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 (Hg0) 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 Hg0 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 Hg0 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 Hg0 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 (Hg0 + Hg2 + PHg) deposited to terrestrial systems and 47.8% of the total mercury deposition to aquatic systems was re‐emitted as Hg0, 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.