Hydroperoxyl radical (HO 2

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL 30, NO 24, 2297, doi:101029/2003gl018572, 2003 Hydroperoxyl radical (HO 2 ) oxidizes dibromide radical anion ( 2 )to bromine (Br 2 ) in aqueous solution: Implications for the formation of Br 2 in the marine boundary layer Brendan M Matthew, 1 Ingrid George, and Cort Anastasio Atmospheric Science Program, Department of Land, Air, and Water Resources, University of California - Davis, Davis, California, USA Received 6 September 2003; revised 4 November 2003; accepted 20 November 2003; published 31 December 2003 [1] The release of photoactive halogen species such as Br 2 from sea-salt particles, snowpack, and sea-ice can have significant effects on chemistry in the marine boundary layer (MBL) Although the reaction of hydroperoxyl radical (HO 2 ) with dibromide radical anion ( Br 2 ) might be a key step in the formation and release of Br 2, there is currently no consensus on whether this reaction produces bromide (Br ) or molecular bromine (Br 2 ) To address this question, we measured the formation of gaseous and aqueous oxidized bromide (primarily Br 2 ) in illuminated bromide solutions as a function of ph Results from these two sets of experimental data are best explained by kinetic models where HO 2 oxidizes Br 2 to Br 2 Using this reaction in a simple aerosol model reveals that the hydroxyl radicalinduced oxidation of particulate bromide, followed by reaction of Br 2 with HO 2, could be an important source of Br 2 in the MBL INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0317 Atmospheric Composition and Structure: Chemical kinetic and photochemical properties; 0365 Atmospheric Composition and Structure: Troposphere composition and chemistry; 4801 Oceanography: Biological and Chemical: Aerosols (0305); 4851 Oceanography: Biological and Chemical: Oxidation/reduction reactions; KEYWORDS: halogen chemistry, oxidation reactions, reactive halogens, sea-salt particles Citation: Matthew, B M, I George, and C Anastasio, Hydroperoxyl radical (HO 2 ) oxidizes dibromide radical anion ( 2 ) to bromine (Br 2 ) in aqueous solution: Implications for the formation of Br 2 in the marine boundary layer, Geophys Res Lett, 30(24), 2297, doi:101029/2003gl018572, Introduction [2] A growing body of evidence indicates that reactive halogen species, such as Br 2, BrCl, and their photoproduced radicals, can play important roles in the chemistry of the MBL In the Arctic, the depletion of tropospheric ozone and hydrocarbons has been linked to reactions involving bromine and chlorine radicals [Foster et al, 2001; Michalowski et al, 2000] In the mid-latitude MBL there is also evidence of halogen radicals [Platt and Hönninger, 2003] and the halogen-associated destruction of O 3 and DMS [Galbally et al, 2000; von Glasow et al, 2002] [3] Based on current models, the most important mechanisms for the formation of Br 2 and BrCl in the MBL appear to be autocatalytic heterogeneous reactions that involve: (i) reaction of gaseous HOBr with Br or Cl in the condensed phase (eg, sea-salt particle) to form Br 2 and/ or BrCl, which then evaporate, (ii) photolysis of gaseous Br 2 and BrCl to yield Br, and (iii) reaction of Br with ozone to eventually form gaseous HOBr [Vogt et al, 1996] Other mechanisms for reactive halogen release have also been suggested, but the relative importance of these pathways is unclear One proposed pathway for the formation of Br 2 is the oxidation of Br by hydroxyl radical ( OH) [Mozurkewich, 1995] As shown in Figure 1, Br 2 is a key intermediate formed from this reaction The subsequent fate of 2 (to produce either Br 2 or Br ) likely plays a significant role in reactive halogen release While superoxide anion ( O 2 ) is known to reduce 2 to Br [Wagner and Strehlow, 1987], there is no consensus on whether HO 2 oxidizes 2 to Br 2 (R1) or reduces it to Br (R2): 2 þ HO 2 :! Br2 þ HO 2 R1 2 þ HO 2 :! 2Br þ O 2 þ H þ R2 [4] As shown in Table 1, the most recent version of the NDRL/NIST Solution Kinetics Database [Ross et al, 1998] lists results from four studies of the reaction of HO 2 with 2 Although the authors of the first three studies reported Br 2 as the product (ie, R1), the NIST database lists Br as the product from these studies (ie, R2) (Table 1) The final listing in the database reflects the results as reported in the fourth study, ie, that both R1 and R2 occur simultaneously (Table 1) [5] Past models that have investigated halogen chemistry in the MBL have either used Br as the product of the reaction of HO 2 with 2 [ie, R2; Herrmann et al, 2000; Sander and Crutzen, 1996] or have included both Br and Br 2 as products, with the Wagner and Strehlow [1987] rate constants reported in Table 1 [ie, R1 and R2; von Glasow et al, 2002] Models that have not included the reaction of HO 2 with 2 [eg, Behnke et al, 1999] have included the 2 self-reaction as a source of Br 2 : 2 þ 2! Br 2 þ 2Br R3 1 Currently at NOAA Aeronomy Laboratory, Boulder, CO and CIRES, University of Colorado, Boulder, CO, USA, Copyright 2003 by the American Geophysical Union /03/2003GL Although Mozurkewich [1995] has examined Br 2 release from sea-salt particles using reaction R1, no other published models allow HO 2 to oxidize 2 to Br 2 with a rate constant on the order of 10 9 M 1 s 1 (as reported in the ASC 14-1

2 ASC 14-2 MATTHEW ET AL: HO 2 OXIDIZES 2 TO Br 2 first three papers in Table 1) Thus these current models might be underestimating the release of Br 2 and other reactive halogens [6] To determine whether HO 2 oxidizes or reduces 2, we measured the ph-dependent release of gaseous Br 2, and the formation of aqueous Br 2 and Br, in illuminated aqueous bromide solutions We then used kinetic models to determine which HO reaction (R1 and/or R2) was most consistent with our experimental results Finally, we examined the potential importance of this reaction as a source of Br 2 to the MBL 2 Experimental [7] Bromide solutions were prepared with Aldrich NaBr (9999%); all other reagents were ACS reagent grade or betteromide solutions contained 100 mm NaBr, 10 mm H 2 O 2 (a photochemical source of OH and HO 2 ), and were adjusted to a specific ph using 10 M H 2 SO 4 or 10 mm sodium tetraborate with 030 M NaOH For each experiment, samples in quartz cells were illuminated with 313 nm light while being stirred constantly [8] In the first type of experiment (Br 2 (g) measurements) the solution in the illumination cell was kept at 15 C, purged with zero grade air (07 L min 1 ), and the evolved gaseous oxidized bromide (Br*(g)) was quantitatively trapped on two sodium-carbonate-coated denuders arranged in series A Teflon filter was placed just after the illumination cell to prevent collection of particulate bromide on the denuders At the end of an experiment, each denuder was extracted with Milli-Q water and the bromide concentration was determined using ion chromatography Based on our modeling, Br 2 accounted for essentially all of the Br*(g) collected on the denuders There was no measurable release of Br 2 (g) (<02 nmol hr 1 ) in a control experiment (ph 40) where the sample cell was purged but not illuminated [9] In the second type of experiment (Br*(aq) measurements), we measured concentrations of aqueous oxidized bromine species (Br*(aq) = Br,Br 2 and HOBr) in illuminated bromide solutions The experimental conditions were the same as described above except that the concentration of bromide was 080 mm, 75 mm allyl alcohol was added to each solution, the temperature was 20 C, and the cell was not purged The formation of Br*(aq) was followed using a Table 1 Reported Products and Rate Constants (k) From Past Studies of the Reaction of 2 with HO 2 NIST Database a Reported Products Original Paper k (M 1 s 1 ) Br Br b Br Br c Br Br d Br 2 +Br Br 2 +Br , e Ref a References: (a) [Ross et al, 1998]; (b) [Rafi and Sutton, 1965]; (c) [Sutton et al, 1965]; (d) [Čerček et al, 1965]; (e) [Wagner and Strehlow, 1987]: authors reported that both Br 2 and Br are formed, with rate constants of and M 1 s 1, respectively chemical probe technique based on the reaction of Br*(aq) with allyl alcohol (AA) to produce 3-bromo-1,2-propanediol (3BPD) [BM Matthew and C Anastasio, manuscript in preparation]: Br* ðaqþ þaa! 3BPD R4 During each experiment, aliquots of illuminated solution were removed at known times and analyzed for 3BPD using gas chromatography [Matthew and Anastasio, 2000] [10] Fits to the experimental data were made using two models 1 written in Acuchem [Braun et al, 1988] The Br*(aq) model consisted of approximately 100 reactions and was used to estimate the rate constant for the reaction of Br 2 with HO 2 (kr1 ) by fitting the model output to the Br*(aq) experimental data All other rate constants in the model were from the literature or were determined experimentally The Br 2 (g) model was identical to the Br*(aq) model, except it had no AA reactions and it included the evaporation of volatile species such as Br 2 to the gas phase 3 Results and Discussion [11] As shown in Figure 2, the rate of Br 2 (g) collection greatly increased with decreasing solution ph The best model fit to these data is obtained when we allow HO 2 to oxidize 2 to Br 2 (R1) with a rate constant of M 1 s 1, ie, when we treat the reaction as reported in the first three papers listed in Table 1 In this best-fit model 997% of the observed bromide on the denuders is from Br 2 at all ph values, while HOBr is negligible If instead of R1 we use R2 (with k = M 1 s 1 ), the model underpredicts the rate of Br 2 (g) formation by factors of 37 and 14 at ph 30 and ph 55, respectively (Figure 2) The final model tested follows the recommendations of Wagner and Strehlow [1987] by using a combination of R1 and R2 with their reported rate constants (Table 1) In this case, the model underpredicts the rate of Br 2 formation by factors of 12 and 10 at ph 30 and ph 55, respectively [12] Based on our model results, the ph dependence observed in Figure 2 is rooted in the acid-base dependence of O 2 (-I) (ie, HO 2 O 2 +H + ;pk a = 48) At ph < 48, HO 2 is the dominant O2 (-I ) species, the rate of formation of Figure 1 Simplified reaction scheme for the oxidation of Br by OH in aqueous solution 1 Auxiliary material is available at ftp://ftpaguorg/apend/gl/ 2003GL018572

3 MATTHEW ET AL: HO 2 OXIDIZES 2 TO Br 2 ASC 14-3 Figure 2 Rate of collection of gaseous Br 2 as a function of ph in an illuminated (313 nm) solution containing 100 mm Br and 10 mm H 2 O 2 Open circles represent the experimental data: errors represent estimated ±1s, based on the average relative standard deviation from replicate runs The lines represent model outputs for three different scenarios: R1 (solid line; HO 2 + Br2! Br 2 + HO 2, k R1 = M 1 s 1 ), R2 (dashed line; HO 2 + Br2! 2 Br + O 2 + H +, k R2 = M 1 s 1 ), and a combination of R1 and R2 (dotted line) using rate constants reported by Wagner and Strehlow [1987; see Table 1] Br 2 (aq) by R1 is significant, and the rate of destruction of Br 2 (aq) by O 2 [Ross et al, 1998] is small: : O 2 þ Br 2! 2 þ O 2 R5 At ph values above 48, O 2 is the dominant O 2 (-I) species, the rates of destruction of Br 2 (aq) and 2 by O 2 are larger, and the formation rate of Br 2 (aq) from R1 is smaller Together, these effects lead to the observed increase in Br 2 (g) release with decreasing ph (Figure 2) [13] Results from the Br*(aq) chemical probe technique, along with the three model fits, are shown in Figure 3 As was the case for Br 2 (g), there is a sharp rise in experimental rates of 3-bromopropanediol formation at low ph that matches the acid-base dependence of O 2 (-I) In addition, the best fit to our Br*(aq) experimental data is obtained when HO 2 oxidizes Br 2 to Br 2 (ie, R1) with a rate constant of M 1 s 1 The two other model scenarios are incapable of capturing the observed ph dependence of Br*(aq) (Figure 3), in agreement with the Br 2 (g) results (Figure 2) [14] Thus the kinetic model with reaction R1 accurately describes both the Br 2 (g) and Br*(aq) experimental data We have also attempted to fit the experimental data without using R1 by altering one or more of the known rate constants, but these alternative models were unsuccessful For example, if we replace R1 with the combination R1 + R2 (and rate constants of Wagner and Strehlow [1987]), the resulting model can be made to fit the Br*(aq) experimental data by using a rate constant for the dissociation of 2 ( 2! Br +Br ) that is 6 times lower than the lowest value reported [Sutton et al, 1965] However, this model is incapable of fitting the Br 2 (g) experimental results Furthermore, models containing R2 (instead of R1) were unable to describe either the Br 2 (aq) or Br 2 (g) experimental data [15] Our finding that HO 2 rapidly oxidizes 2 to form Br 2 suggests that past models of sea-salt particle chemistry might underestimate photoactive halogen release from seasalt particles via radical-induced mechanisms To examine the potential importance of R1 for Br 2 release in the MBL, we constructed a simplified sea-salt aerosol model (see auxiliary material) containing sodium bromide particles (80 mm) as a surrogate for sea-salt particles The particle number density (14 cm 3 ), particle size distribution, aerosol liquid water content ( m 3 (aq) m 3 (air)), and mass transport scheme in this model were from Sander and Crutzen [1996] Concentrations of aqueous OH and HO2 were determined by mass transport from the gas phase ( OH(g) = molecules cm 3,HO 2 (g) = molecules cm 3 ) in conjunction with aqueous phase sinks To examine the sensitivity of model results on the OH accommodation coefficient (a OH ), models were run using a OH of 005 [Herrmann et al, 2000] and [Sander and Crutzen, 1996] H 2 O 2 (aq) was assumed to be in Henry s law equilibrium with 10 ppbv of H 2 O 2 (g) Particle ph was varied in the model to examine differences in the chemistry of fresh sea-salt particles (ph 80), aged remote particles (ph 55), and aged polluted particles (ph 30) [16] As shown in Figure 4, the rates of Br 2 release from the particles were strongly dependent upon ph and were highest in the R1 scenario Compared with the combined R1 + R2 scenario, release rates in the R1 scenario (at the same a OH ) were 4 times higher at ph 30, 10 times higher at ph 55, and 40 times higher at ph 80 In all model runs with the reduction scenario (R2), there was no significant release of Br 2 (<10 4 pptv hr 1 ) For both the R1 and R1 + R2 scenarios, the rate of Br 2 release at a given ph was 8 times faster with a OH = 005 compared to results obtained with a OH = Subsequent model runs with the addition of 54 M Cl in the particles indicate that the presence of chloride has, overall, a minor effect on Br 2 release (eg, 10% decrease in the R1 scenario) because of apparent interactions between oxidized chloride species and Figure 3 Rate of 3-bromopropanediol (3BPD) formation as a function of ph in illuminated (313 nm) solutions containing 080 mm Br, 10 mm H 2 O 2 and 75 mm allyl alcohol Open squares represent the experimental data with error bars corresponding to 90% confidence intervals Lines represent model outputs for the same three scenarios described in Figure 2: R1 (solid line), R2 (dashed line) and R1 + R2 (dotted line)

4 ASC 14-4 MATTHEW ET AL: HO 2 OXIDIZES 2 TO Br 2 Figure 4 Modeled rates of Br 2 (g) release from aerosol bromide particles under marine boundary layer conditions The solid lines represent models where the OH accommodation coefficient (a OH ) is 005, and the dashed lines represent models where a OH is The lines labeled R1 and R1 + R2 correspond to the reaction scenarios described in Figure 2 Rates of Br 2 release in the R2 scenario are too small to be seen on this scale (<10 5 pptv hr 1 ) bromide [B M Matthew and C Anastasio, manuscript in preparation, 2003] [17] The results in Figure 4 indicate that modeled rates of photoactive halogen release from sea-salt particles via OH reactions depend strongly upon whether the reaction of HO 2 with 2 produces Br 2 or Br Furthermore, our model rates reveal that the oxidation of sea-salt bromide by OH could be an important source of gaseous photoactive halogens such as Br 2 in the MBL From a previous model of the remote mid-latitude MBL where reactive halogens play significant roles in the oxidation of S(IV) and destruction of ozone [Vogt et al, 1996], we estimate a total Br 2 release rate (primarily from HOBr autocatalytic reactions) of 03 pptv hr 1 at noon Similarly, in a model of the springtime Arctic a Br 2 (g) source strength of 07 pptv hr 1 was required to explain observed BrO levels and account for the destruction of ground-level ozone within three days [Sander et al, 1997] In our model with the R1 scenario and a OH = 005, the oxidation of bromide by OH leads to Br 2 release rates of 02 pptv hr 1 for aged particles (ph 55; Figure 4) Thus, under favorable conditions, the reaction of OH with bromide might contribute significantly to the formation of Br 2 in the MBL Furthermore, production of OH by photochemical reactions within the drop (eg, by nitrate photolysis) should enhance Br 2 release by this OHinitiated mechanism, although this could be offset somewhat due to radical scavenging by particulate organic compounds (Anastasio et al, manuscript in preparation) 4 Conclusions [18] Results from two different types of experiments indicate that HO 2 oxidizes 2 to form Br 2, in agreement with the original reports [Čerček et al, 1965; Rafi and Sutton, 1965; Sutton et al, 1965] In addition, the rate constant derived from our work ( M 1 s 1 ) is close to the average of the three values reported by these authors ( M 1 s 1 ) Thus the lower rate constant previously reported for the oxidation of Br 2 by HO 2 ( M 1 s 1 ; Table 1) appears to be too small by a factor of 50 Our results also indicate that the reduction of 2 by HO 2 (R2) is of minor significance [19] These findings indicate that numerical models should incorporate the reaction of HO 2 with 2 (using Br 2 as the product and a rate constant of M 1 s 1 ) in order to better predict the release of photoactive halogen species (eg, Br 2 and BrCl) from sea-salt particles Models that do not include the aqueous oxidation of Br 2 by HO 2 might underestimate the release of Br 2 to the gas phase and therefore the importance of halogen radical chemistry Our work also demonstrates that the release of Br 2 in the MBL likely depends on the concentration of OH in sea-salt particles Thus a better understanding of a OH and [ OH(aq)] values is needed if the chemistry in these particles is to be correctly modeled Finally, our results indicate that the OHinduced oxidation of Br might be a significant pathway for Br 2 release that complements the known heterogeneous autocatalytic cycle [20] Acknowledgments This work was funded by a NASA Earth Science Fellowship to BMM and by the National Science Foundation, Atmospheric Chemistry Program (ATM ) The authors also thank Roland von Glasow for helpful input and a preprint copy of his manuscript References Behnke, W, M Elend, U Kruger, and C Zetzsch, The influence of NaBr/ NaCl ratio on the Br catalyzed production of halogenated radicals, J Atmos Chem, 34(1), 87 99, 1999 Braun, W, J T Herron, and D K Kahaner, Acuchem: A computer program for modeling complex chemical reaction systems, Int J Chem Kin, 20, 51 62, 1988 Čerček, B, M Ebert, C W Gilbert, and A J Swallow, Pulse radiolysis of aerated aqueous potassium bromide solutions, in International Symposium on Pulse Radiolysis, edited by M Ebert, J P Keene, and A J Swallow, pp 61 81, Academic, Manchester, England, 1965 Foster, K L, R A Plastridge, J W Bottenheim, P B Shepson, B J Finlayson-Pitts, and C W Spicer, The role of Br 2 and BrCl in surface ozone destruction at polar sunrise, Science, 291(5503), , 2001 Galbally, I E, S T Bentley, and C P Meyer, Mid-latitude marine boundary-layer ozone destruction at visible sunrise observed at Cape Grim, Tasmania, 41 S, Geophys Res Lett, 27(23), , 2000 Herrmann,H,BErvens,H-WJacobi,RWolke,PNowacki,and R Zellner, CAPRAM2 3: A chemical aqueous phase radical mechanism for tropospheric chemistry, J Atmos Chem, 36(3), , 2000 Matthew, B M, and C Anastasio, Determination of halogenated monoalcohols and diols in water by gas chromatography with electron-capture detection, J Chromatogr A, 866(1), 65 77, 2000 Michalowski, B, J S Francisco, S Li, L A Barrie, J W Bottenheim, and P B Shepson, A computer model study of multiphase chemistry in the Arctic boundary layer during polar sunrise, J Geophys Res, 105(D12), 15,131 15,145, 2000 Mozurkewich, M, Mechanisms for the release of halogens from sea-salt particles by free radical reactions, J Geophys Res, 100(D7), 14,199 14,207, 1995 Platt, U, and G Hönninger, The role of halogen species in the troposphere, Chemosphere, 52, , 2003 Rafi, A, and H C Sutton, Radiolysis of aerated solutions of potassium bromide, Trans Faraday Soc, 61, , 1965 Ross, A B, B H J Bielski, G V Buxton, D E Cabelli, W P Helman, R E Huie, J Grodkowski, P Neta, Q G Mulazzani, and F Wilkinson, NIST Standard Reference Database 40: NDRL/NIST Solution Kinetics Database V30, Gaithersburg, MD, 1998 Sander, R, and P J Crutzen, Model study indicating halogen activation and ozone destruction in polluted air masses transported to the sea, J Geophys Res, 101(D4), , 1996 Sander, R, R Vogt, G W Harris, and P J Crutzen, Modeling the chemistry of ozone, halogen compounds, and hydrocarbons in the arctic troposphere during spring, Tellus, 49B(5), , 1997 Sutton, H C, G E Adams, J W Boag, and B D Michael, Radical yields and kinetics in the pulse radiolysis of potassium bromide solutions, in International Symposium on Pulse Radiolysis, edited by M Ebert, J P

5 MATTHEW ET AL: HO 2 OXIDIZES 2 TO Br 2 ASC 14-5 Keene, and A J Swallow, pp 61 81, Academic, Manchester, England, 1965 Vogt, R, R Sander, and P J Crutzen, A mechanism for halogen release from sea-salt aerosol in the remote marine boundary layer, Nature, 383(26), , 1996 von Glasow, R, R Sander, A Bott, and P Crutzen, Modeling halogen chemistry in the marine boundary layer 1 Cloud-free MBL, J Geophys Res, 107, 4341, doi:101029/2001jd000942, 2002 Wagner, I, and H Strehlow, On the flash photolysis of bromide ions in aqueous solutions, Ber Bunsenges Phys Chem, 91, , 1987 C Anastasio, I George, and B M Matthew, Atmospheric Science Program, Department of Land, Air, and Water Resources, University of California Davis, One Shields Avenue, Davis, CA , USA (canastasio@ucdavisedu)