Spatial variability of mercury wet deposition in eastern Ohio: summertime meteorological case study analysis of local source influences.

Extensive exploration of event precipitation data in the Ohio River Valley indicates that coal combustion emissions play an important role in mercury (Hg) wet deposition. During July-September 2006, an intensive study was undertaken to discern the degree of local source influence. Source-receptor relationships were explored by establishing a set of wet deposition sites in and around Steubenville, Ohio. For the three month period of study, volume-weighted mean Hg concentrations observed at the eight sites ranged from 10.2 to 223 ng L(-1), but this range increased drastically on an event basis with a maximum concentration of 89.4 ng L(-1) and a minimum concentration of 4.1 ng L(-1). A subset of events was explored in depth, andthe degree of variability in Hg concentrations between sites was linked to the degree of local source enhancement. Samples collected at sites less than 1 km from coal-fired utility stacks (near-field) exhibited up to 72% enhancement in Hg concentrations over regionally representative samples on an event basis. Air mass transport and precipitating cell histories were traced in order to evaluate relationships between local point sources and receptor sites. It was found that the interaction of several dynamic atmospheric parameters combined to favor local Hg concentration enhancement over the more regional contribution. When significant meteorological factors (wind speed at time of maximum rain rate, wind speed 24 h prior to precipitation, mixing height, and observed ceiling) were explored, it was estimated that during summertime precipitation, 42% of Hg concentration in near-field samples could be attributed to the adjacent coal-fired utility source.