Weak acids enhance halogen activation on atmospheric waters surfaces

Article abstract of DOI:10.1021/jp2021775

We report that rates of I₂(g) emissions, measured via cavity ring-down spectroscopy, during the heterogeneous ozonation of interfacial iodide: I^-(surface, s) + O₃(g) + H⁺(s) →→ I₂(g), are enhanced several-fold, whereas those of IO · (g) are unaffected, by the presence of undissociated alkanoic acids on water. The amphiphilic weak carboxylic acids appear to promote I₂(g) emissions by supplying the requisite interfacial protons H⁺(s) more efficiently than water itself, at pH values representative of submicrometer marine aerosol particles. We infer that the organic acids coating aerosol particles ejected from oceans topmost films should enhance I₂(g) production in marine boundary layers.

Full text of DOI:10.1021/jp2021775

ARTICLE
pubs.acs.org/JPCA
Weak Acids Enhance Halogen Activation on Atmospheric
Water’s Surfaces
Sayaka Hayase, Akihiro Yabushita, and Masahiro Kawasaki*
Department of Molecular Engineering, Kyoto University, Kyoto 615-8510, Japan
Shinichi Enami, Michael R. Hoffmann, and Agustín J. Colussi
W. M. Keck Laboratories, California Institute of Technology, Pasadena, California 91125, United States
S Supporting Information
b
ABSTRACT: We report that rates of I2(g) emissions, measured via
cavity ring-down spectroscopy, during the heterogeneous ozonation of
interfacial iodide: Iꢀ(surface, s) þ O3(g) þ Hþ(s) ff I2(g), are
enhanced several-fold, whereas those of IO (g) are unaffected, by the
3
presence of undissociated alkanoic acids on water. The amphiphilic
weak carboxylic acids appear to promote I2(g) emissions by supplying
the requisite interfacial protons Hþ(s) more efficiently than water itself,
at pH values representative of submicrometer marine aerosol particles.
We infer that the organic acids coating aerosol particles ejected from
ocean’s topmost films should enhance I2(g) production in marine
boundary layers.
’ INTRODUCTION
(which includes acids, alcohols, and phenols) and inorganic
anions, as well as by anthropogenic contaminants, such as spilled
oil and surfactants, via surface tension or ion specific effects.13,15,16
Fine marine aerosols are largely produced from SML in the bursting
of air bubbles generated from the breaking of waves.3 Seawater
inorganic iodide in particular is highly enriched (up to 104 times
relative to bulk ocean water) in fine marine aerosols.3,17 This
is to be expected from anion fractionation during bubble bursting
on the basis of laboratory experiment showing that anion
propensities for the air/water interface increase exponentially with
ion radius F: F(Iꢀ) = 220 pm > F(Brꢀ) = 196 pm > F(Clꢀ) =
181 pm.18,19
Atmospheric model simulations have shown that iodide controls
ozone deposition velocities due to its extreme reactivity.20,21 Recent
experimental reports have suggested that the reaction of O3(g) with
Iꢀ(aq) in micromole per liter to millimole per liter range occurs
primarily at the air/water interface layers,17,22ꢀ26 although the
ozonation of highly concentrated ∼7.3 M deliquesced KI particles
may also be driven by gas accommodation and subsequent reaction
in the bulk phase.27,28 For example, the interfacial Iꢀ concentration
of aqueous microjet decreases by ∼50% after exposure to [O3(g)] =
40 ppmv for ∼10 μs, that is, Iꢀ(s) reacts with O3(g) with an
apparent pseudo-first-order rate constant, k ∼ 105 sꢀ1, which is
∼20 times higher than the rate expected from the bulk reaction
rate constant and the [O3(g)]aq = 4 μM calculated from Henry’s
law constant for ozone in water.26 In a related study based on
The composition of the marine boundary layers (MBLs) is
significantly impacted by iodine chemistry. Iodine-containing
species are involved in ozone depletion, the HOx/NOx cycle, and
the generation of aerosol nuclei.1ꢀ8 Experiments based on cavity
ring-down spectroscopic (CRDS)9,10 detection of gaseous pro-
ducts have revealed that molecular iodine, I2(g), and iodine
monoxide radical, IO (g), are emitted during the heterogeneous
3
oxidation of Iꢀ(aq) by O3(g).11,12 The proposed reaction
mechanism, which accounts both for enhanced I2(g) emissions
at pH <4, and the relatively constant IO (g) emissions observed
3
between pH 2 and 10, proceeds via interfacial IOOOꢀ(surface, s)
and HOI(s) intermediates11,12
IꢀðsÞ þ O3ðgÞ f IOOOꢀðsÞ
IOOOꢀðsÞ f IOꢀðsÞ þ O2
ð1Þ
ð2aÞ
ð2bÞ
ð3Þ
ꢀ
3
IOOO ðsÞ f f IO ðgÞ þ products
IOꢀðsÞ þ HþðsÞ a HOIðsÞ ðpKa ¼ 10:8Þ
HOIðsÞ þ IꢀðsÞ þ HþðsÞ a I2ðsÞ þ H2O
ð4Þ
ð5Þ
I2ðsÞ f I2ðgÞ
Major global processes are influenced by the chemical com-
position of the surface marine microlayer (SML) separating the
ocean from the Earth’s atmosphere.13,14 The SML is enriched in
diverse seawater components, such as natural organic matter
Received: March 7, 2011
Revised:
April 7, 2011
Published: April 22, 2011
r
2011 American Chemical Society
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|

The Journal of Physical Chemistry A
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Table 1. Kinetic and Thermodynamic Data for Relevant Species66
concentration for
saturated surface
coveragea
Henry’s
law constant
(mol kgꢀ1 barꢀ1
kO
(Mꢀ1 sꢀ1
solubility
(mM)
3
reactant
pKa
)
)
(mM)45,58,59
Iꢀ
1.2 ꢁ 109
hexanoic acid
octanoic acid
malonic acid
acetic acid
4.8
4.8
e0.48 ꢁ 10ꢀ3
e2.5 ꢁ 10ꢀ3
7
83.5
1400
770
4 ꢁ 108
4100
>50
4.71
∼5
N/A
2.9 (pKa2 = 5.7)
1.34 ꢁ 103
fully miscible
4.8
e3 ꢁ 10ꢀ3
∼200
a Estimated from reported surface tension measurements.45,58,59
surface-specific glancing-angle Raman spectroscopy, the OH
Raman signals generated on water surfaces exposed to O3(g)
areattenuatedby dissolvediodide witha LangmuirꢀHinshelwood
dependence. This phenomenon has been interpreted as an
evidence that I3ꢀ, one of the products of iodide ozonation,
physically inhibits OH signals from the water’s surface.24
The surface-active organic acids coating the SML and marine
aerosol particles could affect their chemical/physical properties
and, hence, key global processes, such as water evaporation and
uptake/emission rates of atmospheric gases.15 Recent field
measurements show that organic acids represent ∼50% of the
mass of fog waters collected in the US Gulf Coast.29,30 We have
recently reported that I2(g) and IO (g) yields in the ozonation of
3
Iꢀ(aq) are drastically suppressed in the presence of reactive
phenols even at pH g3.11 This finding was ascribed to the rapid
acid dissociation of phenol into phenolate (rather than to a
surface blocking effect) which thereby competes with Iꢀ(aq) for
O3(g) at the air/water interface.11 Since the widespread saturated
organic acids, in contrast with phenols/phenolates, are relatively
inert toward ozone
Figure 1. Gaseous iodine concentrations [I2(g)] during the ozonolysis
of 5 mM aqueous NaI solutions in the absence/presence of added
carboxylic acids: dashed line, pH = 5.8; solid red line, in the presence of
10 mM hexanoic acid (HA) at pH = 4.8; solid blue line, in the presence
of saturated octanoic acid (OA) at pH = 4.8.
RCOOH=RCOOꢀ þ O3ðaqÞ f products
ð6Þ
injection into the gasꢀliquid interaction cell. Product concentra-
tions were monitored with a Nd3þ:YAG pumped dye laser
i.e., k6 , k1 = 1.2 ꢁ 109 Mꢀ1 sꢀ1 (see Table 1), it could have been
anticipated that they would not interfere with iodide oxidation by
atmospheric ozone, reaction 1. In this paper we report, however,
that carboxylic acids significantly enhance I2(g) formation in the
heterogeneous ozonation of Iꢀ on mildly acidic water (pH 2ꢀ5)
because they are more efficient proton donors than water itself,
that is, reactions 7 and 8 are faster than reactions 3 and 4
(Lambda Physik, Scanmate) tuned at 435.6 nm for the IO (g)
3
band head of the A 2Π3/2 r X 2 Π3/2 (v0 = 3, v00 = 0) transition.36
The IO baseline signal was taken at 435.4 nm, a region where
3
IO (g) does not absorb. I2(g) concentration was determined
3
from the BꢀX band absorptions at 435.4 nm,37 which were
calibrated using known I2(g) concentrations in the reaction cell.
The reaction cell was maintained at 100 Torr by means of a rotary
pump, a mechanical booster pump, and a N2(l) trap in tandem, and
filled with NaI(aq) solution ([NaI] = 5 mM (3.0 ꢁ 1018
molecules cmꢀ3) unless otherwise stated, volume = 60 mL) up to
9 ( 1 mm above its bottom. The CRDS detection region is 2 mm
above the solution surface. The average residence time of the gases
circulating through the cell, i.e., the contact time of O3(g) with the
NaI(aq) solution, is ∼0.7 s. To minimize possible secondary
reactions, a freshly prepared solution was used for each data point,
except for rate measurements (Figure 1). We carefully checked that
no secondary wall reactions occurred under the present experimental
condition, e.g., the cell diameter change from 21 to 96 mm did not
influence the results.12 Solution pH was adjusted by adding HCl/
KOH and measured with a pH-meter. Thus, this application of
CRDS enabled us to directly monitor gaseous products released
from the prompt heterogeneous reaction of O3(g) with Iꢀ(aq)
solutions with an adequate wavelength resolution and sensitivity in
less than 1 s. The concentrations of O3(g) were (1.5ꢀ4.8) ꢁ 1015
molecules cmꢀ3. The observed products concentrations were
IOꢀðsÞ þ RCOOHðsÞ f HOIðsÞ þ RCOOꢀ
ð7Þ
HOIðsÞ þ Iꢀ þ RCOOH f I2ðsÞ þ H2O þ RCOOꢀ ð8Þ
Mildly acidic conditions are typical of atmospheric aerosols that
incorporate acidic gases such as carbon dioxide and nitric acid, or
are acidified in situ by the oxidation of dissolved S(IV) even in
clean marine air, according to previous reports on the chemical
composition of atmospheric precipitation.31ꢀ35
’ EXPERIMENTAL METHODS
Cavity Ring-Down Spectroscopy (CRDS). The present
CRDS setup within a gasꢀliquid interaction cell is essentially
the same as shown in our previous works11,12 Ozone was
produced by flowing 1 slm (standard liter per minute) O2(g)
through a high-pressure discharge ozonizer and monitored on-
line by UV absorption by a 253.7 nm Hg lamp prior to its
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The Journal of Physical Chemistry A
ARTICLE
Figure 2. Gaseous iodine monoxide concentrations [IO (g)] as func-
3
tion of initial bulk pH in the ozonolysis of 5 mM aqueous NaI solutions
(blue circles) and with 10 mM hexanoic acid (red triangles).
Figure 3. Gaseous iodine concentrations [I2(g)] as a function of [HA]
in the ozonolysis of (5 mM NaI þ hexanoic acid, HA) solutions at pH
4.8 measured at 20 s after introduction of 4.2 ꢁ 1015 molecules cmꢀ3
(red circles), 3.2 ꢁ 1015 molecules cmꢀ3 (green circles), and 1.5 ꢁ 1015
molecules cmꢀ3 (blue circles) O3(g), respectively. The solid lines are
guides for eye clarity.
(1.8ꢀ3.5) ꢁ 1011 molecules cmꢀ3 for IO(g) and (0.1ꢀ3.1) ꢁ 1014
molecules cmꢀ3 for I2(g). Hexanoic acid, HA (Alfa Aesar), octanoic
acid, OA (Tokyo Chemical Industry), malonic acid, MA (Wako),
acetic acid, (AA) (Wako), and sodium iodide (Sigma-Aldrich) were
used without further purification.
Electrospray Ionization Mass Spectrometry (ESIMS). ESIMS
experiments involved the nebulization of aqueous solutions of a
mixture of NaI and hexanoic acid into the spraying chamber of
an electrospray ionization mass spectrometer (ESIMS, Agilent
1100 MSD Series). Solutions are pumped (50 μL minꢀ1) into the
spraying chamber through a grounded stainless steel needle (100
μm bore) coaxial with a sheath issuing nebulizer N2(g) athigh flow
rates.38 The fast nebulizer gas (typically at vg = 2.5 ꢁ 104 cm sꢀ1
)
shreds the interfacial layers of the much slower liquid microjet
(vj = 11 cm sꢀ1) into microdroplets thatmay carry excess anions or
cations. The production of charged microdroplets from a neutral
liquid is the normal outcome of the charge fluctuations [of
magnitude proportional to (droplet mass)ꢀ1/2] expected in a
statistical breakup process; i.e., droplet charging does not require
the application of an external electric field.39ꢀ41Charged micro-
droplets subsequently evaporate solvent in the chamber while
being drawn to the electrically polarized inlet of the mass spectro-
meter with increasing acceleration: a = (ze/m)E. The latter
statement follows from the fact that the converging electric field
E becomes more intense near the inlet while droplets lose mass m
but retain excess charge ze. Since these microdroplets are the
progeny of nascent droplets shred from the surface of the microjet,
they are enriched with surface-active species. The ESI mass spectra
acquired in these experiments therefore report the composition of
the outermost layers of the microjet. Thus, the present ESIMS
experiment is surface-sensitive. Further experimental details could
be found in previous works.42ꢀ44 HA, MA, and NaI were
purchased from Sigma-Aldrich and used without further
purification.
Figure 4. Gaseous iodine concentrations [I2(g)] during the ozonolysis
of (A) 5 mM NaI (black circles), 5 mM NaI þ 10 mM HA (red
downward triangles) solutions in the presence of 4.8 ꢁ 1015
molecules cmꢀ3 O3(g); (B) 5 mM NaI (black circles), 5 mM NaI þ
10 mM MA (green upward triangles) solutions in the presence of 3.5 ꢁ
1015 molecules cmꢀ3 O3(g), as functions of initial bulk pH. The vertical
dashed lines denote pKa(HA) and pKa2(MA), respectively.
’ RESULTS AND DISCUSSION
introducing O3(g) into the cell the pH of the liquid solution
increased from 4.8 to 10.3 (5.3) in the absence (presence) of
10 mM HA. No discernible pH changes were observed in
experiments under strongly acidic condition (pH = 2.1) in the
absence or presence of 10 mM HA. Figure 3 shows [I2(g)] at t =
20 s after the introduction of various [O3(g)] concentrations as a
function of [HA] over aqueous solutions initially at pH 4.8.
[I2(g)] increases with [HA] up to [HA] ∼ 10 mM, reaching an
asymptotic value of [I2(g)] ∼ 8.5 ꢁ 1013 molecules cmꢀ3 at low
Figure 1 shows that I2(g) emission rates are enhanced 6-fold
by the presence of 10 mM HA or saturated OA during ozonolysis
of a NaI solution at pH = 4.8 ∼ pKa (HA or OA). In contrast,
Figure 2.13 These results imply that the proton, Hþ, is not
IO (g) emission rates remain constant (within 20%) as shown in
3
involved in IO (g) production (see reactions 1 and 2b). It should
3
be emphasized that reaction 2b may not be elementary but
involve several steps.12 In accord with the stoichiometry of iodine
production, 2Iꢀ þ 2Hþ þ O3 = I2 þ H2O þ O2, 180 s after
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ARTICLE
Figure 5. [I2]/[I2]0 ratios in the ozonolysis of 5 mM NaI solutions at
pH ∼ pKa measured at 20 s after O3(g) introduction. [I2] in the presence
of 4 mM acids; [I2]0 in the absence of added acids.
Figure 6. ESI mass spectra signal intensities: hexanoate (HAꢀ, filled red
triangles), Iꢀ (filled red circles) from aqueous (0.5 mM NaI þ 1.0 mM
HA) microjets; malonate (MAꢀ, open blue triangles), Iꢀ (open blue
circles) from aqueous (0.5 mM NaI þ 1.0 mM MA) microjets, as
functions of bulk pH. Connecting lines are visual aids. Note that actual
HAꢀ signal intensities are 10 times larger.
[O3] (1.5 ꢁ 1015 molecules cmꢀ3). At high [O3] (4.2 ꢁ 1015
molecules cmꢀ3), however, [I2(g)] approaches a larger asymp-
totic 1.8 ꢁ 1014 molecules cmꢀ3 value above [HA] ∼ 15 mM.
Since the water’s surface becomes saturated at >50 mM HA, as
indicated by surface tension measurements (Figure S1, Support-
ing Information),45 the results of Figure 3 suggest that in the
ranges investigated I2(g) production is controlled by either [O3]
or the interfacial concentration of undissociated hexanoic acid.
On 10 mM HA solutions, I2(g) production is clearly limited by
[O3] below [O3] ∼ 3.2 ꢁ 1015 molecules cmꢀ3. Above [O3] ∼
4.2 ꢁ 1015 molecules cmꢀ3, however, [I2(g)] production be-
comes proportional to [HA]surface: both increase by 28 ( 5%
between 10 and 25 mM HA. Notice that reaction 9, which
competes with reaction 4 for HOI and depends on [O3]2, is
expected to eventually inhibit I2(g) emission rates at higher
[O3]17,22
‘’normal’’ surfactant effects expected from alkyl chains hindering
the uptake of gaseous reactants on water.15,28,47ꢀ51
The more tensioactive acids, by placing their CO(O)H groups
closer to the interface and providing proton donors accessible to
IOꢀ/HOI at the air/water interfacial layers in the pH range where
water itself is a poorer donor,42,43 should enhance I2(g) emission
rates. The key point is that even relatively weak carboxylic acids are
much stronger acids than water, whether in the bulk liquid or at the
interface. Since the dissociation rate constants of weak acids (pKa)
are given by kd ∼ 1010ꢀpK
s
ꢀ1, water dissociation is a rare event
a
(kd_water ∼ 1010ꢀ14 = 10ꢀ4
s
ꢀ1), compared to carboxylic acid
Hence, in the
dissociation with kd_acid = 1010ꢀ4.8 = 105.2 sꢀ1 52
.
IOꢀ=HOI þ 2O3 f IO3ꢀ þ 2O2ð þ HþÞ
ð9Þ
absence of carboxylic acids or similar proton donors, relatively fast
interfacial reactions that consume protons will rapidly deplete the
acidity of air/water interfacial layers. The slightly water-soluble OA
and HA are expected to populate the air/water interface layers with
their hydrophobic alkyl tails pointing to the gas phase, whereas the
more hydrophilic MA and AA will largely remain in the bulk
liquid.48,50,53ꢀ56 The effects of OA and HA on iodine emission
rates are therefore consistent with surface tension studies (see
Figure S1 in the Supporting Information).45,57,58 Although MA
may not induce significant surface tension reductions below
∼10 mM,59 ESIMS experiments (see below) suggest that the
surface affinity of malonate is comparable with that of iodide.
To probe relative anion populations at the air/water interface
layers, we performed experiments with nebulizer-assisted nega-
tive-ion ESI-MS. We have previously demonstrated that ESIMS
signal intensities are directly proportional to anion concentra-
tions at the topmost layers of the liquid microjets produced from
sample solutions.18,19,60 Figure 6 shows how ESI-MS signals of
carboxylates and Iꢀ obtained from aqueous (0.5 mM NaI þ
1 mM acid) microjets vary as a function of bulk pH from 2 to 7.
Hexanoate (m/z = 115) signals increase with pH from 2 to 7 as
expected from the dissociation of HA (pKa = 4.8).60 In contrast,
Iꢀ (m/z = 127) signals peak at pH ∼3.6 and slightly decrease at
higher pH. It is apparent that hexanoate favorably competes and
displaces Iꢀ from the air/water interface layers. A similar
phenomenon is observed in the case of malonate (pKa1 = 2.9)
at m/z = 103, in which Iꢀ signals plateau above pH 3. From their
relative signal intensities, we estimate that the surface affinity of
Figure 4A shows how I2(g) emission rates depend on the initial
pH in the presence/absence of 10 mM HA. In the absence of HA,
I2(g) emission rates markedly increase below pH ∼4, as reported
previously.11,12 I2(g) emission rates were found to be further
accelerated by HA below pH ∼5, i.e., in the pH e pKa(HA) =
4.8 range where HA is largely present in its undissociated form. This
observation is in line with the fact that the interfacial affinity of
carboxylic acids is larger than their conjugate anions.46 A similar
effect was observed in the case of OA, pKa(OA) = 4.8. Figure 4B
shows the dependence of I2(g) emission rates on initial pH with/
without 10 mM MA (pKa1 = 2.9, pKa2 = 5.7). In this case, I2(g)
emissions were enhanced at pH e6 ∼ pKa2. Since none of these
acids displays appreciable effects on I2(g) emissions above pH ∼7,
we infer that their hydrophobic alkyl chains neither hinder nor
enhance O3(g) uptake. This finding is consistent with our previous
results that neither n-butanol nor salicylic acid affect I2(g)/IO (g)
3
emissions at pH ∼7.11
Figure 5 shows ratios, Ri, of I2(g) emission rates measured in the
presence of 4 mM OA, HA, or AA over those measured in their
absence at pH = 4.8 (= pKa), and the corresponding ratios in the
case of MA at pH = 5.7 (= pKa2). It is apparent these ratios follow
in the order R(OA) > R (HA) . R(MA) > R(AA). AA has a
negligible effect on I2(g) emissions. The same trend is observed
in experiments performed in the presence of 10 mM acids. It
should be emphasized that the observed trend excludes the
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The Journal of Physical Chemistry A
ARTICLE
hexanoate is ∼30 times higher than Iꢀ, whereas malonate is
comparable with Iꢀ, which is known as a significantly surface
active halide proved by other surface specific detection
methods,24,50,61ꢀ64 at pH > pKa. A difference between HA and
MA is likely due to the dissimilar hydrophobicities of the two
acids (see Table 1). As mentioned above, the series R(OA) >
R(HA) . R(AA) represents direct evidence that the hetero-
geneous ozonation of Iꢀ(s) proceeds mainly in the air/water
interfacial layers.22ꢀ25,65 Note that these three carboxylic acids
have the same pKa ∼ 4.8 and the only difference is the surface
affinity caused by the alkyl chain length. The direct correlation
between the relative anion affinities for the air/water interface
determined by ESIMS and the observed I2(g) emission rate
enhancements strongly supports such a view.
’ ASSOCIATED CONTENT
S
Supporting Information. The surface excess versus con-
b
centration of hexanoic acid/octanoic acid plot reproduced from
ref 45. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail:kawasaki@moleng.kyoto-u.ac.jp.
’ ACKNOWLEDGMENT
S.E. is grateful to the JSPS Postdoctoral Fellowships for Research
Abroad. This work was supported by the Ministry of Education,
Japan, a Grant-in-Aid (20245005), and the U.S. National Science
Foundation, Grant AGS-0964853.
On the basis of the proposed reaction mechanism,12 see reactions
1ꢀ5, Hþ is consumed in reactions 3 and 4. Actually, the pH of the
reacted solution increased from 4.8 to 10.3 after exposing 5 mM NaI
to 4.8 ꢁ 1015 molecules cmꢀ3 O3(g) for t = 180 s. Thus, in the
absence of organic acids, the overall process depletes proton and HOI
(pKa = 10.8) concentrations along its course,11 thereby slowing down
reactions 3 and 4. The presence of undissociated organic acids at
pH e5 effectively buffers the system in these time scales, prevents
proton depletion, and sustains I2(g) production rates. By assuming
a universal neutralization rate constant value of kn(Xꢀ þ Hþ)aq
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∼ 1 ꢁ 1010 Mꢀ1
s
ꢀ1, the rate constants for acid dissociation are
a
given by: kd ∼ 1 ꢁ 1010ꢀpK Mꢀ1
s
ꢀ1. Therefore, we estimate
kd ∼ 1.6 ꢁ 105 sꢀ1 for OA, HA, and AA, and 2.0 ꢁ 104 sꢀ1 for the
malonate monoanion.66 This is consistent with previous reports
showing that the protonation of gaseous trimethylamine on
water’s surface is markedly enhanced above pH ∼4 by fulvic
acid, a major component of dissolved organic matter.43 We
conclude that most weak acids will similarly enhance I2(g)
emissions from partially acidified submicrometer marine aerosol
particles throughout the day. The fact that enhancement factors
become negligible below pH 2, where water becomes the
dominant proton donor (because proton at the air/neat water
interface is only available at bulk pH < 4),42ꢀ44,67 provides
further confirmation of the proposed reaction mechanism.
(13) Wurl, O.; Obbard, J. P.; Lam, P. K. S. Mar. Pollut. Bull. 2006,
52, 768.
’ ATMOSPHERIC IMPLICATIONS
We have showed that I2(g) emissions from the heterogeneous
reactions of O3(g) with Iꢀ(aq) are significantly enhanced in the
presence of surface active weak acids under mildly acidic condi-
tion that are typical of fine marine aerosols. The same enhance-
ment may be expected in the case of interfacial ozonation of
bromide or chloride.24,68ꢀ70 A global atmospheric model simula-
tion shows the pH of marine aerosols ranges from 1 to 8 which
strongly depends on the location and season.71 Although accu-
rate pH measurements are still challenging, recent direct mea-
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