem. 66, 3, 167–178(2008).
29. K. A. Read et al. Intra-annual cycles of NMVOC in the tropical marine boundary layer and their
use for interpreting seasonal variability in CO. J. Geophys. Res. 114, D21303 (2009).
30. J. D. Allan et al. Composition and properties of atmospheric particles in the eastern Atlantic and
impacts on gas phase uptake rates. Atmos. Chem. Phys. Discuss. 9, 18331–18374 (2009).
31. L. K. Whalley et al. The chemistry of OH and HO2 radicals in the boundary layer over the tropical
Atlantic Ocean. Atmospheric Chemistry and Physics, 10, 1555–1576 (2010).
32. J. C. Gómez Martin, S. H. Ashworth, A. S. Mahajan, and J. M. C. Plane. Photochemistry of OIO:
Laboratory study and atmospheric implications. Geophys. Res. Lett. 36, L09802 (2009).
33. A. Saiz-Lopez et al.Absolute absorption cross-section and photolysis rate of I2, Atmos. Chem. Phys,
4, 1443–1450(2004).
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11
Figures S1-S5
3 way adjustable tap
Gas transfer lines (PFA, covered with black tape)
MFC Mass flow controller
Figure S1. Experimental set up used to investigate ozone uptake and quantify iodine emissions
by/from iodide solutions and seawater by spectrophotometry. The glass reaction vessel and
traps were covered in foil during all experiments to avoid photolysis of iodine.
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12
Figure S2. Iodination of phenol red (PR, 50 µM) by gaseous HOI, showing the IPB peak at
about 590 nm and the PR “dip” at 430 nm. The blue trace shows the result of trapping HOI
evolved on exposure of 50 µM KI to 500 ppbv O3 at a flow rate of 100 sccm for 1 hr in PR
solution (pH 7) at 5 oC. The red trace is a 2 hr trap of I2 from a permeation oven (250 ppbv I2 at
100 sccm) in PR solution, showing the complete absence of IPB. The green trace is a 50 µM
PR blank. The spectra are all autozeroed against 50 µM PR.
Absorbance
nm
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13
KI solution
IOP Generation Cell
Tungsten lamp
System Control
EPC
FC
E
Nano-DMA + FCE
N2 + O3
O2 + O3 N2
O2 Hg lamp
HV
1 S
enso
r mod
ule
N2
Figure S3. Schematic diagram of the IOP detection system with black arrows showing direction
of air flow. The reaction vessel and IOP generation cell were covered with black cloth to prevent
unwanted photolysis of I2.
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14
Figure S4. Comparison of measured (symbols with error bars representing 1σ variability) and
modelled (lines) iodine disproportionation at a range of pHs in phosphate buffer (NaH2PO4,
Fisher, 99%) prepared in N2-sparged HPLC water and adjusted for pH by titrating with freshly
prepared sodium hydroxide (NaOH, Fisher, 99%). I2 solutions were prepared by overnight
stirring of the required amount of I2 flakes (Analytical grade, Fisher) in degassed HPLC water,
covered by aluminium foil. Disproportionation was studied from an initial [I2] concentration of
1.6 x 10-4
M. [I2](aq) was monitored by spectrophotometric detection at 460 nm (32) and
calibrated as described in section 1.2, where degassed HPLC water was utilised instead of
hexane to dissolve ground I2(s).
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15
Figure S5. Modelled iodine chemistry at Cape Verde using the 1-D model THAMO. The panels
show the diurnal variations of IO, OIO, I2 and HOI in the 1 km high marine boundary layer.
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16
Tables S1-2
Table S1. Kinetic data used in the interfacial model
Aqueous or heterogeneous reaction Rate constant of
forward reaction
Rate constant of
reverse reaction
Reference(s)
Iodide solution and seawater reactions:
1. O3(g)→ O3 (interface, l) 1vd,O3 x (A/V) s-1 See text
2. O3 (interface, l) + I- (+ H+) → HOI 2.0 x 109 M-1 s-1 (pH 8)
Garland et al. 1980; Magi et
al., 1997
3. I2 + I-↔ I3- 6.2 x 109 M-1 s-1 8.9 x 106 s-1 Forward: Lengyel et al., 1993 Back: Palmer et al. 1984
4. I2 (+ H2O) ↔ I2OH- + H+ 3.2 s-1
2.0 x 1010M-1s-1 Lengyel et al., 1993
5. I2 (+ H2O) ↔ H2OI+ + I- 1.2 x 10-1 s-1 1.0 x 1010M-1s-1 Lengyel et al., 1993
6. I2 + OH- ↔HOI + I- 7.0 x 104 M-1 s-1 2.1 x 103 M-1 s-1 Sebők-Nagy and Körtvélyesi 2004
7. I2OH- ↔ HOI + I- 1.34 x 106s-1 4.0 x 108 M-1 s-1 Lengyel et al., 1993
8. H2OI+↔HOI + H+ 9.0 x 108s-1 2.0 x 1010 M-1 s-1 Lengyel et al., 1993
9. HOI + HOI ↔H+ + I- + HIO2 2.5 x 101 M-1 s-1 2.0 x 1010 M-2 s-1 Forward: Schmitz, 2004 Back: Edblom et al. 1987
10. HOI ↔ IO- + H+ 1.0 x 10-1 s-1 1.0 x 1010 M-1 s-1 Paquette et al., 1986
11. HIO2 + HOI ↔IO3- + I- + 2H+ 2.4 x 102 M-1 s-1 1.2 x 103 M-3 s-1
Forward: Furrow, 1987 Back: Schmitz, 2000
12. HOI + IO-↔HIO2 + I- 1.5 x 101 M-1 s-1 Negligible Bischel and von Gunten, 2000
13. I2→ bulk 2kmix
14. HOI → bulk 2kmix
Additional reactions in seawater only3:
15. O3 (interface, g) (+ DOM) →
products 500 s-1 (coastal) 100s-1 (open ocean)
Ganzeveld et al. 2009
16. I2 (+ DOM) → products
7.0 x 10-3 s-1 (coastal) 5.0 x 10-5s-1 (open ocean)
Truesdale et al., 1995a and 1995b
17. 4HOI (+ DOM) → products
7.0 x 10-3 s-1 (coastal) 5.0 x 10-5s-1 (open ocean)
Assumed analogously to R16
18. HOI + Br- + H+↔IBr 4.1 x 1012 M-2 s-1 8.0 x 105s-1 Faria et al.1993
19. HOI + Cl- + H+↔ICl 2.9 x 1010 M-2 s-1 2.4 x 106s-1 Wang et al., 1989
20. I2 + Br-↔ I- + IBr 4.74 x 103 M-1 s-1 2.0 x 109 M-1 s-1 Faria et al.1993
21. I2 + Cl- ↔ I2Cl- 8.33 x 104 M-1 s-1 5.0x 104s-1 5Margerum et al. 1986
22. ICl2-↔ICl + Cl- 6.0 x 105s-1 1.0 x 108 M-1 s-1 6Margerum et al. 1986
23. I- + ICl↔ I2Cl- 1.1 x 109 M-1 s-1 1.5s-1 Margerum et al. 1986
1 For defining d[O3interface(liquid)]/dt, A is surface area of liquid, V is volume of interfacial layer.
2For the stirred laboratory experiments,kmix was fixed at 0.4 s-1. For the unstirred experiments, kmix was
set to zero. In simulating marine conditions we calculate a wind-speed dependent water transfer velocity.
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17
3[Cl-] and [Br-] were fixed at 0.55M and 8.6 x 10-4 M, respectively. Iodate [IO3-] was fixed at
200 x 10-9M for the seawater model runs (this has no effect on volatile iodine emissions). Photooxidation
of I- by O2 and other potential oxidants in seawater including IO3- and nitrate (NO3
-) were reported to be
negligible by Truesdale (2007) and were therefore not included in the model. Pseudo first-order
interfacial O3 loss rates to DOM in coastal and open waters were based on Ganzeveld et al. (2009).
4It is likely that HOI reacts with DOM more rapidly than I2, however even if we assume a reaction rate
100 times faster than that of I2 with DOM, the reduction in the modelled I2 and HOI flux (compared with
using the same rates for I2 and HOI, as shown in the Table) is 0.17% and 0.16%, respectively.
5The forward reaction is an upper limit, the backward reaction is a lower limit. Changing these rate
constants by a factor of 100 (whilst maintaining the equilibrium constant K) has a negligible difference
on the simulated iodine emissions.
6Estimates from Margerum et al. (1986). Changing these rate constants by a factor of 100 (whilst
maintaining the equilibrium constant K) has a negligible difference on the simulated iodine emissions.
Table S1 references
1. Y. Bichsel, U. von Gunten. Hypoiodous acid: Kinetics of the buffer-catalyzed
disproportionation, Water Res. 34, 3197–3203 (2000).
2. E. C. Edblom, L. Gyorgyi, M. Orban, I. R. Epstein. A mechanism for dynamical behaviour in the
Landolt reaction with ferrocyanide, J. Am. Chem. Soc.109, 4876–4880 (1987).
3. T. B. Faria, I. Lengyel, I. R. Epstein, K. Kustin. Combined mechanism explaining nonlinear
dynamics in bromine(III) and bromine(IV) oxidations of iodide ion. J. Phys. Chem. 97, 1164–1171
(1993).
4. S. Furrow. Reactions of iodine intermediates in iodate-hydrogen peroxide oscillators, J. Phys. Chem. 91, 2129–2135 (1987).
5. L. Ganzeveld et al. Atmosphere-ocean ozone exchange: A global modeling study of
biogeochemical, atmospheric, and waterside turbulence dependencies. Global. Biogeochem. Cycles. 23, GB4021 (2009).
6. J. A. Garland, A. W. Elzerman, S. A. Penkett. The mechanism for dry deposition of ozone
to seawater surfaces. J. Geophys. Res. 85, 7488–7492 (1980).
7. I. Lengyel, I. R Epstein, K. Kustin. Kinetics of iodine hydrolysis, Inorg. Chem. 32, 5880–5882
(1993).
8. D. W. Margerum et al. Kinetics of the iodine monochloride reaction with iodide measured by the
pulsed-accelerated-flow method, Inorg. Chem. 25, 4900–4904 (1986).
9. J. Paquette, J. C. Wren, B. L. Ford. The Disproportionation of Iodine (1), in Proceedings of OECD Iodine Workshop, Harwell, AERE R 11974 pp. 29–45 (1986)
10. D. A. Palmer, R. W. Ramette, R. E. Mesmer. Triiodide ion formation equilibrium and activity-
coefficients in aqueous-solution. J. Solution Chem. 13, 673–683 (1984).
11. G. Schmitz. Kinetics of the Dushman reaction at low I− concentrations, Phys. Chem.
Chem.Phys. 2, 4041–4044 (2000).
12. G. Schmitz. Inorganic reactions of iodine(+1) in acidic solutions, Int. J. Chem. Kinet. 36, 480–493
(2004).
13. K. Sebők-Nagy, T. Körtvélyesi. Kinetics and mechanism of the hydrolytic disproportionation of
iodine, Int. J. Chem. Kin. 36, 596–602 (2004).
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18
14. V. W. Truesdale, C. E. Canosa-Mas, G. W. Luther. Disproportionation and reduction of molecular
iodine added to seawater, Mar. Chem. 51, 55–60 (1995a).
15. V. W. Truesdale, G. W. Luther, C. E. Canosa-Mas. Molecular iodine reduction in seawater: An
improved rate equation considering organic compounds, Mar. Chem. 48, 143–150 (1995b).
16. V. W. Truesdale. On the feasibility of some photochemical reactions of iodide in seawater, Mar. Chem. 104, 266–281 (2007).
17. Y. L. Wang, J. C. Nagy D. W. Margerum. Kinetics of hydrolysis of iodine monochloride measured
by the pulsed-accelerated- flow method, J. Am. Chem. Soc.111, 7838–7844 (1989).
Table S2. Transformation of the covariates required to give a linear relationship with the
response (for fixed O3 levels).
Response(y) Covariate(x) Transformation
f(x)
ws 1/ws HOI
I- I !
ws ln(ws)
I2 I
-
€
I −( )1.3
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