Sources of gaseous oxidized mercury and mercury dry deposition at two southeastern U.S. sites

Abstract

Wet deposition measurements have shown that relative to other parts of the US, the southeastern region has the highest mercury (Hg) inputs. The source of this Hg has been investigated by multiple researchers and is suggested to be derived from local, regional and global sources. Here we focus on trying to understand potential sources of Hg to this area during periods dominated by dry gaseous oxidized mercury (GOM) deposition. Dry deposition of GOM to a surrogate surface was measured in conjunction with speciated atmospheric Hg, and ancillary parameters from September 2007 through September 2008 at two sites located within 25 km of coal-fired power plants (CFPPs) near Yorkville, GA, and Pensacola, FL. Mean weekly GOM dry deposition, daily GOM, and daily sulfur dioxide (SO2) concentrations were significantly (P < 0.01) higher at the Yorkville site by factors of 1.5, 2.0, and 1.8, respectively. At both sites, GOM and SO2 concentrations were significantly correlated (P < 0.05) in every season on hourly and daily time scales. Wind rose diagrams showed significantly enhanced GOM and SO2 concentrations when air moved to the sites from the direction of the nearest CFPPs. Most periods of enhanced GOM concentrations ([GOM] > 98th percentile), were also associated with NOy/SO2 ratios that were within 25% of that reported for the local CFPPs (N = 27 of 33 at Yorkville, N = 18 of 26 at Pensacola). During these events, termed Category 1, mean GOM/SO2 enhancement ratios were 2.4 ± 0.1 and 2.3 ± 0.1 pg m−3 ppb−1 for Yorkville and Pensacola, respectively (range = 0.5 to 5.5). The remaining events at both sites (termed Category 2) displayed significantly lower SO2 concentrations, yet GOM concentrations were not significantly different compared to Category 1 events. The potential sources of GOM during the Category 2 events at OLF were investigated using gridded frequency distributions (GFD) of 72-h atmospheric back trajectories. During these periods there was a greater component of air mass transport from the free troposphere, and less precipitation along the trajectory paths compared to GFDs for Category 1 events. GFDs developed for the weeks when GOM dry deposition was in the upper quartile at both sites simultaneously revealed a similar pattern to the GFDs of Category 2 GOM concentration events, that is, greater free tropospheric transport and relatively little precipitation. Although dry deposition inputs are thought to represent <15% of total annual Hg deposition in this region, we suggest that a significant portion of this Hg could be derived from sources outside the local area.