Isotopic Composition of Mercury in the Atmosphere

The cardinal role of the atmosphere in the global dispersion of mercury and its deposition in aquatic environments and on land is well established. Re-emission of mercury from waters and land is the major source of atmospheric mercury. If isotopic variations in Hg are to be used in developing a better understanding of the mercury cycle, experimental studies of the isotopic effects of those processes that induce mercury transformations from one species to another and its transition in and out of the atmosphere are critical. Knowledge of the isotopic composition of atmospheric mercury is also a key. This study has attempted to determine the isotopic signature of Hg in the atmosphere to provide an anchor for geochemical models of mercury. The residence time of Hg0 which comprises of approximately 98% of the total mercury in the atmosphere has been variously estimated to be between 0.6 and 2.0 years. Both the horizontal and vertical mixing times of the troposphere are approximately one month (vertical mixing can be much faster). Accordingly, we suspect that atmospheric mercury might closely approach isotopic homogeneity on a hemispherical scale (mixing times between the northern and southern hemispheres might well exceed a year). We have analyzed atmospheric mercury collected in our local area along the Gulf of Mexico coast at various times, elsewhere in the U.S. and at NOAA Global Baseline Observatories. Mercury is trapped on gold plated sand in a quartz tube. Air is passed through this gold trap by a pump, and a flow meter is used to monitor the flow rate. A desiccant (CaSO4) is used to absorb the moisture from the air and does not interfere with the Hg collection. To ensure that anthropogenic input was at it its minimum, sampling was not conducted in industrialized areas, or near coal powered plants and remote locations were preferred. Once the sampling is complete, mercury is thermally purged from the gold trap, and carried by a helium stream into concentrated nitric acid containing chloride, where it is completely oxidized to Hg (II). Hg (II) is reduced in a CETAC HGX 200 Cold vapor generator, and the evolving cold mercury vapor is analyzed in a NEPTUNE MC-ICP-MS. To minimize the effects of instrumental fractionation, isotope ratios were determined by sample standard bracketing technique and reported in and #61540; (') notation relative to NIST SRM 3133. Where as and #61540;AHg = ', where R= . Samples of atmospheric mercury exhibit a mass dependent fractionation effect with light isotope enrichment in 198Hg/200Hg ratios (relative to NIST-SRM 3133) of the order of 1.3 0.8 ' (2 and #963;), and small but reproducible mass independent fractionation effects with positive 199Hg and 201Hg anomalies of +0.1 to +0.3 '. From this study it is concluded that the atmosphere comes close to an isotopically uniform reservoir of mercury (IUR)Hg, on which local and temporary effects are likely imposed. This (IUR)Hg provides an anchor around which models of the global mercury cycle could be constrained. In atmospheric waters and precipitation, mercury is present in the form Hg (II). Rainwater was collected in Tallahassee, FL for isotopic analysis of mercury. Due to low concentration of mercury in rainwater, samples were pre-concentrated by adding chloride and subsequent evaporation. All samples analyzed show a small positive odd isotope anomaly of the order of 0.213 +/- 0.28 (2 and #963;) ' for 199Hg and 0.18 +/- 0.24 (2 and #963;) ' for 201Hg. A network of Tillandsia usneoides (common name: Spanish Moss), an epiphyte, was collected along the eastern Coastal Plain of the U.S. from northern Florida to North Carolina and analyzed for its isotopic composition to determine its effectiveness as a natural monitor of isotopic composition of the atmosphere in which it grows. The Spanish moss exhibits a clear negative mass independent isotope effect which is distinct from the isotopic composition of the Hg-vapor in the ambient air and Hg (II) in the rain measured. Moreover all samples plot within analytical uncertainty of the theoretical fractionation line for magnetic isotope effect with a and #916;201Hg/ and #916;199Hg ratio of 1:1.1. This clearly indicates towards a biological origin of the isotope effect seen in the moss.