The atmospheric cycling and air-sea exchange of mercury species in the South and equatorial Atlantic Ocean

Measurements of gas-, particle- and precipitation-phases of atmospheric mer cury (Hg) were made in the South and equatorial Atlantic Ocean as part of t he 1996 IOC Trace Metal Baseline Study (Montevideo, Uruguay to Barbados). T otal gaseous mercury (TGM) ranged from 1.17 to 1.99 ng m(-3), with a weight ed mean of 1.61 +/- 0.09 ng m(-3). These values compare well with Pacific O cean data and earlier results from the Atlantic. The open-ocean samples rec orded a distinctive inter-hemispheric gradient, which is consistent with a long-lived trace gas emitted to a greater extent from the Northern than fro m the Southern Hemisphere. Correlations with surface Rn-222 measurements in dicate an influence of regional terrestrial sources on open-ocean TGM conce ntrations. Total Hg in precipitation ranged from 10 to 99 pM (volume-weight ed average: 17.8 +/- 2.9 pM), On average, about 72% of the total Hg was "re active" (i.e., reducible by SnCl2). The data showed an apparent rapid nonli near decrease in concentration with event size ("washout curve"). The wet d epositional flux was estimated at 18-36 nmol m(-2) yr(-1) (4-7 mu g m(-2) y r(-1)), which is slightly lower than that found in mid-continental location s of North America (6-12 mu g m(-2) yr(-1)). Pb-210 analyses indicate a str ong impact of particles on rain Hg concentrations. Particle-phase Hg (range 5-25 fmol m(-3); mean 12 +/- 1 fmol m(-3); 66% "reactive") was comparable to values over the Equatorial Pacific. The dry depositional flux is ca, 0.4 nmol m(-2) yr(-1), or 0.4-1.0% of the wet flux. Particle-phase Hg concentr ations did not change significantly when African dust was present during sa mpling. However, the Hg/Al ratios were consistent with crustal values durin g the dust periods. The residence time of TGM was calculated to be 1.3-3.4 yr in this region, based on standing stock estimates. Incubation of rainwat er added to surface seawater gave reduction rates [i.e., production of elem ental Hg (Hg degrees); 1.6-4.3% d of total Hg added] comparable to addition s of inorganic ionic standards, indicating that Hg+2 from precipitation is reduced in a similar manner in surface waters. Thus, precipitation-phase Hg is generally available for evasion to the atmosphere following deposition to the surface ocean, effectively enhancing the mobility and residence time of Hg at the Earth's surface. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.