Modeled and Observed Atmosphere-terrestrial Exchange of Hg 0 in a Temperate Hardwood Forest Table of Contents

While recent scientific advances have improved our understanding of the fundamental chemistry of mercury (Hg) in the environment, important components of the global Hg budget remain uncertain. Understanding exchange between atmospheric Hg and the terrestrial surface is critical to understanding the fate of anthropogenic Hg inputs in the environment, because the terrestrial surface re-emits historically deposited Hg and can be a source of toxic methylmercury to watersheds. To evaluate the importance of gaseous elemental mercury (Hg) deposition to the terrestrial surface, I analyzed Hg, meteorological, and trace gas data from a forested site in New Hampshire with observed intense warm-season nocturnal depletions in [Hg]. I calculated nightly estimates of Hg dry deposition velocity (Vd) and deposition flux and predicted foliar accumulation of Hg from Hg deposition. I compared observed [Hg] on seasonal and diurnal timescales to the GEOS-Chem global biogeochemical Hg model output, testing the sensitivity of modeled [Hg] to parameterized dry deposition velocity, boundary layer mixing, land fraction, model resolution, and anthropogenic emissions. From this model comparison and analysis of the relationships between Hg concentration and meteorological variables, I attempted to understand the relative importance of stomatal and non-stomatal uptake of Hg at a hardwood forest. The modeled Vd in GEOS-Chem and many globaland regional-scale Hg models is up to an order of magnitude lower than that observed at TF, and is especially too low at night, suggesting that modeled cuticular and/or soil resistance to uptake must be reduced in order to reproduce observed depletions. This asserts the importance of non-stomatal Hg uptake and suggests that dry deposition of Hg is a larger budget term than currently modeled.