Atmospheric Mercury Speciation:  Concentrations and Behavior of Reactive Gaseous Mercury in Ambient Air

Knowledge of atmospheric mercury speciation is critical to understanding its fate once released from point sources. The water-soluble compounds of Hg that exist in flue gases (termed reactive gaseous mercury, RGM) are subject to far greater local removal rates than is elemental Hg vapor, but few ambient air data exist. We developed a method using refluxing mist chambers to quantify the airborne concentrations of RGM in more than 250 ∼1-h samples under ambient conditions and summarize here the results of several RGM sampling campaigns in Tennessee and Indiana from 1992 to 1995. Measured levels of RGM were generally on the order of 50−200 pg/m3, representing about 3% of total gaseous mercury (TGM) and generally exceeding regional particulate Hg concentra tions. RGM exhibits significant correlations (p < 0.05) with temperature, solar radiation, O3, SO2, and TGM, suggesting seasonal trends similar to those of other regional air pollutants. The concentrations of RGM show reproducible diel trends, peaking during midday and decreasing sharply at night. A sharp spike in RGM was measured during a local plume impaction event in Tennessee. Concentration gradients over vegetation suggested a strong ground-level sink for RGM, and RGM concentrations decreased sharply during rain events, as expected for a water-soluble gas. The levels of RGM measured here support the hypothesis that Hg dry and wet deposition may be strongly influenced by the behavior of RGM and that elevated ecosystem exposure may be possible near major point sources of RGM compounds.