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High Mercury Wet Deposition at a Pristine Site in Puerto Rico

James B. Shanley
United States Geological Survey,
Montpelier, VT USA

Measurements of atmospheric wet depostion of mercury are few in tropical latitudes. We report here on two years of wet Hg deposition at a tropical wet forest at 18.3 N latitude in the Luquillo Mountains, northeastern Puerto Rico, USA. Despite receiving unpolluted air masses off the Atlantic Ocean from northeasterly trade winds, the site averaged 27.9 g m-2yr-1 wet Hg deposition, or about 30% more than Florida and the Gulf Coast, the highest deposition areas of the USA. These high Hg deposition rates are driven in part by high rainfall, which averaged 2855 mm yr-1. The volume-weighted mean Hg concentration was 9.8 ng L-1, somewhat lower than the southeastern USA, but high concentrations occurred even in high-rainfall weeks; there was no significant relation between Hg concentration and weekly rainfall depth. During a two month period, we measured atmospheric concentration and speciation of Hg at sea level and on a mountain ridge (750 m) above the deposition station (476 m). Particulate Hg and reactive gaseous mercury (RGM) concentrations were low, consistently < 12 pg m-3. Given this apparent lack of RGM at ground level to support the high wet Hg deposition, we suggest that RGM is being scavenged by precipitation and/or cloud water either after formation in the mixing layer, or by high cloud tops scavenging RGM from above the mixing layer. Photooxidation of Hg0 from the global pool is likely more intense in the tropics and helps to maintain a pool of RGM. The high wet Hg deposition found at this tropical site unimpacted by point sources makes a case for monitoring other tropical locations.