Northeast United States and Southeast Canada natural mercury emissions estimated with a surface emission model

Most mercury emission inventories only include anthropogenic emissions and neglect the large contribution of the natural mercury cycle due to difficulty in spatially estimating natural emissions and uncertainties in the natural emissions process. The Mercury (Hg) Surface Interface Model (HgSIM) has been developed to estimate the natural emissions of mercury, for inclusion in a more complete mercury emissions inventory. The model used a 3422 cell, 36 km on each side, gridded domain and 1 h time steps. The emissions over land are modeled as a function of the land cover, evapotranspiration, and temperature. The emissions over water are modeled as a function of the concentration gradient, the mixing of the air and water, and the temperature. The spatially distributed model is shown to account for the extreme spatial variability across the Northeast (NE) US and Southeast (SE) Canada. Estimates of natural mercury flux from uncontaminated surfaces are presented for a 2 week period in July. The total natural emissions for the domain, 4,434,912 km(2), was 2101.5 kg over the 2 week simulation. The highest total natural emissions were 820 ng m(-2) from the Atlantic Ocean in the SE part of the domain and the lowest total natural emissions were 74 ng m(-2) in the urban areas with little vegetation. The flux estimates from vegetation canopies, averaged over the 14 days, ranged from 0.0 ng m(-2) h(-1) during the night time hours when transpiration ceased to 4.46 ng m(-2) h(-1) during the afternoon in a mixed deciduous-coniferous forest. The range of the air-water flux was between 0.5 and 2.73 ng m(-2) h(-1) over the model domain with the higher emission rates corresponding to windier and warmer areas. The soil emissions ranged from near 0 to 2.3 ng m(-2) h(-1) with the higher rates corresponding to warmer agricultural regions. (C) 2004 Elsevier Ltd. All rights reserved.