Article pubs.acs.org/est
Production of Gas Phase NO2 and Halogens from the Photochemical Oxidation of Aqueous Mixtures of Sea Salt and Nitrate Ions at Room Temperature Nicole K. Richards and Barbara J. Finlayson-Pitts* Department of Chemistry, University of California, Irvine, California 92697-2025, United States S Supporting Information *
ABSTRACT: Nitrate and halide ions coexist in a number of environmental systems, including sea salt particles, the Arctic snowpack, and alkaline dry lakes. However, little is known about potential synergisms between halide and nitrate ions. The effect of sea salt on NO3− photochemistry at 311 nm was investigated at 298 K using thin films of deliquesced NaNO3-synthetic sea salt mixtures. Gas phase NO2, NO, and halogen products were measured as a function of photolysis time using NOy chemiluminescence and atmospheric pressure ionization mass spectrometry (API-MS). The production of NO2 increases with the halide-to-nitrate ratio, and is similar to that for mixtures of NaCl with NaNO3. Gas phase halogen production also increased with the halide-to-nitrate ratio, consistent with NO3− photolysis yielding OH which oxidizes halide ions in the film. Yields of gas phase halogens and NO were strongly dependent on the acidity of the solution, while that of NO2 was not. An additional halogen formation mechanism in the dark involving molecular HNO3 is proposed that may be important in other systems such as reactions on surfaces. These studies show that the yield of Br2 relative to NO2 during photolysis of halide-nitrate mixtures could be as high as 35% under some atmospheric conditions.
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INTRODUCTION Approximately 70% of the Earth’s surface is covered by oceans, making the exchange of marine aerosols and trace gases between the marine boundary layer (MBL) and the troposphere potentially important for atmospheric processes.1,2 Sea salt aerosol (SSA) is generated from wave action3−8 and is a large source of halide ions in the atmosphere. The dominant halide ion in seawater is chloride, with smaller amounts of bromide ([Cl−]/ [Br−] ∼660:1), and even smaller amounts of iodide ([Cl−]/[I−] ∼106:1).9−11 However, sea salt particles often do not have the same chemical composition as their oceanic source12,13 because of reactions during transport in air. These processes are especially significant in polluted regions where higher levels of oxidized nitrogen species such as N2O5, NO2 and HNO3 react to form halogen gases,1,2,14−19 leaving nitrate ions in the particles20−23 and changing the Cl−/Br− ratio.13 Mixtures of SSA and nitrate ions are also found on the Arctic and Antarctic snowpacks because of the deposition of oxides of nitrogen generated at lower latitudes.24,25 Nitrate ion concentrations in such mixtures can be quite high in SSA, ∼100−300 mM26 and on the order of μM on the snowpack.27 The [Cl−]/[Br−] ratio is often lower than that in fresh sea salt aerosols, as small as ∼100:1 around the Dead Sea28 and 50:1 (averaged from December to February)200:1 (averaged from March to May) in freshly fallen snow in the Arctic.29 Freezing of seawater will also lead to enhanced bromide ion concentrations compared to chloride ions; for example, Koop et al. calculated the concentration of chloride and bromide ions at © 2012 American Chemical Society
240 K, and found that chloride ions will be concentrated during freezing by a factor of 11 but bromide ions by a factor of 38.30 Nitrate photolysis is known to be a major source of OH and NOy in the condensed phase including on the Arctic snowpack31−39 and in laboratory studies of frozen nitrate solutions (reactions 1a and 1b from refs 40−46; reaction 2 from ref 47): NO3− + hν( 99.4%), Mg(NO3)2·6H2O (Sigma Aldrich, 98.0%), NaBr (Fluka, >99.5%), and synthetic sea salt (Instant Ocean) were made using Milli-Q water (18.2 MΩ cm, pH 5.5). Typical concentrations of Cl− and Br− in natural seawater are 0.55 M and 8.1 × 10−4 M respectively. This can be reproduced by dissolving 35.17 g of synthetic sea salt (SSS), which contains 19.2 g of Cl− (0.55 mols) and 0.06 g of Br− (8.1 × 10−4 mols), in 1 L of water. The amount of SSS was varied so that the total halide ion concentration, [Cl− + Br−], ranged from 0.083 to 2 M, whereas the ratio of chloride to bromide ions remained at 660. A series of experiments were also carried out using SSS with added NaBr to obtain ratios of [Cl−]/[Br−] as small as 5 while maintaining a total concentration of 2 M. For solutions that were more concentrated in bromide ions, the amount of SSS was decreased so that the combined [Cl− + Br−] concentration was 2 M in the nebulizer. The nitrate ion concentration in the nebulizing solution was held constant for all experiments at 0.25 M and the chloride and bromide ion concentrations and mole fractions were varied by adding the appropriate amounts of SSS and NaBr. Experiments were also conducted with added HNO3 (ACS, >70%, 15.7 M) or NaOH (Fisher, Certified, 1 M) to alter the initial pH of the aqueous thin film. The Pitzer molality-based model101,102 was used to calculate the change in [Cl−]/[Br−] due to HCl, HNO3 and HBr degassing at low pH prior to photolysis. The net loss of Cl−, NO3−, and Br− as function of pH was determined to be insignificant (≤0.3%), and the [Cl−]/[Br−] ratio remained at 188. The total halide mole fraction, χhalide, was used throughout to define the relative concentrations of available chloride and bromide anions compared to nitrate:
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EXPERIMENTAL SECTION Photolysis experiments were carried out in 230 L Teflon (51 μm FEP) reaction chambers. The method for coating the Teflon reaction chambers with salt solutions is described in detail elsewhere.67 The coated chambers were filled with synthetic air (NOx < 0.001 ppm, SO2 < 0.001 ppm, Scott-Marrin Inc., Riverside, CA) that had flowed through a water bubbler to give a final relative humidity of 75−78% measured using a relative humidity−temperature probe (Vaisala, HMP 338). Irradiation of the thin salt film on the walls of the Teflon chamber was carried out using 14 externally mounted narrowband UVB lamps (λmax ∼ 311 nm) whose output overlaps with the nitrate n →π* absorption band. All experiments were conducted at 298 ± 2 K. Gas phase products were measured as a function of time using (a) a nitrogen oxides (NOy) analyzer with chemiluminescence detection (Thermo Electron Corp., model 42 C) for NO2 and NO and (b) a dual quadrupole mass spectrometer with atmospheric pressure ionization (API-MS) (Perkin-Elmer Sciex, API-300) operating in negative ion mode for detecting Br2, Cl2, I2, BrCl, and IBr. The signal intensities of selected parent-daughter ion pairs were measured at m/z 158/79 and 160/81 for Br2, 70/35 and 72/35 for Cl2, 254/127 for I2, 114/79 and 114/35 for BrCl, and 206/127 and 208/81 for IBr. Calibration of the nitrogen oxides analyzer was performed using known mixtures of NO2 (4.57 NO2 ppmv in O2 free N2, ScottMarrin Inc.) or NO (101.3 ppmv of NO in O2 free N2, NO2 < 0.5 ppmv, Scott Marrin Inc.) in N2 (Oxygen Service Co., UHP, 99.999%) at levels similar to those detected during photolysis. Long path (64 m) FTIR (Mattson Research Series) was used to probe for HONO formation; gas phase HONO could cause artificially high NO2 estimates using the NOy analyzer since it
χhalide =
[Cl−] + [Br −] [Cl ] + [Br −] + [NO−3 ] −
(eq 1)
Errors in concentrations are reported as 2s, where s is the sample standard deviation defined as N
s=
∑i = 1 (xi − x ̅ )2 N−1
(eq 2)
where N is the number of samples and was 3−5 depending on the measurement.103 10448
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Figure 1. (a) NO2 production and (b) Br2 production as a function of time during photolysis. Symbols correspond to different molar ratios of total halides for mixtures of SSS/NaNO3 having [Cl−]/[Br−] = 188. Experimental conditions are 75−78% RH, 298 K in air, constant initial NO3−, and pH 5.5. Open squares are for pure NaNO3. Error bars are 2s of replicate experiments.
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RESULTS AND DISCUSSION Figure 1 shows that both gaseous NO2 and Br2 increased as a function of photolysis time and with χhalide for constant initial nitrate and a fixed initial [Cl−]/[Br−] ratio of 188 at pH 5.5. There was an induction time for Br2 production that will be shown later to depend on the pH of the aqueous film. Small amounts of BrCl, expected as an intermediate in mixed chloride-bromide systems,66,69,104,105 were also measured at concentrations that were consistently 6% of Br2. This has been observed in other lab studies where dihalogen anion (e.g., BrCl−) precursors to the mixed halogen gases have also been observed upon photolysis of seawater.69 Mass spectra were examined to determine if other halogen-containing species such as BrNO, BrNO2, and BrONO2 were formed by comparing the ratio of m/z 81 to m/z 160. A ratio of m/z 81 to m/z 160 that is higher than 0.5 would indicate such species are also formed. However there was no appreciable difference from authentic Br2 mass spectra, confirming that Br2 is the major gas phase halogen product. Even though Cl− is the most concentrated halide ion in sea salt, Cl2 was not detected nor were other chlorine-containing species (e.g., ClNO, ClNO 2 , and ClONO2), in agreement with a number of previous studies of similar systems. For example, George and Anastasio65 did not detect chlorine products when they measured gaseous halogen compounds from the photolysis of aqueous solutions of chloride, bromide, and nitrate ions. Frinak and Abbatt56 studied the reaction of gas phase OH with aqueous solutions of NaCl/NaBr at 269 K and observed Cl2 only when bromide ions had been depleted from the solution. Figure 2 summarizes the rates of NO2 formation as a function of χhalide for two different [Cl−]/[Br−] ratios, 660 and 188, reflecting those in unreacted sea salt particles9−11 and in polar regions,29,30 respectively. Also shown for comparison are data for mixtures of NaCl with NaNO3 published earlier.98 The rate of gas phase NO2 production increased with increasing χhalide, as observed earlier for simple NaCl/NaNO3 and NaBr/ NaNO3 binary mixtures, consistent with enhanced photochemistry at interfaces.67,98 The rates of NO2 formation for the nitrate mixtures with synthetic sea salt are within experimental error of those for NaCl/NaNO3, and are independent of the [Cl−]/[Br−] ratio. The photochemical rate of NO2 production from nitrate ion photolysis is the same in the complex synthetic sea salt mixture as in simple binary NaCl/NaNO3 mixtures. This confirms that the additional cations and anions in sea salt
Figure 2. Rates of production of NO2 from mixtures of SSS with added nitrate and bromide ions as function of χhalide for [Cl−]/[Br−] = 188 (orange), 660 (yellow), and published results by Wingen et al.98 on NaCl/NaNO3 (blue) normalized to the same nitrate concentration in the nebulizing solution as used here. Experimental conditions are 75−78% RH, 298 K in air, constant initial NO3− concentration, and pH 5.5. The open bar is for pure NaNO3. All error bars are 2s of replicate experiments.
do not significantly alter nitrate ion photolysis product yields, at least at the concentrations found in seawater (see SI for additional discussion). The rates of Br2 formation (RBr2) obtained from the time dependence of Br2 after the induction periods are plotted in Figure 3. Also shown in Figure 3 is the ratio of the rate of Br2 formation to that of NO2 (RNO2). This ratio, RBr2/RNO2, provides the stoichiometry for the formation of Br2 relative to NO2, and increases from 0.03 to 0.09 as the mole fraction of added halide ions increases. Since the ratio of Cl− to Br− remains constant in these experiments, the relative contributions of the OH + Br− and OH + Cl− reactions to the chemical processes in the film do not change. The increase in Br2 relative to NO2 suggests that under these conditions, halide ions increasingly compete with NO2 and NO2− to trap the OH,48 and/or possibly the O−.67,106,107 Earlier work67 suggested that there was significant clustering of bromide with nitrate ions, which could lead to Br− trapping some of the O− as it was generated, in competition with the O−−H2O reaction. George and Anastasio65 showed that when aqueous solutions of 3.5 M NaCl, 0.1 M NaNO3 and varying amounts of NaBr were irradiated, total gaseous bromine products reached a 10449
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from 4 to 9 of the production of OH, which is generated from the same primary photolysis step as NO2. The stoichiometry for Br2 relative to NO2 given by RBr2/RNO2 ranges from 0.06 at an initial pH of 7.5 to 1.4 at a pH of 0.5. If reactions 1−3 are responsible for generating the OH that oxidizes Br− based on the chemistry summarized in Table S1 of the SI, a maximum value of RBr2/RNO2 = 0.5 is expected. However, RBr2/RNO2 at high acidity is clearly much larger, suggesting additional mechanisms of oxidation of bromide ions must come into play. Under most experimental conditions, the major gas phase NOx product was NO2, with much smaller amounts (about a factor of 14 less) of NO. However, as the pH was lowered, increasing amounts of NO were also generated (Figure 4c). This observation, combined with the unexpectedly high yield of Br2, suggests that the two may be mechanistically linked. This is supported by the fact that photolysis of thin films of NaNO3 alone acidified to a pH of 0.5 with HNO3 showed no increase in NO production. It is noteworthy that the rates of Br2 and NO production increased by 2.5 ± 0.2 ppb min−1 and 2.0 ± 0.3 ppb min−1 respectively, when the pH dropped from 7.5 to 0.5. If the production of NO and excess Br2 are linked, it is in a manner that yields approximately one NO for each Br2. Nitric acid is known to oxidize bromide ions in the dark. Indeed, thin films of SSS/NaNO3 in the dark at pH 0.5 after several days produced 6 ppb Br2 and 15 ppb NO. The overall stoichiometry for the oxidation of Br− by HNO3109 is given by
Figure 3. Rates of production of Br2 (left axis, black squares) and RBr2/RNO2 (right axis, red triangles) as a function of χhalide for a ratio [Cl−]/[Br−] = 188. Experimental conditions are: 75−78% RH, 298 K in air, constant initial NO3−, and pH 5.5. All error bars are 2s of replicate experiments.
plateau above 2 mM Br−. They estimated that the ratio of product bromine (as Br) to OH formed in the photolysis was 0.25 at χhalide = 0.97. Converting our data to bromine atoms rather than Br2 gives RBr/RNO2 ∼ 0.2 at χhalide = 0.9, similar to their measurements.65 Figure 4a,b shows that decreasing the initial pH eliminated the induction time and increased the rate of production of gaseous Br2, with no significant change in the rate of NO2 production (Figure 4c). The rate of BrCl production also increased as a function of increasing acidity, with its yield relative to Br2 remaining constant at 6%. This change in Br2 and BrCl with pH is expected based on known oxidation reactions for chloride and bromide in solution.56,65,66,104,105,108 Small amounts of gaseous IBr were also detected at pH 0.5 and 2.5, consistent with Abbatt et al.66 who observed IBr from irradiation of frozen halide-nitrate mixtures at pH 2.5. Chlorine production was not observed even at pH 0.5 where formation would be expected to be promoted by H+ (see Table S1 of the SI). These results are in agreement with similar studies by Frinak and Abbatt56 in which Cl2 production was not measured at pH 0.5 for [Cl−]/[Br−] = 1 × 101 − 1 × 104. The lack of dependence of NO2 on pH is in agreement with the experiments of Zellner et al.50 on bulk room temperature photolysis of nitrate ions; they observed no dependence on pH
HNO3 + 2Br − + 2H+ → Br2 + HONO + H 2O
(4)
Nitrous acid further oxidizes Br−,109,110 so the thermal reaction in solution is autocatalytic. The individual steps in reaction 4 are not well understood, but if the initial interaction of HNO3 with Br− proceeds via attack at a terminal oxygen atom in HNO3, the likely initial products would be BrONO + OH− (or H2O since the solution is acidic). In a strongly acidic solution, BrONO may react further, for example, to form NO+ and HOBr which ultimately forms Br2. Another fate may be photolysis. Gaseous BrONO has a large absorption cross section111 at the wavelengths (∼311 nm) used here. Photolysis would be expected to generate NO + BrO, with further reactions of BrO involving Br− generating Br2. Such a mechanism would generate equal amounts of Br2 and NO without changing the NO2, consistent with the experimental observations.
Figure 4. (a) Br2 as a function of time for pH from 7.5 to 0.5; (b) rate of Br2 formation after the induction period (left axis, black squares) and RBr2/ RNO2 (right axis, red triangles); and (c) NO2 (black squares) and NO (blue diamonds) production as a function of initial pH of the nebulizing solution for [Cl−]/[Br−] = 188 at χhalide = 0.9. Experimental conditions are: 75−78% RH, 298 K in air, and constant initial NO3−. All error bars are 2s of replicate experiments. 10450
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Arctic conditions where the ratio is 50:1,29 it could be as high as 35% of the NO2 if the chemistry observed here also applies to the snowpack Our experiments also show evidence for a photochemically enhanced production of Br2 and NO via a nitric acid initiated oxidation. Given that molecular HNO3 may exist on some atmospheric surfaces113−116 and at the air−water interface,117−122 a contribution to Br2 formation from this pathway should also be considered.
Studies of the production of NO2 and Br2 as a function of the ratio of chloride to bromide were also carried out over a range of [Cl−]/[Br−] from 5 to 660. While the rate of NO2 formation remained constant at 1.75 ± 0.32 ppb min−1 under all conditions, the rate for Br2 production changed dramatically. Figure 5 shows the corresponding rates of Br2 formation as well
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ASSOCIATED CONTENT
S Supporting Information *
A table containing a list of aqueous phase chemical reactions of halogen species, including rate constants; Figure S1 compares the rate of NO2 production from the photolysis of thin films of SSS/NaNO3, SSS/KNO3, and SSS/Mg(NO3)2. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
Figure 5. Br2 production (left axis, black squares) and RBr2/RNO2 (right axis, red triangles) as function of [Cl−]/[Br−] at constant χhalide = 0.9. Experimental conditions are: 75−78% RH, 298 K in air, constant initial NO3− and pH 5.5. Error bars are 2s of replicate experiments.
*Phone: 949-824-7670; fax: 949-824-2420; e-mail: bjfinlay@ uci.edu. Notes
The authors declare no competing financial interest.
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as RBr2/RNO2 as a function of [Cl−]/[Br−] at a fixed total mole fraction of χhalide = 0.9. Given the rate constants for the OH reaction with Cl− and Br−, 4.3 × 109 and 1.1 × 1010 M−1 s−1, respectively (see SI), OH is initially trapped primarily by Cl− to form HOCl− even at the smallest ratio of [Cl−]/[Br−] = 5, whereas at a [Cl−]/[Br−] = 200, the chloride reaction is favored by a factor of ∼80 over that with Br− (this assumes the bulk phase rate constants apply throughout the system, including at the interface). While the yield of Br2 drops off rapidly as the relative amount of bromide ion decreases, the rate of Br2 production is still 0.03 ± 0.02 ppb min−1 at halide ratios characteristic of fresh sea salt aerosols. This is in part due to the fact that the HOCl− decomposes rapidly back to reactants (see Table 1 in SI) and reacts only slowly with Cl−. The reaction of HOCl− with H+ leads to Cl atoms and subsequently Cl2−, which in the presence of bromide ions leads to Br2 (see Table 1 in SI) via well-known interhalogen chemistry.64,69,100,104,112 The stoichiometry at smaller values of [Cl−]/[Br−] (i.e., relatively more Br−), where the trapping of OH and the chlorine intermediates by Br− is efficient, approaching 0.5 Br2 per NO2. As discussed earlier, this is the maximum expected if every OH generated from NO3− photolysis ultimately oxidizes Br− to form Br2. As the [Cl−]/[Br−] ratio increases, the yield of Br2 relative to NO2 decreases. In this case, OH is almost always trapped by reaction with Cl− to form the adduct HOCl−. The source of Br2 is then expected to be from the trapping of chlorine intermediates by Br− (see Table 1, SI). Under atmospheric conditions, the generation of NO2 via photolysis of NO3− could be increased by a factor of 2 or more in the presence of halide ions from sea salt. We have shown here that the complex mixture in sea salt has the same efficiency in enhancing NO2 production as mixtures of chloride ions with nitrate. Thus in terms of NO2 production, NaCl/NaNO3 mixtures are reasonable mimics for sea salt. However, this does not capture the production of Br2, which is highly dependent on the [Cl−]/[Br−] ratio as well as pH. Under conditions representative of the Dead Sea, for example where [Cl−]/[Br−] ∼ 100:1,28 Br2 could be generated by the chemistry observed here in about 15% yield compared to the NO2 formed, and under some
ACKNOWLEDGMENTS This research was supported by the National Science Foundation (Grant Nos. 0909227 and 0836735). We thank Lisa M. Wingen and Paul M. Nissenson for insightful conversations, and James N. Pitts Jr., Karen M. Callahan, Theresa M. McIntire, and Veronique Perraud for helpful comments on the manuscript. We are grateful to Simon Clegg for providing the model to calculate the effect of pH on the initial ratio of Cl− to Br− in the particles.
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