Modification of Ozone Deposition and I2 Emissions at the Air

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Modification of Ozone Deposition and I2 Emissions at the Air− Aqueous Interface by Dissolved Organic Carbon of Marine Origin Marvin D. Shaw*,† and Lucy J. Carpenter Department of Chemistry, University of York, Heslington, York, UK Y010 5DD S Supporting Information *

ABSTRACT: The reaction between gaseous ozone (O3) and aqueous iodide (I−) at the surface microlayer (SML) is believed to be a major chemical contributor to the oceanic dry deposition of O3 over open ocean waters and has also recently been shown to produce environmentally significant quantities of gaseous molecular iodine (I2). Here we investigate how this reaction is affected by the presence of dissolved organic carbon (DOC) of marine origin, using a heterogeneous flow reactor and detection of gaseous I2 by solvent trapping and UV/vis spectroscopy. Ozone deposition measurements over coastal seawater implied an O3 reactivity (λ) toward coastal marine DOC of ∼500 (420−580) s−1, 2−5 times higher than that toward iodide at typical ocean concentrations (∼0.5−1 × 10−7 M). We added varying amounts of highly concentrated DOC extracted from coastal seawater to I− solutions (1 × 10−5 M) such that the relative reactivities of DOC and I− toward O3 (λDOC/λI) were in the expected range for natural seawater. The evolution of gaseous I2 and the loss of aqueous I− both reduced as DOC concentrations increased, with an overall suppression of I2 emissions of about a factor of 2 under conditions of λDOC/λI representative of open ocean waters (0.5−1). A kinetic model of the SML suggested that neither competition of DOC with I− for reaction with interfacial O3, nor direct loss of I2 and hypoiodous acid (HOI) through reaction with increasing quantities of DOC, can fully explain these results. We conclude that the suppression of I2 emissions by DOC is largely a physical effect arising from a decrease in the net transfer of I2 from the aqueous to gas phase, as suggested by recent laboratory studies.

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unstable trioxide intermediate (IOOO−).17,19 More recently, we showed that because HOI is in equilibrium with I2 in surface seawater (reaction 4) but heavily in favor of HOI, gaseous emissions of the latter outweigh those of I2 at ambient I− concentrations.23 The production of gaseous HOI and I2 via ozonation of marine iodide has global implications as it provides a potentially ubiquitous mechanism for O3 induced halogen emission from the world’s oceans.23 HOI formation:

INTRODUCTION Tropospheric iodine chemistry depletes ozone,1−4 perturbs HOx (OH + HO2) and NOx (NO + NO2)1,5 cycles and can generate new aerosol particles in coastal environments,6,7 influencing atmospheric oxidizing capacity, human health and climate. Volatile organic iodine compounds (VOIC) and molecular iodine (I2) are important sources of reactive iodine in the marine boundary layer (MBL).1,4,8−10 Seaweeds are strong emitters of both VOIC and I2 in some coastal environments,7,11,12 and VOIC are ubiquitously found in the open ocean, as a result of phytoplanktonic activity10,13,14 and potentially abiotic processes.15,16 The short atmospheric lifetime of I2 prevents coastal biogenic emissions influencing open ocean concentrations of reactive iodine. Recent observations over the tropical Atlantic indicate however that organic iodine fluxes are insufficient to explain observed gaseous iodine monoxide (IO) concentrations in this region, contributing only 20−25% of reactive iodine to the tropical MBL.3,10 These field and model studies suggest an additional “missing” ubiquitous ocean reactive iodine source. Experimental studies have suggested oceanic sources of I2 from the oxidation of I− in surface seawater by gaseous O3.17−23 Reactions 1−6 show the proposed formation of I2 and hypoiodous acid (HOI) at the surface microlayer (SML) from the ozonation of I− and subsequent reactions involving an © XXXX American Chemical Society

I−(aq) + O3(g) → IOOO−(interface)

(1)

IOOO−(interface) → IO−(aq) + O2 (aq)

(2)

IO−(aq) + H+ → HOI(aq)

(3)

net: I−(aq) + O3(g) + H+ → HOI(aq) + O2 (aq)

HOI/I2 equilibra: HOI(aq) + I− + H+ ⇌ I 2(aq) + H 2O

(4)

Received: March 15, 2013 Revised: August 28, 2013 Accepted: September 4, 2013

A

dx.doi.org/10.1021/es4011459 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

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I 2(aq) ⇌ I 2(g)

(5)

HOI(aq) ⇌ HOI(g)

(6)

administered into the reaction vessel using a gastight syringe (Samco) via a Luer lock tap. The parallel resistance approach to oceanic ozone deposition (discussed in the SI), has been used extensively and found to explain laboratory and field measurements of ozone deposition velocities reasonably well,25,39 although capturing the large variability in values (deposition velocities to surface seawater reported in the literature vary significantly with values ranging from 0.01 to 0.12 cm s−1)39−42 remains a challenge. We use the resistance approach as the basis for interpretation of laboratory results in this paper, where the deposition velocity vD is the inverse of the sum of the aqueous phase (rs) and aerodynamic (ra) resistances to ozone deposition:41,43

Note that the disproportionation reaction 4 is a complex reaction of several steps.24 As well as providing a viable route for I2(g) and HOI(g) production, the reaction of O3(g) with I− in the SML contributes a significant fraction of the chemical loss of O3(g) to the ocean surface and thus to dry deposition.25 The total annual global sink of O3(g) through dry deposition to land and sea surfaces is estimated to be comparable to the tropospheric source of ozone from stratospheric-tropospheric exchange (600−1000 Tg yr−1).25,26 Current estimates, based on chemistry transport model analyses, indicate that oceanic ozone deposition accounts for approximately one-third of this total annual global sink.25,26 Marine water is covered by an organic rich SML of 50 ± 10 μm thickness at low wind speeds.27,28 Chlorophyll is one of the many large unsaturated biological molecules located and enriched within this layer,29 and it has been suggested that molecular as well as organo-halides can be formed via the photo-oxidation of halides by chlorophyll cations (or excited state chlorophyll).30,31 In this study, however, we concentrate on the effect of dissolved organic carbon (DOC) on the yield of I2 following the dark reaction of O3 + I−. DOC may reduce the yield of I2 by (i) competing with I− for reaction with O3 at the surface thus modifying ozone deposition,20,25,32,33 (ii) reacting directly with I2 and/or HOI in the interfacial layer,34−36 or (iii) acting as a physical barrier to gas exchange, by forming an insoluble layer,37 or as a secondary liquid phase providing resistance to gas exchange,38 or by enhancing the solubility of I2 at the aqueous surface due to decreased interstitial layer polarity.21 The latter was suggested by Reeser and Donaldson21 to explain their observation of strongly increased partitioning of I2 to the aqueous phase in the presence of an octanol coating. Organic amphiphiles other than octanol may also have similar effects given that the surface interstitial layer is likely to contain a complex mixture of amphiphillic species. It has also been suggested that amphiphillic weak carboxylic acids enhance interfacial I2(g) production under mildly acidic conditions by supplying the required interfacial protons for reaction 3.22

vd = (ra + rs)−1

(7)

Being relatively insoluble, atmospheric ozone deposition is determined (∼95%) predominantly by the aqueous phase resistance (rs) in the surface seawater which is a function of its chemical loss rate (λ, s−1) in seawater:40,44 rs = H /(λD)1/2

(8)

λ = k iC i

(9)

where H is the dimensionless Henry’s law constant, D is the molecular diffusivity of O3 in water (m2 s−1), ki is the second order kinetic rate constant and Ci is the reactant concentration. We define the thickness of the SML over which the fast O3 + I− reaction can occur as the reacto-diffusive length, δ.23,45 δ = √ (D / λ )

(10)

Ozone deposition velocities were determined from the observed ozone uptake rate constant (kobs) assuming pseudo first order rate kinetics at 70 ppbv O3, eq 11. Equation 1243 was used to estimate the half-life of ozone within the reaction vessel (t1/2(surface), where t1/2(measured) and t1/2(control) are the measured half-life of ozone during exposure to DOC/I− solutions and the empty reaction vessel, respectively, and the 5/6th is an empirically determined factor.43 The vD of ozone was calculated using eq 1343 where V is the headspace volume of the reaction chamber (2.0 × 10−3 m3) and A is the surface area of the liquid surface (4.9 × 10−3 m2).

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EXPERIMENTAL DETAILS Heterogeneous Flow Reactor and Measurement of Ozone Deposition and I2 Emission. Supporting Information (SI) Figure S1 shows the experimental set up used for the study of I2 production from iodide solutions buffered at pH 8 (293K). The system was designed to supply ozone to the reaction vessel (2 L round-bottom flask, Cole Palmer) over 1 × 10−5 M [I−] phosphate buffer solutions (PBS) containing 0− 3.1 × 10−4 M (0−4.4 ppm) DOC. The entire reaction vessel was covered in aluminum foil to prevent I2 losses from photolysis. Ozone was produced from dry hydrocarbon-free air by its exposure to a commercial ozone generator (185 nm excitation, UVP). The O3 concentrations and flow rate through the system were metered and monitored, respectively, by a mass flow controller (MKS technologies) and ozone monitor (model 49i ozone analyzer, Thermo Scientific). Prior to each run, the system was equilibrated with ozone at a flow rate of 0.2 L min−1 until a constant mixing ratio of ∼70 ppb ±2.5% was observed. The PBS (20 mL) containing DOC and iodide was then

−ln[O3] = kobst − ln[O3]0

(11)

t1/2(surface)−1 = t1/2(measured)−1 − 5/6t1/2(control)−1

(12)

vD = (V /A) × t1/2(surface)−1

(13)

Molecular iodine evolved from the reaction of ozone with iodide was trapped in hexane (≤−50 °C) at 100% trapping efficiency46 using a 25 mL capacity midget bubbler (Supelco) then quantified spectrophotometrically. Residual moisture evolved from the reaction vessel was removed from the gas stream prior to I2 enrichment by using two spiral condensers in series held at 0 and −10 °C. Following each experimental run of 60 min, both the PBS and hexane were removed and analyzed for I−, triiodide (I3−) and I2 using a dual beam Lambda 25 UV−vis spectrophotometer (Perkin-Elmer) with quartz cells of a 10 cm light path. Triodide was not quantitatively determined in any aqueous solutions containing DOC due to strong interferences observed in the 250−400 nm region. A calibration curve was plotted daily at 223.0 and 522.5 nm to B



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alanine (99% purity, BDH supplies) was used at successive carbon concentrations to generate a 5-point calibration curve. The required DOC concentrations in PBS during the heterogeneous flow reactor experiments were prepared by adding 20−160 μL of the DOC extract (555 ppm, 3.8 × 10−2 M) concentrated from seawater to the PBS (20 mL). Modeling Iodine Emissions: The Interfacial Model. Full details of the interfacial model are given in the SI of Carpenter et al.23 Briefly, we treat the SML as a box, assuming no horizontal advection but mixing vertically with bulk mixed layer water at a fixed SML turnover time (representative of constant stirring). Iodine production is initiated by the (measured) gas phase flux of O3 into the interfacial layer, with the depth of the interfacial layer defined as the reacto-diffusive length for O3 (δ). Concentrations of [I−], [H+] and [OH−] were fixed for each model run and we use a detailed treatment of aqueous iodine dynamics, including iodide oxidation, iodine disproportionation and iodine reduction. Calculation of mass transfer velocities for aqueous−air transfer of I2 and HOI were calculated from an empirical formulation relevant for still aqueous solutions.49 For modeling the effect of DOC on surface seawater iodine emissions, we simulated three scenarios. Scenario A only included the combined effect of the O3 + DOC reaction in the SML plus the impact of DOC on O3 deposition velocity (which increases with increasing [DOC]). Scenario B included the conditions in scenario A and also included direct reactions of both I2 and HOI with DOC at rates representative of measured I2 loss in coastal seawater34 of 7 × 10−3 s−1 per ppm DOC (equivalent to ∼1 × 102 M−1 s−1). Scenario C was the same as Scenario B except the rate of HOI + DOC was set at a much faster rate of 1 × 104 M−1 s−1, similar to rates of HOI loss to substituted phenols and corresponding phenolates.36

quantitatively determine iodide in PBS at pH 8, and I2 trapped in hexane, respectively. All Teflon tubing used in the experimental setup was heated to ∼60 °C for the duration of each experiment to minimize I2 wall losses. A series of control experiments were conducted over an experimental run time of 60 min to determine I2 wall losses as a function of gaseous I2 concentration (10−40 ppb). Molecular I2 wall losses were dependent on concentration according to the relationship 0.18 ppb lost per ppb I2. These wall loss corrections were applied to all gaseous I 2 determinations. Solution Preparation. PBS (pH 8) was prepared by dissolving sodium dihydrogen phosphate (NaH2PO4, Fischer, 99%) in degassed HPLC water (800 mL). The final pH of the solution was adjusted by titrating freshly prepared aqueous sodium hydroxide (NaOH, Fischer, 99%) solution while stirring using a pH meter, prior to making the final volume up to 1 L. The pH of the PBS solutions were measured and found to be within a suitable tolerance (±2%) on a weekly basis. The preparation of solutions of aqueous I− and I2 in hexane were prepared by overnight stirring of KI (analytical grade, Fischer) in PBS and I2 flakes (Riedel-de Haen) in hexane (HPLC grade, Fischer), respectively. Solutions of I2 in hexane were covered in aluminum foil and stored at ambient temperature to reduce degradation. Primary I2 and I− stock standards were reprepared on a monthly basis whereas secondary standards (used to plot the calibration curve) were reprepared every 2 weeks. Both primary and secondary standards encountered no measurable degradation during these periods. Marine DOC Extraction and Determination. The DOC extract was prepared from coastal seawater (10 L, Bridlington coast, UK, 54.09°N, 0.20°W) collected in airtight glass bottles (4 × 2.5 L) and wrapped in aluminum foil before transportation to the lab. The bottles were refrigerated and stored at 2−8° for ∼12 h prior to preparation. The seawater was first degassed using zero grade N2 while being heated to 70 °C for 1 h to adequately remove oxygen and any volatile halogenated compounds. The seawater used for DOC extraction was then filtered through a 0.45 μm cellulose nitrate membrane filter (Whatman) under vacuum and acidified with 10 M HCl to pH 2−3 to remove any aqueous inorganic carbon and arrest any biological processes from occurring.47 Preconcentration of the organic content was achieved by passing the prepared seawater through a conditioned (with 5 mL methanol) styrene divinyl benzene (SDB) polymer SPE cartridge (Strata-X, 500 mg, Phenomenex) under vacuum at a flow-rate not exceeding 5 mL min−1. Such devices are designed for a high sample throughput and the retention of a wide range of nonpolar aliphatic and aromatic DOC. Recently, Dittmar et al.48 found that SDB polymer sorbents used for SPE could extract 43−62% of marine DOC for sample volumes of 0.5−50 L. Once loaded, the sorbent was washed using 5% methanol (5 mL) in HPLC water and dried under vacuum ∼ (1 min). The organic constituents were eluted with methanol (10 mL) and concentrated to dryness (Biotage V10 solvent concentrator) and resuspended in 10 mL HPLC water. All preparation and preconcentration steps were completed within 24 h of sample collection. Quantitative determination of the DOC extract was carried out using a flash 2000 CHNS elemental analyzer (Thermo Scientific). Aliquots of the acidified extract (250 μL) were introduced to the instrument and analyzed in duplicate. L-

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RESULTS AND DISCUSSION Ozone Deposition over Iodide Solution, Coastal Seawater, and DOC Extracts. The previous discussion suggests that ozone deposition and I2(g) evolution rates at the SML are influenced by the pH dependent ki (O3 + I− reaction) as well as the competing reactions of O3 and potentially I2/HOI with DOC. Reported values for ki vary by a factor of 2. In buffered solutions of [I−] (3−56 × 10−5 M) at pH 6.7 (298 K), Liu et al.50 determined a ki of 1.2 × 109 M−1 s−1, which should theoretically represent an overestimation of the reaction at pH 8 since the reaction is believed to be both pH and temperature dependent. Garland et al.51 determined a ki of 2.0 × 109 M−1 s−1 from O3 uptake rates on seawater (1 × 10−7 M) at pH 8 (298 K). Magi et al.52 measured a value of 2.4 × 109 M−1 s−1 in nonbuffered [I−] (0.5−3.0 M) solutions at 293 K; these may not be representative of natural seawater conditions as the aqueous pH was not maintained during the experiments. We measured ozone deposition rates in triplicate at 5 and 10 M KI in PBS (pH 8, 20 mL, 293 K), SI Figure S2. The logarithm of the ozone concentration exhibited a linear relationship with time (R2 > 0.99), indicating pseudo first order kinetics. Our values suggest a ki of 1.4 ± 0.2 × 109 M−1 s−1 at 293 K (using eqs 3 and 4 in the SI to calculate ki from λI). Using the activation energy of the O3 + I− reaction (73.08 kJ) determined by Magi et al.,52 produces a ki of 2.2 ± 0.9 × 109 M−1 s−1 at 298 K, which is close to the ki of 2.0 × 109 M−1 s−1 at 298 K derived by Garland et al.44 from similar experiments. Figure 1 compares measured surface resistances, rs, to O3 over iodide solutions with those over iodide-spiked coastal C



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tropical waters of around 1.0 × 10−7 M.25,57 Thus, our laboratory measurements suggest that an additional components of coastal seawater, most likely DOC, is an important chemical control of O3 deposition over coastal waters. In order to calculate the reactivity of our SPE-extracted DOC to ozone, O3(g) loss was measured over 1 × 10−5 M I− in PBS (pH 8, 20 mL) containing 0−3.1 x10−4 M (or 0−4.4 ppm) DOC. The initial O3 mixing ratio was 70 ppbv and losses were observed over 60 min at a flow rate of 0.2 L min−1. Figure 2

Figure 1. Surface resistances (rs) to ozone as a function of [I−], all at pH 8, from measurements of O3 loss rates over iodide solutions and seawater spiked with iodide. Filled gray symbols show observed rs values in iodide solution: Gray diamonds, this work (293 K); gray squares, this work corrected to 298 K (see text); gray circles, data from Garland et al. (298 K).51 Empty symbols denote observed rs values over seawater: Diamonds, this work (293 K); white squares, this work corrected to 298 K (see text); circles, data from Garland et al. (298 K).51 The black line is a guide to the eye and shows predicted rs values for iodide solution using a ki = 2.2 × 109 M−1 s−1 (298 K). The dotted black line includes reactions of O3 + DOC using a constant integrated reactivity of 500 s−1,(λDOC,) and ki = 2.2 × 109 M−1 s−1 (298 K). Errors on the y-axis for data presented in this work represent the standard deviations of triplicate measurements and range between 3−19% and 6−15% for seawater and iodide solution measurements, respectively.

Figure 2. Measured gaseous ozone concentration with respect to time following addition of 1 × 10−5 M iodide in PBS (pH 8) with varying DOC concentrations. Black diamonds, no [DOC]; white triangles, [DOC] of 7.7 × 10−5 M (or 1.1 ppm) ; gray squares, [DOC] of 3.1 × 10−4 M (or 4.4 ppm). The y error bars (±5%) are calculated from the propagation of the quoted MFC error (2.5%), the calibration error and the standard deviation of replicate analyses.

seawater as a function of [I−], using data from both our experiments and those derived by Garland et al.51 from similar experiments (298 K). Also shown are our data corrected to 298K using the activation energy of the O3 + I− reaction determined by Magi et al.52 At low I−(aq) bulk concentrations, the resistances over iodide-spiked seawater diverge and become significantly lower than those over iodide solutions. This is likely due to the presence of aqueous organic material in seawater. A number of sea surface organic substituents are known to react with ozone32,53,54 and thus organics are likely to play an important role in mediating oceanic ozone deposition, particularly in biologically productive regions.13 Observed oceanic O3 deposition velocities measured by eddy covariance from both fixed tower platforms41,55 or by turbulent aircraft measurements40,56 are approximately double that calculated from the presence of iodide alone in coastal waters.25,32 This is consistent with recent laboratory measurements by Martino et al.,33 who demonstrated that natural DOC (sourced from the Suwannee River) and iodide at naturally occurring concentrations contribute similarly to the chemical enhancement of ozone deposition to surface waters. As shown in Figure 1 (dotted black line), a prescribed O3 + DOC constant reactivity (λDOC) for coastal regions of 500 s−1 in addition to an O3 + I− reactivity of 2.2 × 109 M−1 s−1 at 298 K from this work, brings the measured rs values in coastal seawater in this and previous laboratory work51 into line with the calculated values. A range of pseudo first order loss rates was calculated from the standard deviation of the measured rs values over coastal seawater spiked with I− (open symbols) from the calculated λDOC = 500 s−1 curve (sm, 15.6%) (dotted black line). The range required to explain the coastal O3 deposition equates to 420−580 s−1. This is roughly equivalent to the chemical loss due to 2.5 × 10−7 M surface [I−], rather higher than typical concentrations even in

shows semilog plots of the ozone concentration against time for the experiments using pure iodide solution, 3.9 × 10−5, 7.7 × 10−5, and 3.1 × 10−4 M (or 0.6, 1.1, 4.4 ppm) DOC, all at pH 8 in PBS. These experiments were carried out in triplicate similarly to those conducted on iodide in PBS and seawater. Ozone deposition velocities, vd, as a function of [DOC] were calculated from these data and eqs 7−9, thus allowing determination of the total chemical reactivity toward aqueous O3 (λ) as a function of [DOC]. Subtracting the (constant) contribution from iodide gives the λDOC term, which is plotted in Figure 3 against [DOC] to derive a bimolecular rate constant for the aqueous O3 + DOC reaction of 2.6 ± 0.8 × 107 M−1 s−1

Figure 3. Calculated bimolecular rate constant for the observed aqueous O3 + DOC reaction, using DOC extracted by SPE from coastal seawater. Error bars on y-axis (±30%) were calculated from the standard deviation of the linear regression (sm) of duplicate analyses at each [DOC]. D



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Figure 4. a. Measured aqueous I− and gaseous I2 evolution from 1 × 10−5 M iodide in PBS (pH 8) at varying DOC/I− reactivity on exposure to 70 ppbv O3 for 60 min. The y error bars are calculated from the propagation of the quoted MFC error (2.5%), the calibration error and the standard deviation of replicate analyses, and the x error bars represent the error in the calculated DOC reactivity determined from the surface resistance measurements. b. Modeled gaseous I2 emission for Scenario A (gray dashed line), B (solid black line) and C (gray line with crosses). See text for details.

over the DOC concentration range of 3.9 × 10−5−3.1 × 10−4 M (or 0−4.4 ppm). Table S1, in the SI, summarizes the mean and range of calculated chemical reactivities of marine organic compounds toward gaseous ozone. Our inferred rate constant for the extracted DOC is similar to experimentally derived rate constants for the dimethyl sulfide DMS + O3 reaction (8.6 × 108 M−1 s−1)37 and the “free” chlorophyll + O3 reaction (6 × 107 M−1 s−1),32 and lower than the phenolate + O3 reaction (1.4 × 109 M−1 s−1);20 these have all been invoked as possible contributors to O3 deposition over marine waters. However, over the DOC concentration range of 3.9 × 10−5−3.1 × 10−4 M (or 0−4.4 ppm) used in this work, this equates to an ozone reactivity of 1820 ± 560 s−1 per 7 × 10−5 M (or 1 ppm) DOC (the uncertainty was calculated from the standard deviation of the slope of ozone surface resistances against [DOC]). With typical DOC values in seawater of 0.7−2.1 × 10−4 M (or 1−3 ppm),58 this is approximately an order of magnitude higher than previous estimates for ozone loss to DOC over open ocean and coastal waters (100−500 s−1).25 It is therefore clear that the prepared DOC extract was not representative of ubiquitous marine DOC, but represented the more reactive fraction toward oxidation by O3 (possibly the result of a greater retention of nonpolar reactive fractions during SPE extraction). The DOC could have also decomposed to more reactive forms during storage (which was up to 1 month, in the dark) or preparation (thermal degradation during solution degassing at 70 °C). Nevertheless, we take advantage of this high DOC reactivity to examine the effect of DOC on I2 emissions in a

chemical regime where the relative DOC/I− reactivity toward O3 is similar to that in the marine environment. Dependence of Gaseous I2 Evolution and Aqueous Iodide Loss on DOC/I− Reactivity. As discussed in the Introduction, DOC is highly likely to decrease iodine emission from the sea-surface via a number of potential mechanisms. Reaction with O3 in the SML provides competition with reaction 1: This has already been demonstrated in principle by Hayase et al.20 who reported that the presence of aqueous phenol suppressed both I2(g) and IO (g) emissions from ozonized NaI solutions under basic conditions, through rapid reaction of phenolate (C6H5O−) (pKa 9.95) with gaseous ozone. Alternatively, the direct reduction of iodine by DOC in seawater has been suggested to be rapid,34,35 that is, of the order of minutes in coastal seawater. Potential iodinating species for DOC include I2, HOI and OI− and H2OI+.59,60 HOI is believed to be the species involved in the well-known iodoform reaction with α-methyl carbonyl molecules.59 Enolization is the rate limiting step of such reactions, and in the pH range 6.8−8.6 Bischel and von Gunten59 reported an HOI consumption rate constant of 0.9 M −1 s −1 for acetaldehyde, the most reactive carbonyl tested, 3 × 101−6 × 105 M−1 s−1 for substituted phenols and 102−106 faster for the corresponding phenolates formed at high pH (>5). In seawater, Truesdale34,35 measured pseudo first order loss rates of total iodine (assumed to be I2, HOI and OI−) to DOC of 4.7 to 7.9 × 10−3 s−1 in coastal waters and 3.0 × 10−6 to 5.7 × 10−5 s−1 in open ocean waters. E



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Assuming the chemical reactivity of O3(g) in seawater is first order with respect to both [I−(aq)] and [DOC (aq)], then it is the ratio of λDOC/λI that is important when considering their competitive reaction toward ozone at the sea surface. Using the calculated λDOC values from the ozone deposition measurements, the calculated ratio of λDOC/λI in these experiments (at 1 × 10−5 M [I−]) was between 0.1 and 0.9. For comparison, in open ocean regions where reaction with iodide dominates the chemical reactivity of seawater toward O3,25 the ratio is likely to be ∼0.5−1. In coastal regions, assuming an O3 reactivity of ∼500 s−1 toward DOC, the λDOC/λI would most likely be in the range of 1−5. Martino et al.33 observed λDOC/λI reactivities of approximately 1 using [I−] and Suwannee River Natural Organic Material (NOM) concentrations representative of average oceanic conditions of 5−15 × 10−8 M I− and 4−8 × 10−5 M DOC. The experiments described to examine the effect of increasing DOC concentration on O3 loss were repeated to quantify the changes in gaseous I2 evolution and aqueous I− concentration as a function of λDOC /λI, and are shown in Figure 4a. The average amount of gaseous I2 trapped during the experiments was equivalent on a mean molar basis to ∼75% of the aqueous iodine lost in the form of iodide. As I2 wall losses were taken into account using a series of control experiments, the remaining ∼25% in the iodine budget is likely to be due to the formation of other species such as aqueous iodate (IO3−),18 I3−61 and HOI23 at concentrations below the spectrophotometer limit of detection, with possibly some contribution from the direct loss of I2/HOI via reaction with DOC59 giving volatile or dissolved organic species. Increasing the DOC concentration in the KI solution led to a similar decrease in the aqueous I− lost and the amount of gaseous I2 evolved. The interfacial model was used to explore different reasons for this result. Scenario A tested the possibility that the decrease in the aqueous I− lost and I2 produced was caused by decreased oxidation of I− by ozone within the SML due to competitive reactions of O3 with organic species. We used rate constants for O3 + DOC calculated from the ozone deposition experiments, that is, 2.6 × 107 M−1 s−1 to calculate both the loss of interfacial O3 and the increased O3 uptake. Scenario B tested the additional effect of direct reduction of I2 and HOI by DOC in the interfacial layer using rate constants of I2 and HOI loss derived from measurements in seawater and scenario C uses an approximately 100 times greater rate of the HOI + DOC reaction similar to those with phenols at high pH, as described in the Experimental Details section. Figure 4b shows that model Scenarios A and B showed very little change in I2 production as [DOC] increased. For Scenario A, this is because the increased uptake rate of O3 with higher [DOC] almost compensates for the increased rate of O3 loss in the interfacial layer. Scenario B shows that the additional loss of iodine through reaction with DOC at rates representative of natural waters makes almost no difference to the I2 emission (at these high levels of [I−]). Scenario C could be argued as the more representative rate of the experiments because the DOC extract added would likely be biased toward phenolic components, and shows a similar trend as the measurements. The model was also used to calculate the fractional I− loss as a function of DOCthis is a linear function of the interfacial O3 concentration. The calculated fractions however decrease only slightly from 1 (with no added DOC) to 0.994 (at 3.1 × 10−4 M DOC), since there is very little change in the calculated interfacial O3 concentration with [DOC], as discussed above.

The fractions are also exactly the same for all scenarios A−C, because the additional iodine + DOC reactions do not influence the interfacial [O3]. Thus, the interfacial model suggests that a purely chemical interpretation of both the I− and I2 results is not possible. An alternative, physical explanation is feasible. It is well recognized that the presence of organic surfactants within DOC can provide direct resistance to sea-air mass transfer37 and/or represent a secondary, less polar, liquid phase providing resistance to gas exchange.38 Of direct relevance to our results, Reeser and Donaldson21 studied the production of I2 produced in the ozone−iodide interfacial reaction in the presence of octanol films, and found that increasing octanol coverage did not strongly affect the total amount of I2 produced, but decreased the amount of I2 in the gas phase compared to I2 in solution. Note that an increasing amount of I2 in solution would increase the loss rate of I− due to the rapid formation of I3−, and thus may explain our observations of decreasing I− loss with increasing [DOC]. The increased concentrations of I3− could not be confirmed due to the presence of strongly absorbing species at 250−400 nm in solutions spiked with DOC. Thus, our results may be indicative of processes that affect sea−air flux in general. In terms of the ozone−iodide−DOC system, they show that at the upper end of λDOC/λI reactivities of 0.6−0.9 typical of coastal seawater, I2 emission was suppressed by a factor of 2. The SML model indicated that O3 reactions with environmentally relevant DOC could not explain these results, in contrast to the laboratory results of Hayase et al.20 using highly reactive (with O3(g)) phenolate. We further note that increased uptake of O3 to the SML occurs at higher [DOC], almost compensating for increased O3 loss through DOC reactions, although contributing significantly to O3 deposition to coastal waters. Reactions of I2/HOI reactions with DOC also appear to be too slow to affect the ozone-iodide system. An additional resistance to gas exchange by DOC may instead explain these and our previous observations19 where ozone-induced I2 emissions were about a factor of 2 lower from iodide-spiked coastal seawater compared to the same concentration of buffered iodide solution.

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ASSOCIATED CONTENT

* Supporting Information S

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Corresponding author. Present Address †

M.D.S.: Now at Lancaster Environment Centre, Lancaster University, Lancaster, UK LA1 4YW Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank J. Lee, University of York, for loan of the O3 generator and monitor. M.D.S thanks NERC for the award of his PhD studentship. This work was funded through SOLAS grant NE/D006538/1. F



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ABBREVIATIONS DOC dissolved organic carbon MBL marine boundary layer SML surface microlayer PBS phosphate buffer solutions SPE solid phase extraction device HPLC high performance liquid chromatography DMS dimethylsulfide SDB styrene divinylbenzene

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