Description and initial simulation of a dynamic bidirectional air‐surface exchange model for mercury in Community Multiscale Air Quality (CMAQ) model

Emissions of elemental mercury (Hg 0 ) from natural processes are estimated to be as large as or larger than anthropogenic emissions and thus represent a critical process which must be accurately described in the modeling of the transport and fate of mercury. Recent ecosystem‐scale measurements indicate that a fraction of recently deposited mercury is recycled back into the atmosphere quickly, and that an atmospheric compensation point exists at background ambient concentrations. Modeled Hg 0 emissions from natural sources are typically uncoupled from dry deposition estimates and unconstrained by air‐biosphere gradient processes. A module has been developed for the Community Multiscale Air Quality (CMAQ) model to parameterize concentration‐dependent processes of bidirectional mercury exchange. The Hg 0 air‐surface exchange was modeled as a function of a dynamic compensation point. The compensation point is modeled as a function of sources and sinks of Hg 0 in vegetation and soil media using partitioning coefficients. A box model simulation was run for five months and a CMAQ simulation with bidirectional (BIDI) and without bidirectional (BASE) mercury exchange was run for the month of July 2002. The BASE case modeling scenario estimated that 8.5% of the total mercury (Hg 0 + Hg 2 + PHg) deposited to terrestrial systems and 47.8% of the total mercury deposition to aquatic systems was re‐emitted as Hg 0 , while the re‐emission ratios were 70.4% and 52.5% in the base case. The BIDI case was in better agreement with recent estimates of mercury cycling using stable isotopic mass balance experiments.