Mercury Wet Deposition: Variability Associated With Precipitation Type

Atmospheric mercury (Hg) exists in three forms: gaseous elemental mercury (GEM), oxidized mercury in gaseous (GOM) and particulate form (HgP). Of the three forms, GOM and HgP are effectively removed by wet deposition. Observed wet deposition patterns in the Southeastern Gulf Coast region of the United States and Puerto Rico show enhanced wet removal disproportionate to local surface emissions of mercury. Prior studies suggest that enhanced wet deposition at these sites could be due to thunderstorm scavenging of oxidized mercury from the free troposphere. Further, these studies also suggest that the efficiency of wet deposition removal of mercury is dependent on the nature of the precipitation system. In this context, this study examines mercury wet deposition as a function of precipitation system type. Observations from Mercury Deposition Network (MDN), consisting of integrated mercury wet deposition from multiple rainfall events, are processed to develop a subset of measurements where the rain sample originated from a single event. Radar observations from the WSR-88D network are then analyzed for each of these events and the associated precipitation systems are assigned to one of the following classes: Supercells, Disorganized, Quasi Linear Convective Systems (QCLS), Extratropical Cyclones and Tropical Cyclones. The power law fit for the disorganized convection events shows the general dilution effect (decrease of mercury concentration with increase of rainfall). Mercury concentration in rainfall for other types of precipitation systems were compared to the disorganized thunderstorm dilution curve. For rainfall accumulations more than 40mm, all precipitation systems exhibit less efficiency of removal of mercury. Lowest mercury concentrations were found in rainfall from extratropical cyclones with stratiform rain, even after accounting for the differences in amount of rainfall between different events.