Seasonal and Diurnal Variations of Atmospheric Mercury across the U.S. Determined from AMNet Monitoring Data

Speciated atmospheric mercury data collected over the period from 2008 to 2010 at the Environmental Protection Agency and National Atmospheric Deposition Program Atmospheric Mercury Network (AMNet) sites were analyzed for its spatial, diurnal and seasonal characteristics across the U.S. Specifically, particulate bound mercury (PBM), gaseous oxidized mercury (GOM) and gaseous elemental mercury (GEM) data was used to determine the mercury chemical distributions and temporal variations, as well as the potential reasons that cause the similarities and differences among monitoring sites. Data analysis showed that the median values of GEM, GOM and PBM were 1.33- 2.02 ng m-3 (149 - 226 ppqv), 0.47-12.4 pg m-3 (0.05- 1.4 ppqv) and 1.27-13.7 pg m-3 (0.14-1.5 ppqv), respectively. GEM at most monitoring sites had similar median values and seasonality, except for Salt Lake City (elevation 1297 m), which showed much higher values (median = 2.02 ng m-3). The median values of GOM were close to the values of previous rural sites measurements in U.S., and the highest value was found in Salt Lake City. Discernable differences in median values among sampling sites can be found (up to 30 ppqv from monthly medians), which may be caused by diverse local emission sources. On average, GEM was higher in spring and winter, and lowest in fall. GOM was higher in spring and summer and PBM was highest in winter. At some monitoring sites, GEM, GOM and PBM demonstrated distinct diurnal variations. We will further utilize meteorological data to better understand the transport of atmospheric mercury and the photochemistry reactions among mercury species in the U.S. Overall, our analysis of the AMNet data should provide a more detailed understanding of atmospheric mercury that can be used to better inform regional and global models.