Reactive Uptake of Ozone by Chlorophyll at Aqueous Surfaces

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Environ. Sci. Technol. 2008, 42, 1138–1143

Reactive Uptake of Ozone by Chlorophyll at Aqueous Surfaces DANIEL CLIFFORD,† D . J . D O N A L D S O N , * ,† MARCELLO BRIGANTE,‡ BARBARA D’ANNA,‡ AND C H R I S T I A N G E O R G E * ,‡ Department of Chemistry, University of Toronto, 80 St.George Street, Toronto, Ontario, Canada M5S 3H6, and Université Lyon 1, Lyon, F-69626, France, CNRS, UMR5256, IRCELYON, Institut de recherches sur la catalyse et l’environnement de Lyon, Villeurbanne, F-69626, France

Received July 23, 2007. Revised manuscript received November 27, 2007. Accepted November 27, 2007.

We report the results of two complementary studies of the heterogeneous reaction between gas-phase ozone and aqueous chlorophyll. In the first experiment, the chlorophyll is present at the air–water interface and its concentration is measured as a function of time, using laser-induced fluorescence, to obtain the surface kinetics. Under most experimental conditions, these are well described using a Langmuir–Hinshelwood formalism. The second experiment was carried out in a wettedwall flowtube apparatus and measured the uptake coefficient of ozone by the chlorophyll solution. The uptake coefficient decreases with increasing ozone concentration, consistent with the surface mechanism found in the fluorescence experiment. The two experiments agree that the uptake coefficient for ozone by such chlorophyll samples is ∼2–5 × 10-6 with unpolluted boundary layer ozone concentrations. At low wind speed, the reaction between ozone and chlorophyll at the sea surface may represent the driving force for ozone deposition at the ocean surface, significantly increasing its deposition velocity there.

Introduction Understanding the sources and sinks of ozone in the marine boundary layer is critical to the ability to model the local oxidizing potential, and thus, among other things, the formation of new particles via oxidation of sulfur-containing compounds released at the sea surface. Recently, there have been efforts to establish whether halide species (in particular, Cl- and Br-) may be oxidized by gas-phase oxidants at or near the ocean surface, releasing photochemically active halogen compounds such as BrCl or HOBr into the atmosphere, which may then participate in catalytic ozone destruction cycles (1, 2). Recent results from the FinlaysonPitts (1, 3) and our (4) groups have established that ozone itself may participate in a heterogeneous oxidation of bromide ions at the air-aqueous interface, releasing photochemically active bromine compounds. Although there is now growing understanding about oxidative halogen release as a gas-phase reactive sink for * Address correspondence to either author. E-mail: jdonalds@ chem.utoronto.ca (D.J.D.); [email protected] (C.G.). † University of Toronto. ‡ Université Lyon 1. 1138

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boundary layer ozone, ozone’s loss rate to the sea surface by dry deposition remains uncertain. Reported values of the surface resistance to ozone uptake span a range of almost an order of magnitude (5); a recent analysis suggests a value of about 1000 s m-1 (6, 7). Wind speed (6–8) and turbulence of the water (5, 9) both influence the exact value, over a factor of about 3. However, models for the deposition which ignore chemical reactions at the interface region underestimate the observed deposition velocities (7, 8). Chang et al. considered several candidates for reactive enhancement of the ozone uptake and suggested that the oxidation of iodide was a likely contributor to the observed deposition rates (8). Another possible reactive sink for ozone at the sea surface is reaction with the biogenically derived compounds present there. Essentially all bodies of marine and fresh water are covered by an organic film (the “surface microlayer”, SML) of 1–1000 µm thickness. Two recent monographs review the current state of knowledge of the physical, chemical, and biological properties of these films (10, 11). Chemical analysis of the organics at the sea surface (11–20) has shown that amphiphiles derived from oceanic biota (fatty acids, fatty alcohols, sterols, amines, etc.) are enriched in this microlayer. Ozone is known to undergo efficient heterogeneous reaction with unsaturated fatty acid amphiphiles (21–23) and aromatics (24, 25) present at the air-aqueous interface. Chlorophyll is one of many large unsaturated biological molecules found in the ocean and sea-surface microlayer. Its structure is illustrated in Figure S1 of the Supporting Information. Because of the presence of double bonds in such molecules, they are expected to be quite reactive toward trace atmospheric oxidants, ozone in particular. Because marine biota are ubiquitous in the ocean, the reaction of chlorophyll and other such large unsaturated biomolecules with ozone may be important for ozone concentrations and chemistry in the marine boundary layer. This would particularly be the case during periods of high biological activity, such as those that occur during algae or plankton blooms, during which chlorophyll concentrations increase. Indeed, concentrations of chlorophyll-a have been found to be enriched in the SML by an average factor of 1.9 versus concentrations in subsurface seawater (26), and also enriched in the surface microlayer covering a bog pond (27). In the following, we report experimental measurements of the heterogeneous reaction of gas-phase ozone with chlorophyll present at the air–water interface. Two sets of data are reported: at higher ozone concentrations, laserinduced fluorescence monitoring of the surface chlorophyll concentration is used to determine the heterogeneous reaction kinetics as a function of ozone concentration. A Langmuir–Hinshelwood mechanism describes the observations very well, implying that the reaction takes place in the water surface region. The second experiment uses a wettedwall flowtube apparatus to measure the loss of gas-phase ozone upon its exposure to thin aqueous films containing chlorophyll. The dependence of the uptake coefficient on ozone concentration also implies a surface reaction. At atmospherically important ozone concentrations, the reactive uptake coefficient is ∼2–5 × 10-6; this value is also implied by the higher ozone concentration data.

Experimental Details 1. Chlorophyll Extraction. Two different methods were employed to extract chlorophyll from spinach leaves. The first was used for water surface experiments and consisted of grinding the leaves with a mortar and pestle, until they had reached a pastelike consistency. A small amount of 10.1021/es0718220 CCC: $40.75

 2008 American Chemical Society

Published on Web 01/12/2008

acetone was then added to the ground spinach, and it was mixed well and further ground then gravity-filtered to remove any leaf residues. The extract resulting from this procedure was a strongly green colored solution, which fluoresced in the red when exposed to visible light. To determine the concentration of chlorophyll in this extract, we measured an absorption spectrum of a dilute solution of extract in acetone against pure acetone and compared it to a reference spectrum (28). The ratio of absorbance at 418 nm to that at 660 nm was found to be approximately equal for both the reference spectrum and that from the sample. The absorbance at 418 nm was divided by the molar extinction coefficient of chlorophyll to estimate the concentration in the solution. A different extraction method was used for wetted-wall flowtube experiments, since ozone was directly monitored using UV–vis spectroscopy at λ ) 254 nm and was affected by interference of gas-phase acetone (λmax ) 260 nm). Spinach leaves were powdered and extracted with methanol at 293 K for one day. The methanol infusion was extracted with hexane, giving an alcoholic fraction rich in carotenoids and a hexane fraction containing chlorophyll. The hexane was removed in vacuo and the resulting extract was dissolved in methanol, then stored in the dark at 279 K. The chlorophyll concentration was determined by UV–visible absorption spectroscopy. 2. Water Surface Fluorescence Experiments. The kinetics setup used is a modification of one previously employed to study the heterogeneous reaction of ozone with surfacebound PAH’s (29, 30) and bromide (4). It consists of a 3-neck round-bottom flask that is filled with 100 mL of aqueous solution. Two drops of the concentrated spinach extract in acetone were placed on the sample surface using a Pasteur pipet. This was observed to spread rapidly across the surface, leaving a visible green layer at the interface. In the absence of ozone, this layer of extract at the surface persisted for times considerably longer than those required for reaction, unless the sample was vigorously mixed. The 355 nm output of a pulsed (10 Hz) frequency-tripled Nd:YAG laser was passed through a quartz window on the side of the flask and impinged upon the surface at a glancing angle, ∼75° from the surface normal. Fluorescence excited by the laser probe was collected perpendicular to the incident radiation by a liquid light guide suspended in a stopper in the center neck of the flask, approximately 1 cm above the surface. The collected radiation was passed through a monochromator and detected by a red-enhanced photomultiplier tube. The resulting time-resolved fluorescence decay signal was averaged over 4 laser shots in a digital oscilloscope and the data was read from the oscilloscope by a custom LabVIEW program running on a PC. A 40 ns ‘slice’ around the peak of the decay curve was averaged and saved for later analysis. Ozone was generated by flowing 1.0 L min-1 of highpurity oxygen at atmospheric pressure past an ozone generating lamp. The resulting mixture (containing 1 × 1014 to 1 × 1016 molecules cm-3 ozone) flowed through a 10 cm long quartz-windowed absorption cell, then into a side neck of the reaction flask, and was exhausted through the opposite neck. The concentration of ozone entering the reaction cell was determined by measuring the attenuation of light from a 254 nm mercury pen-lamp passed through the absorption cell, using a photodiode detector (31). Each kinetics measurement run started with a baseline fluorescence intensity measurement at 674 nm, then ozone flow into the flask was started. A fluorescence intensity measurement was made every 4 s thereafter, until all of the chlorophyll had reacted. After reaction, the color of the sample surface changed from green to clear. A blank run was carried out with the ozone generating lamp switched off, but with

oxygen flow continued, to ensure that no chlorophyll decay was observed in the absence of ozone. Experiments were done using a number of aqueous substrates. For most experiments, high purity 18 MΩ deionized water was used. To approximate a natural seawater substrate, 1 mol L-1 sodium chloride solutions were prepared by dissolving a known amount of salt in the deionized water. In some experiments, 0.1 mol L-1sodium bromide was added to this salt solution and the pH of the solution was raised to approximately 8 with sodium hydroxide. Some experiments were also performed using solutions to which base was not added; these had a pH in the range of 5–6. An Orion model 501 pH meter equipped with a gel-filled combination electrode calibrated around the pH range of interest was used for all pH measurements. 3. Wetted-Wall Flowtube (WWFT) Experiments. The WWFT technique has been described in previous works (32–34). Uptake kinetics were determined from the measured loss rate of gas-phase O3 flowing along a vertically aligned flowtube which was 70 cm long and 1.0 cm internal diameter, yielding a maximum exposed surface area of 220 cm2. The flowtube was kept at constant temperature by circulating thermostatted water through an outer jacket. Under our operating conditions, the mass transport equation in cylindrical coordinates can be applied. The observed loss of O3 from the gas phase was used to obtain the reactive uptake coefficient, γ, defined as the probability that a collision of a gaseous O3 molecule with the liquid surface leads to reactive loss within the liquid. At the bottom of the tube, the reagent solution was pumped out to waste or to a sample vial for further analysis. The reagent solution containing chlorophyll extract was injected at the top of the WWFT using a peristaltic pump, and evenly distributed over the inside walls using an annular reservoir dispenser system made of Teflon, leading to a homogeneous film flowing downward on the inner walls of the flowtube. Ozone was generated by flowing 250 mL min-1 of air through a UV light source (UVP-Ozone Generator) and used without further dilution. Ozone was introduced at the top of the flowtube, which was maintained at 1 bar total pressure. The decay of ozone was followed by extracting the reacted gas into a movable injector positioned at one of five different tube positions corresponding to 10, 30, 50, 60, and 70 cm of gas-liquid interaction lengths. These positions correspond to contact times ranging from 2 to 13 s, given the flow velocity in the WWFT. The first 10 cm was not used in order to establish a laminar flow profile and to allow the relative humidity to reach 100%. After this “injection zone”, the gas flows are well-established and the vapor pressure is in equilibrium with the liquid film, allowing kinetic measurements to be made under reproducible conditions. The gas extracted from the flowtube passed through a 1 m long optical absorption cell. The ozone concentration was monitored by optical detection at 254 nm using a spectrograph coupled to a CCD camera. Typical experimental conditions employed O3 concentrations in the range of 6.2 × 1011 to 2.5 × 1012 molecules cm-3. 4. Chemicals. Sodium chloride (ACS grade), sodium bromide (99.9%, Aldrich), air gas (ALPHAGAZ Air 1), and oxygen gas (99.9%, BOC Gases) were used as received without any further purification. Acetone, methanol, and hexane used for chlorophyll extraction were purchased from Merck (HPLC grade).

Results 1. Surface Fluorescence Experiments. A resolved fluorescence spectrum of the spinach extract at the water surface was obtained by manually scanning the monochromator in 2 nm increments over the range of 520-750 nm, and VOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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air-aqueous interface, including polycyclic aromatic hydrocarbons, oleic acid salts, and bromide anions. The Langmuir–Hinshelwood mechanism (35) was found to describe the kinetics very well in each case. It posits that reaction occurs in two dimensions between coadsorbed species, at least one of which is in rapid equilibrium between bulk and surface phases. It is now established that ozone adsorbs on the aqueous surface (4, 22, 24, 25, 29, 30, 36); its surface concentration is given by [O3(surf)] )

N surf[O3(g)] B + [O3(g)]

(1)

where Nsurf is the maximum number of surface sites available to ozone, and B represents a ratio of desorption to adsorption rate coefficients involving both bulk phases (37). The dependence of the measured chlorophyll loss rate, kobs, on the gas-phase ozone concentration then becomes kobs )

FIGURE 1. (a) Fluorescence intensity of spinach extract at the water surface (measured at 674 nm) vs time following introduction of gas-phase ozone at two concentrations. The lines show fits to a single exponential decay. (b) First-order loss rate constants (derived from the slopes of plots such as those shown in Figure 1a) as a function of gas-phase ozone concentration. Data from “pure” and salt water are both shown with error bars indicating the 1 σ limits on 3–7 separate measurements. The solid line shows a fit of the data to a Langmuir–Hinshelwood form, eq 2. recording the fluorescence intensity at each point. For this spectrum, the oscilloscope was set to average the displayed signal over 64 laser shots. A single fluorescence peak was observed with a maximum at 674 nm, matching closely that of chlorophyll a dissolved in methanol (28). A spectrum is displayed in Figure S2 of the Supporting Information. The red fluorescence from the surface was visible to the naked eye under dimmed room lighting. Immediately following the introduction of ozone into the vessel, the chlorophyll fluorescence intensity at 674 nm decreased, rapidly diminishing to the background level. Figure 1a shows semilog plots of the fluorescence intensity vs. time for two different gas-phase ozone concentrations. Under all conditions reported here, the excellent fit of the data to single exponential functions indicated first-order kinetics with respect to the chlorophyll. The slopes of such plots give the pseudofirst order rate constants for the reaction; plotting these against the gas-phase ozone concentration over the range ∼1 × 1014 to 1 × 1016 molecules cm-3 yields the result displayed as the points in Figure 1b. The same nonlinear dependence of the chlorophyll loss on gas-phase ozone concentration was obtained for all the aqueous substrates used. The result shown in Figure 1b is suggestive of a surfacemediated reaction mechanism. Indeed, we (4, 24, 30) and others (22, 25) have reported such kinetic behavior for the reactions of ozone with several other reagents present at the 1140

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k IIN surf[O3(g)] B + [O3(g)]

(2)

where k II represents the two-dimensional reaction rate coefficient. The solid line in Figure 1b shows a fit of the data to this general form, taking A ) k IINsurf as a single parameter. As discussed elsewhere (30), the B parameter describes gas-surface partitioning and is different for pure water than for water coated with a monolayer of organic. The value we determine here (4.0 × 1014 molecules cm-3) is very similar to that obtained for the reaction of ozone with PAHs on octanol- or octanoic acid-coated aqueous surfaces, ∼5 × 1014 molecules cm-3 (24, 30). We argued in (30) that differences among the A parameters extracted for ozone surface reactions with various species probably reflect differences in the 2D rate coefficients, more than the maximum number of sites which could be occupied by adsorbed ozone. The A parameter obtained from the fit to the data in Figure 1b is (3.6 ( 0.1) × 10-2, about 10 times larger than those for the reactions with PAHs (30, 38) and comparable to that reported for the reaction with oleate (22). 2. Wetted-Wall Flowtube Experiments. These experiments were carried out using lower ozone concentrations, between 6.2 × 1011 and 2.5 × 1012 molecules cm-3, which are typical of atmospheric amounts. The ozone loss as a function of the gas-solid exposure time was fit to a single exponential loss to obtain an apparent pseudofirst order coefficient (kobs) for the ozone decay. Figure 2a shows the ozone decay on a film of pure water (filled circles), chlorophyll solution at pH 8.6 (hollow circles), and chlorophyll solution in synthetic seawater at pH 8.6 (filled triangles) as a function of the exposure time. It is obvious from this figure that the uptake rate in the presence of chlorophyll is drastically increased compared to that seen with water or synthetic seawater (Figure 2b) alone, indicating that chlorophyll introduces a new reactive pathway (in or on the solution) which enhances the uptake of O3. We are confident that this is not due to reaction of ozone with bromide at the surface (4); under our conditions, such reaction should yield an uptake coefficient of ∼1 × 10-7, compared to the values of ∼1 × 10-6 we observe here (vide infra). The pseudofirst order coefficient derived from the loss rate is related to the ozone uptake coefficient (γ) though the following equation kobs ) γ 〈 c〉/2r

(3)

where r is the radius of the tube and 〈c〉 is the ozone mean thermal velocity, given by (8RT/πM)0.5. Figure 2b shows the uptake coefficients of two different chlorophyll solutions as a function of ozone concentration, as well as that obtained on a 0.5 mM solution of NaBr. The results from both

FIGURE 3. Ozone uptake coefficient as a function of the square root of chlorophyll concentration in salt-free solutions at [O3(g)] near 1 × 1012 molecules cm-3. The dashed line shows a fit to eq 4, indicating a nonphysical, negative intercept in this case. The solid line displays a fit to eq 5.

FIGURE 2. (a) Ozone concentration (from absorbance at 254 nm) vs time of exposure to the chlorophyll solution in the flowtube for three conditions: solid circles, no chlorophyll; hollow circles, chlorophyll solution at pH 8.6; filled triangles, chlorophyll solution in synthetic seawater at pH 8.6. The initial ozone concentration is ∼1 × 1012 molecules cm-3 in all cases. (b) Uptake coefficient for ozone onto chlorophyll solutions, as a function of ozone concentration in the flowtube. The hollow circles display results on salt-free solutions and the filled circles show the uptake on synthetic seawater solutions. The squares represent the uptake coefficient on 0.5 mM NaBr solutions. The dashed line represents our detection limit i.e., 1 × 10-7; all uptake coefficients on pure water lie below this detection limit. chlorophyll solutions show an increase in γ as the ozone concentration is lowered. The result from the bromide solution shows that the uptake is barely above our detection limit in the absence of chlorophyll. The uptake coefficient of ozone onto both aqueous solutions of chlorophyll increases significantly (by a factor of 2–4) with decreasing ozone concentration below about 1.2 × 1012 molecules cm-3. This dependence of the uptake coefficient on ozone concentration was also reported for the reaction of ozone with benzo[a]pyrene on soot surfaces (39) and implies a surface-mediated reaction mechanism. The way in which the uptake coefficient depends upon the bulk chlorophyll concentration is also suggestive of a surface reaction (or a surface component to the reaction). Under conditions where liquid phase reaction drives uptake from the gas phase, the uptake coefficient is expected to be of the form (40) γ)

4HO3RT√krxn[chl]DO3 〈c〉

(4)

where HO3 is the Henry’s law constant for ozone, R is the gas constant, DO3 gives ozone’s aqueous phase diffusion coefficient, T is the temperature, and [chl] represents the bulk chlorophyll concentration. Figure 3 shows a plot of the uptake coefficient versus the square root of the aqueous

chlorophyll concentration, as suggested by eq 4. The dashed line shows clearly that the uptake coefficients indeed exhibit a square root dependence on chlorophyll concentration over a certain concentration range, but do not describe the behavior of the uptake at very low concentrations, when γ must go to a small positive value. From the linear portion of the plot, one can estimate the aqueous phase second order rate coefficient for the reaction between ozone and chlorophyll to be ∼6 × 107 M-1 s-1 at 293 K using eq 4 with H ) 1.13 × 10-2 M atm-1 and D ) 1.176 × 10-5 cm2 s-1 (41). However, the fact that eq 4 does not provide a satisfactory description of the kinetics at low chlorophyll concentrations suggests that the bulk-phase reaction is not the exclusive loss process for ozone. Given the results presented above, it is reasonable to postulate a surface reaction occurring as well as that in the bulk. The behavior shown in Figure 3 has been noted previously when there is a competition between bulk and surface reaction (42–44). Hu et al. (44) showed that in systems where there is a surface component to the reactive uptake, there is a transition between a surface controlled regime (in this case at low chlorophyll concentration) and a bulk regime (at high chlorophyll concentration). In such a case, the uptake depends linearly on the reactant concentration (surface reaction) and then switches to bulk control, as indicated by eq 5 γobs )

4HRT√kbulkD 〈c〉

+ γsurf

(5)

where the surface reaction between chlorophyll and ozone is represented using γsurf. At low chlorophyll concentrations, its surface concentration is expected to be a linear function of the bulk concentration. The solid line in Figure 3 shows a fit to the data assuming a chlorophyll concentration dependence which is both linear and square root, as suggested by eq 5. This function shows a better fit than that given by the dashed line, consistent with a surface component to the uptake of ozone by solutions containing chlorophyll. Figure 2 displays a very small enhancement in the uptake coefficient for synthetic seawater solutions, at ozone concentrations below about 1 × 1012 molecules cm-3. This enhancement is not apparent from results at higher concentrations in the flowtube experiment, or at any concentrations (all higher than 1 × 1012 molecules cm-3) in the surface experiments. Because reaction with bromide is not responsible for the increased ozone loss rate, we postulate that the surface concentration of chlorophyll is higher in the synthetic seawater solutions due to a salting-out effect (45), thereby increasing the reaction rate there. The rate enhancement would be most pronounced at the lower ozone concentraVOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Ozone uptake coefficients as a function of [O3(g)] derived from the surface fluorescence experiments (solid circles) and flowtube experiments (hollow circles). The break in the X-axis separates low (fewer than 1 × 1012 molecules cm-3) and high (more than 1 × 1014 molecules cm-3) ozone concentrations. tions, because that is where the surface reaction is relatively more important. Another possibility is that the presence of chlorophyll somehow increases the bromide concentration at the surface, although there is no experimental or theoretical evidence for this effect (46).

Discussion The work described here represents the first (to our knowledge) joint determination of heterogeneous reaction kinetics made by monitoring both the surface and the gas-phase reagent concentrations. In both experiments reported here, a range of ozone concentrations was used, but there was a reproducible trend toward lower ozone loss rates with higher ozone concentration in the WWFT experiments, and saturation in the loss rate of chlorophyll at higher ozone concentrations in the surface fluorescence experiments. These observations are consistent with the reaction following a Langmuir–Hinshelwood mechanism (39). To make a direct comparison of the two separate sets of results, we transformed the kinetic data shown in Figure 1b to uptake coefficients, in a similar manner to that reported previously (29, 39). Here, rather than calculating the uptake coefficient as the fraction which yields chlorophyll loss of the total collision rate of ozone with the exposed surface, the relevant collision rate is taken to be that with a single chlorophyll molecule, having an estimated collision cross-section of 60 A2 on the basis of its structure. Figure 4 displays the resulting curve of γ vs [O3(g)], as well as the results from the flowtube experiments determined at low ozone concentrations. The two experiments yield essentially identical values of the uptake parameter under atmospherically relevant ozone concentrations: γ ≈ 2-5 × 10-6 for ∼1 × 1012 molecules cm-3 of ozone. Interestingly, this value is roughly comparable to that for the heterogeneous ozonation of oleate at the water surface, which is reported to be about 1 × 10-5 (22), though it is about 10 times larger than the value for ozone + anthracene at that interface (29). However, both of these values may be contrasted with values near 1 × 10-3 for ozone reactive uptake by aerosol particles composed of neat unsaturated fatty acids (47), similar values for the reactive uptake of NO3 by films of saturated hydrocarbons (48) and values of 0.1–1 for uptake of OH by oleate-coated aqueous particles (22). However, most of these experiments were performed with high organic concentration, or pure organic substrates, which may accelerate the uptake and partly explain the differences. Understanding aqueous surface processes is important in order to correctly describe ozone deposition to the ocean surface. Chang et al. (8) investigated the impact of ozone 1142

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surface reactivity on its dry deposition rate to the surface, based on reaction rates of species known to react with ozone in seawater. They concluded that reaction with iodide was the most important process driving the chemical deposition of ozone at low wind speeds, with a high variability which depends on the iodide concentration at the surface (20–400 nM). It is therefore important to compare our findings with the uptake of ozone on iodide solutions. Magi et al. (49) reported uptake coefficients for that case ranging from 0.0037 to 0.0116 for iodide concentrations ranging from 0.5 to 3 M. Extrapolating such uptake coefficients (using eq 4) to oceanic iodide concentrations yields an uptake coefficient of ca. 4 × 10-6, i.e., on the same order of magnitude as those reported here on chlorophyll solutions. Therefore our study highlights that in addition to the chemical species previously thought to provide a reactive sink for ozone at the ocean surface, chlorophyll may also be a key player, adding a very large variability to the deposition process. Oceanic chlorophyll concentrations are reported to be highly variable, of the order of a few micromolar up to 10 µM (50); in this case, the associated uptake coefficient might be as high as 1 × 10-5 and the ozone reaction with chlorophyll would represent the most important driving force for O3 deposition. Using these uptake coefficients, it becomes possible to estimate the ozone deposition velocity on ocean surfaces, which is a complex process involving gas-phase (possibly turbulent) mass transport and molecular process close to the interface. The deposition velocity (νd in m s-1) strongly depends on wind speed but levels off at low wind speed, reaching a constant value that is governed by chemical processes at the air-water interface. It is therefore at low wind speed that the effect of the ozone + chlorophyll reaction can be best highlighted. Indeed, at low wind speeds νd can be expressed as νd )

√krxn[chl]DO

3

HO3RT

1 ) γ 〈 c〉 4

(6)

Using this expression, values of the deposition velocity ranging from ∼1 × 10-5 to ∼1 × 10-3 m s-1 can be estimated, the range being determined by the estimated range of chlorophyll concentrations. The reaction between ozone and chlorophyll significantly increases the deposition velocity by up to a factor of 3 compared to estimates solely based on the reaction with iodide (8) and may therefore be a significant ozone sink, enhancing vd up to wind speeds of about 20 m s-1. Clearly, such chemical processes need to be considered further in modeling the ozone budget close to oceans. Of course, the real impact depends on the actual physical state of chlorophyll (being associated or not with phytoplankton, being free at the ocean’s surface, etc.) but nevertheless we have demonstrated that reaction with organic compounds may be extremely important for assessing the ozone deposition and variability at the molecular level.

Acknowledgments The surface fluorescence experiments in Toronto were funded by NSERC (Canada). C.G. acknowledges the French Research Ministry for a postdoctoral fellowship provided to M.B.

Supporting Information Available Table S1 and Figures S1 and S2 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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