Abstract. Global models of atmospheric mercury generally assume that gas-phase OH and ozone are the main oxidants converting Hg 0 to Hg II and thus driving mercury deposition to ecosystems. However, thermodynamic considerations argue against the importance of these reactions. We demonstrate here the viability of atomic bromine (Br) as an alternative Hg 0 oxidant. We conduct a global 3-D simulation with the GEOS-Chem model assuming gas-phase Br to be the sole Hg 0 oxidant (Hg + Br model) and compare to the previous version of the model with OH and ozone as the sole oxidants (Hg + OH/O 3 model). We specify global 3-D Br concentration fields based on our best understanding of tropospheric and stratospheric Br chemistry. In both the Hg + Br and Hg + OH/O 3 models, we add an aqueous photochemical reduction of Hg II in cloud to impose a tropospheric lifetime for mercury of 6.5 months against deposition, as needed to reconcile observed total gaseous mercury (TGM) concentrations with current estimates of anthropogenic emissions. This added reduction would not be necessary in the Hg + Br model if we adjusted the Br oxidation kinetics downward within their range of uncertainty. We find that the Hg + Br and Hg + OH/O 3 models are equally capable of reproducing the spatial distribution of TGM and its seasonal cycle at northern mid-latitudes. The Hg + Br model shows a steeper decline of TGM concentrations from the tropics to southern mid-latitudes. Only the Hg + Br model can reproduce the springtime depletion and summer rebound of TGM observed at polar sites; the snowpack component of GEOS-Chem suggests that 40% of Hg II deposited to snow in the Arctic is transferred to the ocean and land reservoirs, amounting to aCorrespondence to: C. D. Holmes (cdholmes@post.harvard.edu) net deposition flux to the Arctic of 60 Mg a −1 . Summertime events of depleted Hg 0 at Antarctic sites due to subsidence are much better simulated by the Hg + Br model. Model comparisons to observed wet deposition fluxes of mercury in the US and Europe show general consistency. However the Hg + Br model does not capture the summer maximum over the southeast US because of low subtropical Br concentrations while the Hg + OH/O 3 model does. Vertical profiles measured from aircraft show a decline of Hg 0 above the tropopause that can be captured by both the Hg + Br and Hg + OH/O 3 models, except in Arctic spring where the observed decline is much steeper than simulated by either model; we speculate that oxidation by Cl species might be responsible. The Hg + Br and Hg + OH/O 3 models yield similar global budgets for the cycling of mercury between the atmosphere and surface reservoirs, but the Hg + Br model results in a much larger fraction of mercury deposited to the Southern Hemisphere oceans.

[1] Snow lying on sea ice could be a potentially important source of sea salt aerosol, as small snow particles, rich in salts, can be easily lifted into the air though blowing-snow events. Using a measured distribution of snow salinity on Antarctic sea ice and a blowing snow sublimation parameterization, we derive a method for estimating sea salt aerosol production, and bromine release, during blowing-snow events. Compared with sea salt aerosol production rates from the open ocean, we find that the aerosol production rate from snow can be more than an order of magnitude larger per unit area under typical weather conditions. The large sea ice cover may thus enhance the supply of sea salt to the polar lower atmosphere. This is consistent with observations of sea salt aerosol seasonality and with the ice-core record. This large emission of sea salt from snow also implies an additional tropospheric bromine source in these regions.

[1] An off-line three-dimensional tropospheric chemical transport model, parallelTropospheric Off-Line Model of Chemistry and Transport (p-TOMCAT), has been extended by incorporating a detailed bromine chemistry scheme that contains gas-phase reactions and heterogeneous reactions on both cloud particles and background aerosols. Bromine emission from bromocarbon photo-oxidation and from sea-salt bromine depletion and bromine removal through dry and wet deposition are included. Using this model, tropospheric bromine chemistry and ozone budgets are studied. The zonal mean of the inorganic gas-phase bromine compounds (Br x ) is calculated to be high (4-8 pptv) in the lower troposphere of the midlatitudes to high latitudes in each hemisphere, with decreasing trends with altitude (down to $2-3 pptv in the upper troposphere). The lowest Br x (<2 pptv) is over low latitudes, corresponding to small sea-salt Br emission and a high rate of precipitation scavenging. A mean lifetime of $5 days is obtained for the tropospheric Br x . Sea-salt emission plays the dominant role in total Br x in the lower troposphere while organic Br-containing compounds are important in upper layers. High daytime BrO mixing ratios (>1 pptv) are found over the high-latitude ocean surface, corresponding to high tropospheric column BrO values of up to 1.6 Â 10 13 molecules/cm 2 in the monthly mean. The addition of bromine chemistry to the model leads to a reduction in tropospheric ozone amounts by 4-6% in the Northern Hemisphere and up to $30% in the Southern Hemisphere high latitudes. The net ozone loss depends not only on total Br x , but also on solar irradiance, especially at high latitudes. The hydrolysis reaction of bromine nitrate, which occurs on cloud and aerosol surfaces (BrONO 2 + H 2 O aq ! HOBr + HNO 3 ), has a significant influence on ozone chemistry through its effect on NO x as well as on reactive BrO and Br.