A regional scale modeling study of atmospheric transport and transformation of mercury. II. Simulation results for the northeast United States

This paper presents the results of a simulation study of the transport, transformation, and deposition of atmospheric mercury (Hg) in the northeastern United States for 5 consecutive days in summer 1997, using the newly developed regional scale air quality model described in part I. Hourly ambient concentrations, in-cloud transformations, and deposition fluxes were predicted for each of the three mercury species: Hg(0), Hg(II), and Hg(p). The simulation results showed that surface level Hg concentrations over the land varied diurnally with the boundary layer structure. The Hg concentrations over the coastal ocean water were strongly influenced by the concentrations over land because of atmospheric advection by the prevailing westerly winds in the region. Over 90% of the ambient air concentration of Hg was in the form of Hg(0) vapor. Approximately half of the total Hg in wet deposition originated from ambient Hg(0), while the predominant species in dry deposition was Hg(II). The simulation predicted more dry deposition (31 kgday−1) than wet deposition (18 kgday−1) over the region for the simulation period. The re-emission of Hg(0) from land and water surfaces (46 kgday−1) was of the same magnitude as the total anthropogenic emission (55 kgday−1) within the region. Both precipitating and non-precipitating clouds were shown to be important media for in-cloud transformation of Hg(0). It was predicted that transformation and redistribution caused a net gain of Hg(p) at the upper troposphere during rain events. Over the region of simulation, the amount of Hg(0) transformation in non-precipitating clouds was 30% higher than in precipitating clouds. In co-existing non-precipitating clouds, the amount of Hg(0) transformation in the lower and upper troposphere was approximately equal.