EVIDENCE ON THE ORIGIN OF ATMOSPHERIC MERCURY

One goal of the National Atmospheric Deposition Program operated by the USGS/USEPA is to determine variations in the deposition of mercury from the atmosphere, during precipitation events. Weekly samples of precipitation are collected across the USA (our station is in Culpeper, VA). The average mercury concentration is about 7.5 ng/L; lower concentrations are usually in the winter and spring. The greater concentrations were observed in rainfall following several weeks without rain (lower observed after several weeks of frequent rain). The highest mercury concentration happened just before Hurricane Isabel passed through central Virginia in September of 2003, and the lowest was in the week after. Comparisons with other East Coast sampling sites do not identify particular sources of mercury, such as coal-burning electrical power plants. The source of mercury in rain and snow in central Virginia is likely a "global pool" that is transported in the atmosphere over long distances for weeks and months before it is deposited.