NEAR REAL TIME OBSERVATIONS OF THE REMOVAL OF AMBIENT REACTIVE GASEOUS MERCURY BY PRECIPITATION

J. Timothy Dvonch*, Gerald J. Keeler, James A. Barres, Frank J. Marsik, Mary Lynam, Elizabeth Malcolm (The University of Michigan Air Quality Laboratory, Ann Arbor, MI 48109-2029, USA); Matthew S. Landis (USEPA-National Exposure Research Laboratory, Research Triangle Park, NC 27711, USA); Robert K. Stevens (Florida Dept. of Environmental Protection at USEPA, Research Triangle Park, NC 27711, USA).

Since recent flue gas measurements of mercury (Hg) emissions from anthropogenic point sources have indicated that a large percentage of the Hg emitted is in a water soluble, reactive gaseous form (RGM), extending our understanding of the speciation of mercury in ambient air is of prime importance for predicting its atmospheric transport and removal rates.

Using a recently developed automated instrument (Tekran 1130-P Mercury Speciation Unit), speciated measurements of gaseous mercury in ambient air were collected in near real time during several field campaigns conducted in 1998-1999. Measured levels of RGM in Baltimore, MD (September, 1998) exceeded 200 pg/m3 during plume impaction by a nearby municipal waste incinerator, as may be expected since previous in-stack measurements of Hg from waste incinerators have shown that 75-95% of the Hg is emitted as RGM. However, RGM levels below detection (< 1 pg/m3) were observed in Michigan (September, 1998) and in the Florida Everglades (February, 1999) during precipitation events, suggesting an efficient removal of the highly water soluble RGM.

To more comprehensively quantify the removal of RGM by precipitation, measurements were collected at a site in Ann Arbor, MI from June to October, 1999. Continuous rainfall measurements, with a temporal resolution of 30 minutes, were compared with concurrent 60 minute integrated semi-continuous ambient RGM measurements. On average, a 53% decrease in the concentration of RGM was observed after the onset of precipitation, as the mean concentration of 14.7 ± 6.8 pg/m3 just prior to precipitation was reduced to a concentration of 6.8 ± 3.3 pg/m3 just after precipitation began (N = 20 events).

The observations of ambient RGM presented here support the hypothesis that RGM emitted from point sources may remain in an RGM form downwind of the source and may also be efficiently removed from the atmosphere by precipitation.