Abstract
Abstract. Atmospheric mercury depletion events (AMDEs) during polar springtime are closely correlated with bromine-catalyzed tropospheric ozone depletion events (ODEs). To study gas- and aqueous-phase reaction kinetics and speciation of mercury during AMDEs, we have included mercury chemistry into the box model MECCA (Module Efficiently Calculating the Chemistry of the Atmosphere), which enables dynamic simulation of bromine activation and ODEs. We found that the reaction of Hg with Br atoms dominates the loss of gaseous elemental mercury (GEM). To explain the experimentally observed synchronous depletion of GEM and O3, the reaction rate of Hg+BrO has to be much lower than that of Hg+Br. The synchronicity is best reproduced with rate coefficients at the lower limit of the literature values for both reactions, i.e. kHg+Br≈3×10−13 and kHg+BrO≤1×10−15 cm3 molecule−1 s−1, respectively. Throughout the simulated AMDEs, BrHgOBr was the most abundant reactive mercury species, both in the gas phase and in the aqueous phase. The aqueous-phase concentrations of BrHgOBr, HgBr2, and HgCl2 were several orders of magnitude larger than that of Hg(SO3)22−. Considering chlorine chemistry outside depletion events (i.e. without bromine activation), the concentration of total divalent mercury in sea-salt aerosol particles (mostly HgCl42−) was much higher than in dilute aqueous droplets (mostly Hg(SO3)22−), and did not exhibit a diurnal cycle (no correlation with HO2 radicals).
Highlights
Mercury is a prominent environmental pollutant which can form toxic compounds and bioaccumulate in aquatic organisms and food chains
Atmospheric mercury depletion events (AMDEs) during polar springtime are closely correlated with brominecatalyzed tropospheric ozone depletion events (ODEs)
We found that the reaction of Hg with Br atoms dominates the loss of gaseous elemental mercury (GEM)
Summary
Mercury is a prominent environmental pollutant which can form toxic compounds and bioaccumulate in aquatic organisms and food chains. These models have been commonly used to study the long-range transport of mercury They contain chemical reactions of Hg species related to SO2 and chlorine but they do not consider bromine chemistry. Since the concentration of the BrO radical will be many times that of the Br atoms in the O3-rich troposphere, BrO may be an important oxidant for GEM Their model used prescribed fluxes of Br2 and BrCl, and the mechanism did not consider aqueous-phase reactions. Hedgecock et al (2008) published a simulation of AMDEs with AMCOTS and a chemistry scheme based on MOCCA (the predecessor of MECCA) They used measured rate constants for the reaction of Hg with Br but did not consider aqueousphase species. In the gas and the aqueous phase including bromine chemistry with a fully coupled gas/aqueous chemistry mechanism
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