Constraints from observations and modeling on atmosphere-surface exchange of mercury in eastern North America

Atmosphere-surface exchange of mercury, although a critical component of its global cycle, is currently poorly constrained. Here we use the GEOS-Chem chemical transport model to interpret atmospheric Hg⁰ (gaseous elemental mercury) data collected during the 2013 summer Nitrogen, Oxidants, Mercury and Aerosol Distributions, Sources and Sinks (NOMADSS) aircraft campaign as well as ground- and ship-based observations in terms of their constraints on the atmosphere-surface exchange of Hg⁰ over eastern North America. Model-observation comparison suggests that the Northwest Atlantic may be a net source of Hg⁰, with high evasion fluxes in summer (our best sensitivity simulation shows an average oceanic Hg⁰ flux of 3.3 ng m^-2 h^-1 over the Northwest Atlantic), while the terrestrial ecosystem in the summer of the eastern United States is likely a net sink of Hg⁰ (our best sensitivity simulation shows an average terrestrial Hg⁰ flux of -0.6 ng m^-2 h^-1 over the eastern United States). The inferred high Hg⁰ fluxes from the Northwest Atlantic may result from high wet deposition fluxes of oxidized Hg, which are in turn related to high precipitation rates in this region. We also find that increasing simulated terrestrial fluxes of Hg⁰ in spring compared to other seasons can better reproduce observed seasonal variability of Hg⁰ concentration at ground-based sites in eastern North America.