Identifying changes in source regions impacting speciated atmospheric mercury at a rural site in the eastern United States

To investigate the effectiveness of emission reductions on the concentrations of gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM) at a rural site in Maryland (MD08), long-term (2005-14) measurements of speciated atmospheric mercury were analyzed using concentration-weighted trajectory (CWT) analysis. CWT results suggested that the number of major source regions contributing to GEM, GOM, and reactive mercury (RM 5 GOM 1 PBM) over the eastern United States and southeastern Canada declined over time. Across much of these regions, source contributions in 2011-14 decreased by up to 20% for GEM, by greater than 60% for GOM, and by 20%-60% for PBM compared to 2006-08, largely because of the decreases in power-plant mercury emissions since 2009. Changes in the spatial distribution of the source regions were also observed over time. Increases in source contributions of GEM after 2011 over the northeastern United States and southeastern Canada were predominantly from emission increases in metal and steel production and forest fires. Source contribution increases in PBM were more widespread, which can be attributed potentially to mercury transformation processes in the air or wood combustion rather than industrial sources.