Mercury air/surface exchange kinetics of background soils of the Tahquamenon River watershed in the Michigan Upper Peninsula

Air/surface exchange of mercury was investigated over background soils at five sites in the Tahquamenon River watershed in the Michigan Upper Peninsula in the summer of 1998. Measurements of Hg fluxes were performed during midday periods using the ORNL Teflon dynamic flux chamber. Mean Hg emission fluxes were 1.4 +/-0.3-2.4 +/-1.0 ng m(-2) hr(-1) for three shaded forest sites and 7.6 +/-1.7 ng m(-2) hr(-1) for an open field site. Hg dry deposition was observed at a heavily shaded forest site over wet soils (mean = -0.3 +/-0.2 ng m(-2) hr(-1)). The overall mean Hg flux was 1.4 +/-1.4 ng m(-2) hr(-1) for the four shaded forest sites. The Hg fluxes observed at these sites are similar to those found at other northern background sites. Significant, rapid response of Hg emission to solar radiation was observed over these background soils. Artificial irrigation over these soils induced immediate and measurable increases in Hg emission. Soil temperature was found to be less influential to Hg air/surface exchange over these heavily shaded forest background soils than we have seen elsewhere.