High Mercury Wet Deposition at a “Clean Air” Site in Puerto Rico

Abstract

Atmospheric mercury deposition measurements are rare in tropical latitudes. Here we report on seven years (April 2005 to April 2012, with gaps) of wet Hg deposition measurements at a tropical wet forest in the Luquillo Mountains, northeastern Puerto Rico, U.S. Despite receiving unpolluted air off the Atlantic Ocean from northeasterly trade winds, during two complete years the site averaged 27.9 μg m(-2) yr(-1) wet Hg deposition, or about 30% more than Florida and the Gulf Coast, the highest deposition areas within the U.S. 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), and was highest during summer and lowest during the winter dry season. Rainout of Hg (decreasing concentration with increasing rainfall depth) was minimal. The high Hg deposition was not supported by gaseous oxidized mercury (GOM) at ground level, which remained near global background concentrations (<10 pg m(-3)). Rather, a strong positive correlation between Hg concentrations and the maximum height of rain detected within clouds (echo tops) suggests that droplets in high convective cloud tops scavenge GOM from above the mixing layer. The high wet Hg deposition at this "clean air" site suggests that other tropical areas may be hotspots for Hg deposition as well.

Subtropical subsidence and surface deposition of oxidized mercury produced in the free troposphere

“…Precipitation amounts and the contribution of UT and MT together explain 55 % of the spatial variation in the observed Hg flux, while individually they explain 25 and 42 % of the spatial variation, respectively. This is consistent with previous studies that have shown higher Hg wet deposition flux in convective thunderstorms that can scavenge Hg(II) present at high altitudes (Guentzel et al, 2001;Shanley et al, 2015;Holmes et al, 2016;Kaulfus et al, 2017).…”

“…During the Nitrogen, Oxidants, Mercury, and Aerosol Distributions, Sources, and Sinks (NOMADSS) aircraft campaign, the highest Hg(II) concentrations (300-680 pg m −3 ) were observed in clean and dry air (CO < 75 ppbv and relative humidity (RH) < 35 %) originating in the subsiding air masses of the Pacific and the Atlantic subtropical anticyclones Shah et al, 2016). Furthermore, higher concentrations of Hg in precipitation are observed in thunderstorms reaching higher altitudes (Guentzel et al, 2001;Shanley et al, 2015;Holmes et al, 2016;Kaulfus et al, 2017), and higher Hg wet and dry deposition fluxes are associated with transport from the free troposphere Gustin et al, 2012;Huang and Gustin, 2012;Sheu and Lin, 2013).…”

Sedimentary records of mercury stable isotopes in Lake Michigan

“…Atmospheric deposition is an important input of Hg to Great Lakes (Landis and Keeler, 2002) and sea waters (Mason and Sheu, 2002). Precipitation scavenging oxidized Hg species (e.g., Hg II g and Hg p ) in the atmosphere is most likely the primary carrier for atmospheric Hg signals in offshore lake sediments (Shanley et al, 2015). Previous studies have reported mean d Hg (mean: ∼0.25‰)in precipitation collected in the Great Lakes region (Gratz et al, 2010;Chen et al, 2012;Demers et al, 2013;Sherman et al, 2011Sherman et al, , 2015.…”

Total- and monomethyl-mercury and major ions in coastal California fog water: Results from two years of sampling on land and at sea

“…Hg 0 is distributed globally and the atmosphere is the main transport pathway to sensitive ecosystems far from anthropogenic and natural sources (Fitzgerald et al, 1998;Steffen et al, 2015;Shanley et al, 2015). Once oxidized in the atmosphere, Hg 0 converts to gaseous oxidized Hg (Hg II ) and particulate bound Hg II which have much shorter atmospheric lifetimes (hours to days) and these species readily deposit to water and land surfaces (Lindberg et al, 2007).…”