Effect of bromine chemistry on the atmospheric

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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, D23309, doi:10.1029/2008JD010262, 2008

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Effect of bromine chemistry on the atmospheric mercury cycle Christian Seigneur1,2 and Kristen Lohman1 Received 13 April 2008; revised 18 September 2008; accepted 3 October 2008; published 13 December 2008.

[1] Bromine chemistry is believed to play a major role in the atmospheric oxidation of

elemental mercury (Hg0) to divalent mercury (HgII). However, its effect on the speciation of atmospheric mercury (Hg) has been mostly ignored in models of the fate and transport of mercury. We investigate the effect of bromine chemistry on Hg speciation for various reaction kinetics and environmental conditions with a box model. Bromine chemistry was also added to the chemical mechanism of a global chemical transport model, and simulations of the global mercury cycle were conducted with different sets of bromine reaction kinetics. The global model simulations conducted with bromine chemistry lead to Hg0 concentrations that are consistent with observations only if the pressure dependence of the kinetics of the oxidation of Hg0 by Br is taken into account. Bromine chemistry is found to reduce the overall lifetime of Hg by about 10%.

Citation: Seigneur, C., and K. Lohman (2008), Effect of bromine chemistry on the atmospheric mercury cycle, J. Geophys. Res., 113, D23309, doi:10.1029/2008JD010262.

1. Introduction [2] Halogen species are responsible for mercury (Hg) depletion events (MDEs) in the Arctic [Schroeder et al., 1998; Lindberg et al., 2002] and Antarctic [Ebinghaus et al., 2002a, 2002b] where gaseous elemental Hg (Hg0) is rapidly oxidized to gaseous divalent Hg (HgII), which subsequently deposits to the ground. In addition, bromine may also convert Hg0 to HgII in the marine boundary layer thereby leading to rapid deposition of some Hg to the ocean [Mason and Sheu, 2002; Laurier et al., 2003], in the vicinity of salted lakes such as the Dead Sea, Israel [Peleg et al., 2007], and near the tropopause where depletion of Hg0 [Talbot et al., 2007] and high concentrations of particulate Hg (Hgp, most likely adsorbed HgII) [Murphy and Thomson, 2000; Murphy et al., 2006] have been observed. Global chemical transport models (CTMs) differ in their simulation of Hg0 oxidation in the free troposphere [Bullock et al., 2008]. Among global CTMs, GEOS-Chem leads to the highest HgII concentrations in the free troposphere and shows some agreement with available measurements by simulating the oxidation of Hg0 by ozone (O3) and hydroxyl radicals (OH) only, i.e., without accounting for bromine chemistry [Selin and Jacob, 2008]. However, GEOS-Chem could not reproduce some observations of high HgII concentrations in the free troposphere, thereby suggesting that other oxidation pathways beside oxidation by O3 and OH may be at play [Swartzendruber et al., 2006]. [3] Depending on the research laboratory, the kinetics of the reactions of Hg0 is faster with chlorine atoms (Cl) than 1 Atmospheric and Environmental Research, Inc., San Ramon, California, USA. 2 Now at Centre d’Enseignement et de Recherche en Environnement Atmosphe´rique, Joint Laboratory ENPC/EDF R&D, Universite´ Paris-Est, Marne la Valle´e, France.

Copyright 2008 by the American Geophysical Union. 0148-0227/08/2008JD010262$09.00

with bromine atoms (Br) [Ariya et al., 2002] or of comparable magnitude [Donohoue et al., 2005, 2006]. In any case, the concentrations of the chlorine atoms are significantly lower than the concentrations of the bromine atoms, in part because the photolysis of Br2 is faster than that of Cl2 (thereby leading to faster production of Br than Cl) and the lifetimes of stable chlorine products (HOCl and ClONO2) are longer than those of the corresponding bromine products (HOBr and BrONO2) (thereby leading to slower regeneration of Cl atoms than Br atoms) [Calvert and Lindberg, 2003]. Consequently, the chemical kinetics of halogen reactions with Hg0 indicates that the major reactions are the oxidation of Hg0 by Br atoms and possibly also BrO radicals [Calvert and Lindberg, 2003; Ariya et al., 2004]. There is, however, still considerable uncertainty in the kinetics of the reaction of Hg0 with Br and BrO, as well as the concentrations of these two bromine species in the atmosphere. [4] Major sources of bromine include halons (used mostly in fire extinguishers), methyl bromide (originating from anthropogenic sources such as industrial fumigation and natural sources such as biomass burning), and sea salt [Khalil et al., 1993; Sander et al., 2003; Yang et al., 2005]. Holmes et al. [2006] investigated the Hg0 atmospheric lifetime due to reaction with Br. They used Br concentrations from a global model simulation of bromine species in the troposphere [Yang et al., 2005]. They concluded that the reaction of Hg0 with Br was a major, and possibly dominant, global sink for Hg0 and that it was particularly important in cases where Br concentrations were high and the temperature was low. They recommended further investigation of the effect of bromine chemistry on the Hg global cycle using a three-dimensional chemical transport model. However, current three-dimensional global and hemispheric models of atmospheric Hg do not generally take into account the gas-phase oxidation of Hg0 by bromine compounds [Dastoor and Larocque, 2004; Travnikov, 2005;

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SEIGNEUR AND LOHMAN: BROMINE CHEMISTRY OF MERCURY

Figure 1. Kinetic rate parameter (molec1 cm3 s1) of the oxidation of Hg0 by Br as a function of temperature. Seigneur et al., 2006; Selin et al., 2007]. It is, therefore, of interest to investigate how the global mercury cycle is affected by the bromine reactions and whether our current understanding of mercury chemical kinetics is consistent with the available observations. Accordingly, we present here an analysis of the effect of the gas-phase bromine reactions on Hg speciation. First, we investigate this bromine-Hg chemistry for conditions conducive to Hg0 oxidation (i.e., Arctic, Antarctic, marine boundary layer, salted lakes and upper atmosphere) using a box model. Next, we investigate the effect of bromine chemistry on the global cycling of atmospheric Hg using a global chemical transport model for mercury (CTM-Hg).

2. Bromine Chemistry of Mercury [5] The following chemical kinetic mechanism is considered for the oxidation of Hg0 by Br and BrO. ðR1Þ

Hg0 þ Br ) HgBr

ðR2Þ

HgBr ) Hg0 þ Br

ðR3Þ

HgBr þ X ) HgII

ðR4Þ

Hg0 þ BrO ) HgO þ Br

ðR5Þ

Hg0 þ BrO ) HgBr þ O

X represents chemical species such as Br, BrO, Br2, OH and O3. There is considerable uncertainty in the kinetics of these reactions and such uncertainties must be taken into account when investigating the effect of those reactions on the Hg global cycle. [6] The overall kinetics of the oxidation of Hg0 by Br to form HgII compounds was studied experimentally by Ariya et al. [2002] using a relative rate approach; a kinetic rate parameter of 3.2 1012 molec1 cm3 s1 was reported at T = 298 K. It is likely that under laboratory conditions (i.e., high Br concentrations) the HgBr reactions were displaced toward the formation of HgBr2 (R3) rather than decomposition to Hg0 and Br (R2). Therefore the experimental value can be considered to be that of k1. Donohoue et al. [2006] studied the kinetics of the reaction of Hg0 with Br using a

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direct measurement with a pulsed laser photolysis-pulsed laser induced fluorescence technique and derived a threebody kinetics that depends on pressure and temperature, k1 = 3.6 1013 P (T/298)1.86 molec1 cm3 s1. They placed an uncertainty of 50% on this measurement due mostly to the uncertainty in determining the Br concentration. At 1 atm and 298 K, the rate parameter is 3.6 1013 molec1 cm3 s1. [7] Three theoretical estimates of the kinetics of k1 have been made. Khalizov et al. [2003] used collision theory to estimate a value for k1 = 1.01 1012 exp(209/T) molec1 cm3 s1, i.e., 2.0 1012 molec1 cm3 s1 at 298 K. Goodsite et al. [2004] used Rice-RamspergerKassel-Marcus (RRKM) theory to derive k1 = 1.1 1012 (T/298)2.37 molec1 cm3 s1. Recently, Shepler et al. [2007] used the quasi-classical trajectory method (QCT) to estimate k1 = 9.8 1013 exp(401(1/T 1/298)) molec1 cm3 s1; they estimated the uncertainty of their estimate to be a factor of 1.7. [8] Figure 1 shows the temperature dependence of these various k1 values. At 298 K, there is about one order of magnitude difference between the experimental value of Ariya et al. [2002] and that of Donohoue et al. [2006]; the theoretical estimates lie within that range. If we take into account the uncertainty estimates, the lower bound of the theoretical value of Shepler et al. [2007] is commensurate with the upper bound of the direct rate measurement of Donohoue et al. [2006] at about 5.5 1013 molec1 cm3 s1. The temperature dependence of the kinetics shows a clear increase in the kinetic rate parameter as the temperature decreases. From 298 K to 258 K, the kinetics increases by a factor ranging from 1.1 [Khalizov et al., 2003] to 1.4 [Goodsite et al., 2004]. [9] The kinetics of the decomposition reaction has been estimated by Goodsite et al. [2004] by theoretical considerations of equilibrium with k1: k2 = 1.2 1010 exp(8357/ T) s1. This kinetics shows a strong temperature dependence with the decomposition rate being about two orders of magnitude slower at 258 K than at 298 K; therefore, the oxidation to HgII is favored at lower temperatures. The reaction of HgBr with another species can lead to the formation of a stable HgII species such as HgBr2 (reaction with Br, BrO or Br2), HgBrO (reaction with O3 or BrO), HgBrOH (reaction with OH) or HgBrOBr (reaction with BrO). Goodsite et al. estimated k3 = 2.5 1010 (T/298)0.57 molec1 cm3 s1 for reaction of HgBr with Br or OH using RRKM theory. [10] Skov et al. [2004] used ambient data from mercury depletion events at Station Nord in Greenland to derive empirical estimates of the overall kinetics of the oxidation of Hg0 by Br to HgII. To that end, they used a relative rate approach where the oxidation of ozone by Br (which has a well established kinetics) was the reference. They estimated a pseudofirst-order rate parameter of 0.8 1012 molec1 cm3 s1 at 233 K and 1.2 1012 molec1 cm3 s1 at 263 K (this rate parameter is equivalent to k1 if [Br] 106 cm3 at 263 K and Br 104 cm3 at 233 K, which was the case here). These values are commensurate with the k1 values reported in Figure 1; however, the temperature dependence is not consistent with experimental and theoretical estimates of the overall oxidation of Hg0 by Br and,

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Table 1. Environmental Conditions Used in the Box Model Simulations Scenario

Pressure (atm)

Temperature (K)

Hg0 (ng/m3)

Br (cm3)

BrO (cm3)

O3 (molec cm3)

OH (cm3)

H2O2 (molec cm3)

Arctic, springtime Marine boundary layer Dead Sea, Israel Upper troposphere

1.0 1.0 1.05 0.2

233, 263 288 310 220

1.6 1.6 2.0 0.3

107, 108 5 105, 2.5 106 2 107, 2 108 5 105

7.5 108 5 107 2 109 5 106

6 1011 1012 1.5 1012 1.5 1012

106 106 106 106

6 109 1.5 1010 2.5 1010 5 108

therefore, may reflect uncertainties in the empirical approach rather than a true temperature dependence. [11] The kinetics of the BrO reaction(s) is available from one experimental laboratory study [Raofie and Ariya, 2003] and a wide range was reported for this kinetics (from 1015 to 1013 molec1 cm3 s1). The thermodynamics of the homogenous gas-phase reaction is not favorable because it is significantly endothermic [Tossell, 2003; Shepler et al., 2007] but the atmospheric heterogeneous reaction cannot be ruled out.

3. Box Model Simulations [12] Box model simulations of the mercury-bromine chemical mechanism were conducted to investigate the sensitivity of the Hg concentrations to (1) the kinetic rate parameters and (2) environmental variables such as the Br and BrO concentrations and ambient temperature for various environmental conditions. Two distinct sets of bromine kinetics were added to an existing Hg chemical kinetic mechanism [see Seigneur et al., 2006, Table 2]. The first set uses the kinetics of Goodsite et al. [2004], which provides an internally consistent set of kinetic parameters for the Br reactions, and a midrange value of 1.5 1014 molec1 cm3 s1 for the BrO kinetic following Ariya et al. [2002]. The second set uses the lower bounds of the available kinetics. Thus the direct rate measurement of Donohoue et al. [2006] for the oxidation of Hg0 by Br is used, the value of k2 is modified to maintain the same k1/k2 ratio at 298 K (since k2 was estimated based on a theoretical estimate of this equilibrium), the value of k3 is kept unchanged [Goodsite et al., 2004], and the lower bound (1015 molec1 cm3 s1) of the kinetics of the oxidation of Hg0 by BrO estimated by Raofie and Ariya [2003] is used. The second kinetic set includes pressure dependence for the oxidation of Hg0 by Br whereas the first kinetic set does not. We present box model simulations for four distinct environmental settings: (1) springtime Arctic (or Antarctic) during a Hg depletion event, (2) marine boundary layer at a midlatitude, (3) Dead Sea, Israel during a partial Hg0 depletion event, and (4) upper troposphere. All these simulations were for gas-phase conditions (i.e., no aqueous-phase was present). Table 1 summarizes the box model inputs for these scenarios. 3.1. Arctic Springtime Scenario [13] Skov et al. [2004] stated that sea ice temperature needs to be below 269 K for Hg depletion events to occur, which is consistent with the data of Lindberg et al. [2002], who show a mercury depletion event occurring at T = 269 K and none at higher temperatures. Accordingly, temperatures

HCl (molec cm3) 5 5 5 5

108 109 109 108

of 263 and 233 K were used for the Arctic scenario based on Hg depletion events reported by Skov et al. The Br and BrO concentrations (see Table 1) are those used by Ariya et al. [2004] in their box model calculations of Arctic mercury depletion events. The Br concentrations are consistent with estimates of 3 107 and 108 cm3 derived from the average Hg0 oxidation kinetics and atmospheric lifetimes of 3 and 10 h during Arctic Hg depletion events [Skov et al., 2004]. Also, Boudries and Bottenheim [2000] calculated a Br concentration of 1.4 107 cm3 during an ozone depletion event at Alert, in the Canadian Arctic. The BrO concentration is consistent with spectroscopic measurements conducted in the Arctic [Haussmann and Platt, 1994] and Antarctic [Friess et al., 2004], which ranged up to 30 ppt (8 108 cm3). An O3 concentration of 20 ppb was used [Lindberg et al., 2002]; although the O3 concentration decreases significantly during a Hg depletion event (since Br and BrO also react with O3), the kinetics of the oxidation of Hg0 with O3 is slow and has little effect on these simulation results. A typical hydroxyl radical (OH) concentration of 106 cm3 was used. Hydrogen peroxide (H2O2) and hydrogen chloride (HCl) concentrations of 0.2 ppb and 0.02 ppb [Calvert and Lindberg, 2005], respectively, were used. [14] Figure 2 presents the results of the box model simulations of mercury chemistry using an initial Hg0 concentration of 1.6 ng/m3 [Lindberg et al., 2002; Ebinghaus et al., 2002a, 2002b], a constant Br concentration of

Figure 2. Concentration of Hg0 as a function of time for environmental conditions typical of Arctic spring (T = 233 and 263 K; [Br] = 107 and 108 cm3) for kinetic set 1: kinetics of Goodsite et al. [2004] for k1 and midrange kinetics of Raofie and Ariya [2003] for k4.

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by Skov et al. [2004]. The data reported by Lindberg et al. [2002] show half-lives of Hg0 ranging from 2.5 to 5 hours, i.e., lifetimes of 3.5 to 7 hours. Therefore the box model simulations conducted here suggest that both kinetics are consistent with the Hg0 concentration temporal profiles observed during mercury depletion events in the Arctic during springtime, although the slow kinetics requires the high Br concentration to lead to realistic results.

Figure 3. Concentration of Hg0 as a function of time for environmental conditions typical of Arctic spring (T = 233 and 263 K; [Br] = 107 and 108 cm3) for kinetic set 2: kinetics of Donohoue et al. [2006] for k1 and lower bound of kinetics of Raofie and Ariya [2003] for k4. 107 molecules/cm3, a constant BrO concentration of 7.5 108 molecules/cm3 and a temperature of 263 K for the first kinetic set. The high Br simulation differs from the base simulation by a Br concentration of 108 molecules/cm3 and the low-temperature simulation differs by a temperature of 233 K. For the base simulation, the Hg0 lifetime is 11 hours (i.e., a half-life of 8 hours). If the Br concentration is increased tenfold, the lifetime of Hg0 decreases to 1.8 hours (i.e., a half-life of 1.2 hours). The increase in the Br concentration leads to an increase in the kinetics that is greater than proportional because Br is involved with both k1 and k3 in the overall kinetics (see reactions above). If the temperature is decreased by 30 K, the consumption of Hg0 occurs much faster. This result is due to the fact that the kinetics of the three-body reaction of Hg 0 with Br increases with decreasing temperature (see Figure 1) and the decomposition reaction of HgBr is very sensitive to the temperature and decreases by about an order of magnitude, whereas the formation of HgII by reaction of HgBr with Br is not very sensitive to temperature (it increases slightly as the temperature decreases). No temperature dependence is available for the BrO reaction. The lifetime of Hg0 at 233 K is 9 hours (i.e., a half-life of 6.3 hours). The results for a high Br concentration at 233 K show a Hg0 lifetime of 1.3 hours. These results exemplify the sensitivity of the mercury-bromine chemistry to both bromine concentrations and temperature. [15] Results are presented in Figure 3 for the second kinetic set with the same conditions as above (i.e., base simulation, lower temperature and higher Br concentration). The slower kinetics leads to a lifetime of Hg0 of 53 hours at 263 K with the lower Br concentration. With a higher Br concentration, the Hg0 lifetime decreases to 6 hours; with both a lower temperature and higher Br concentration, it is 4.8 hours. [16] The kinetics of Hg0 oxidation in the model simulations is dominated by bromine chemistry. Lifetimes of Hg0 during mercury depletion events at Station Nord, Greenland, have been reported to be on the order of 3 to 10 hours

3.2. Marine Boundary Layer Scenario [17] For a midlatitude marine boundary layer scenario, we assumed a sea surface temperature of 288 K [e.g., von Glasow et al., 2002]. von Glasow et al. [2002] simulated concentrations of about 5 107 cm3 for BrO in the marine boundary layer during daytime. Measurements during a cruise in the Atlantic Ocean showed BrO concentrations of about 1 ppt (2.5 107 cm3) for a couple of days near the Canary Islands, with levels