Gaseous elemental mercury exchange with low mercury containing soils: Investigation of controlling factors

Deposition of atmospheric elemental Hg (Hg0) to soils may be an important pathway for the transfer of Hg0 to terrestrial ecosystems. In this study a laboratory method was applied to investigate the role of soils with natural background Hg concentrations (<0.1 μg/g) as a source or sink for atmospheric Hg0 and to identify factors influencing Hg0 exchange between these soils and air. Air–soil Hg0 exchange was measured for a variety of dry substrates (8 pure soil constituents and 35 soils) under controlled experimental conditions. Fluxes measured using the pure soil constituents indicated that the mineralogical nature of the soil particles may play an important role in the sorption of atmospheric Hg0. In individual tests for 26 of the 35 natural soils, statistically significant linear correlations were found between Hg0 flux and air Hg0 concentration. Mercury flux under light conditions was typically higher than that in the dark, and soil air compensation points (CP, the air Hg0 concentrations at which there is no net Hg0 exchange) were significantly higher under light exposure. When all soil data were combined, at low air concentrations (2.8 ± 0.8 ng/m3) soils emitted Hg0 to the air in light conditions (mean flux: 1.3 ± 1.0 ng/m2 h) and adsorbed Hg0 in dark conditions (mean flux: −1.1 ± 1.2 ng/m2 h); while at elevated air Hg0 concentration (5.8 ± 1.0 ng/m3) deposition was the dominant flux (mean flux of −2.1 ± 1.6 and −4.6 ± 1.25 ng/m2 h in the light and dark, respectively). At air Hg0 concentrations similar to the ambient air (<5 ng/m3; 1.7 ± 1.6 ng/m3), Hg0 concentration in the air, light, and soil Hg concentration were significantly correlated with air–soil Hg0 exchange, while at higher air Hg0 concentrations (greater-or-equal, slanted5 ng/m3; 8.2 ± 2.2 ng/m3) that might be found in urban areas, soil Hg concentration, pH, and organic matter were the primary factors correlated with Hg0 flux. This study indicates that natural background soil may be a source or sink of atmospheric Hg0 depending on environmental parameters and soil physical and chemical properties.