Factors affecting the measurement of mercury emissions from soils with flux chambers

Air-surface exchange of mercury (Hg) above an arid geothermal area was meas ured with three parallel flux chamber experiments. The different experiment al designs were intercompared with each other, with regard to the magnitude of the measured Hg fluxes and their response to environmental changes. Qua litatively, the measured Hg fluxes agreed well throughout the diurnal. cycl e, and in their response to environmental events and experimental manipulat ions, but quantitatively,there were significant discrepancies between the i ndividual flux results. On average, the three designs yielded Hg fluxes agr eeing within a factor of 2, but even more pronounced differences were obser ved during midday high emission periods and during apparent nighttime depos ition events. The chamber flushing rate appears to have a very significant impact on the measured fluxes and on the response behavior to environmental change. This study demonstrates that both experimental differences and sma ll-scale regional variability introduce large uncertainty in the estimation of natural Hg air surface exchange by different flux chamber techniques. A lso, the impact of environmental parameters on Hg air-surface exchange was studied. Rain events led to a strong increase in the Hg emissions, even whe n the covered soil remained dry, suggesting that the apparent chamber footp rint is larger than the actually covered area. Exclusion of sunlight led to decreases in Hg emissions. Statistical analysis revealed the strongest cor relations between the measured Hg fluxes and radiation and wind speed. Weak er correlations were observed with air and soil temperature and wind direct ion (probably due to local Hg sources). Fluxes were also inversely correlat ed with relative humidity.