Air–snowpack exchange of bromine, ozone and mercury in the springtime Arctic simulated by the 1-D model PHANTAS – Part 2: Mercury and its speciation

摘要

Atmospheric mercury depletion events (AMDEs) refer to a recurring depletionof mercury occurring in the springtime Arctic (and Antarctic) boundary layer,in general, concurrently with ozone depletion events (ODEs). To close some ofthe knowledge gaps in the physical and chemical mechanisms of AMDEs and ODEs,we have developed a one-dimensional model that simulates multiphase chemistryand transport of trace constituents throughout porous snowpack and in theoverlying atmospheric boundary layer (ABL). This paper constitutes Part 2 ofthe study, describing the mercury component of the model and its applicationto the simulation of AMDEs. Building on model components reported in Part 1("In-snow bromine activation and its impact on ozone"), we have developed achemical mechanism for the redox reactions of mercury in the gas and aqueousphases with temperature dependent reaction rates and equilibrium constantsaccounted for wherever possible. Thus the model allows us to study thechemical and physical processes taking place during ODEs and AMDEs within asingle framework where two-way interactions between the snowpack and theatmosphere are simulated in a detailed, process-oriented manner. Model runsare conducted for meteorological and chemical conditions that represent thespringtime Arctic ABL characterized by the presence of "haze" (sulfateaerosols) and the saline snowpack on sea ice. The oxidation of gaseouselemental mercury (GEM) is initiated via reaction with Br-atom to form HgBr,followed by competitions between its thermal decomposition and furtherreactions to give thermally stable Hg(II) products. To shed light onuncertain kinetics and mechanisms of this multi-step oxidation process, wehave tested different combinations of their rate constants based on publishedlaboratory and quantum mechanical studies. For some combinations of the rateconstants, the model simulates roughly linear relationships between thegaseous mercury and ozone concentrations as observed during AMDEs/ODEs byincluding the reaction HgBr + BrO and assuming its rate constant to be thesame as for the reaction HgBr + Br, while for other combinations theresults are more realistic by neglecting the reaction HgBr + BrO.Speciation of gaseous oxidized mercury (GOM) changes significantly dependingon whether or not BrO is assumed to react with HgBr to form Hg(OBr)Br.Similarly to ozone (reported in Part 1), GEM is depleted via bromine radicalchemistry more vigorously in the snowpack interstitial air than in theambient air. However, the impact of such in-snow sink of GEM is found to beoften masked by the re-emissions of GEM from the snow following thephoto-reduction of Hg(II) deposited from the atmosphere. GOM formed in theambient air is found to undergo fast "dry deposition" to the snowpack bybeing trapped on the snow grains in the top ~1 mm layer. Wehypothesize that liquid-like layers on the surface of snow grains areconnected to create a network throughout the snowpack, thereby facilitatingthe vertical diffusion of trace constituents trapped on the snow grains atmuch greater rates than one would expect inside solid ice crystals.Nonetheless, on the timescale of a week simulated in this study, the signalof atmospheric deposition does not extend notably below the top 1 cm of thesnowpack. We propose and show that particulate-bound mercury (PBM) isproduced mainly as HgBr₄2− by taking up GOM into bromide-enrichedaerosols after ozone is significantly depleted in the air mass. In theArctic, "haze" aerosols may thus retain PBM in ozone-depleted air masses,allowing the airborne transport of oxidized mercury from the area of itsproduction farther than in the form of GOM. Temperature dependence ofthermodynamic constants calculated in this study for Henry's law andaqueous-phase halide complex formation of Hg(II) species is a critical factorfor this proposition, calling for experimental verification. The proposedmechanism may explain observed changes in the GOM–