THE SOURCES AND COMPOSITION OF MERCURY IN PACIFIC-OCEAN RAIN

Here we report measurements of total Hg (Hg(T)), reactive Hg (Hg(R)), and methylmercury (MMHg) in precipitation from the equatorial Pacific Ocean, collected during a cruise in January and February 1990, and from a mid-continental location in the rural temperate lacustrine northcentral Wisconsin environs. The concentrations of Hg(T) (14.4 +/-6.5 pM), Hg(R) (8.9 +/- 4.5 pM) and MMHg (< 50 fM) found in equatorial Pacific rain were less than the average concentrations found in Wisconsin. In general, the results indicate that although particulate Hg is a small fraction of the total atmospheric Hg burden, it is the major contributor to Hg in precipitation. Furthermore, deposition could be an important source of Hg(R) to the equatorial Pacific Ocean. In contrast, deposition is not a significant source of MMHg to either the equatorial Pacific Ocean or the remote seepage lakes of Wisconsin. This implies that methylated mercury is formed in situ in these systems.