ATMOSPHERIC MERCURY IN NORTHERN WISCONSIN - SOURCES AND SPECIES

The atmospheric chemistry, deposition and transport of mercury (Hg) in the Upper Great Lakes region is being investigated at a near-remote s ampling location in northern Wisconsin. Intensive sampling over two ye ars and various seasons has been completed. A multi-phase collection s trategy (gas-, particle- and precipitation-phases) was employed to gai n insight into the processes controlling concentrations and chemical/p hysical speciation of atmospheric Hg. Additional chemical and physical atmospheric determinations (e.g. ozone, particulate constituents, met eorology) were also made during these periods to aid in the interpreta tion of the Hg determinations. For example, correlations of Hg with oz one, sulfur dioxide and synoptic-scale meteorological features suggest a regionally discernible signal in Hg. Comparison to isosigma backwar d air parcel trajectories confirms this regionality and implicates the areas south southeast and northwest of the site to be sources for Hg. Particle-phase Hg (Hg-p) was found to be approximately 40% in an oxid ized form, or operationally defined as ''reactive''. However, this was quite variable from year-to-year. Hg-p and other particle constituent s (esp, sulfate) show significant correlation and similarity in behavi or (concentration ratios in precipitation and in particles). These obs ervations are part of the growing evidence to support the hypothesis t hat precipitation-phase Hg arises in large part from the scavenging of atmospheric particulates bearing Hg. Observed concentrations of rain and particle-Hg fit broadly the theoretical expectations for nucleatio n and below-cloud scavenging. Significant increases in the Hg/aerosol mass ratio appear to take place during transport. Enrichment of aeroso ls is taken as evidence of gas/particle conversion which could represe nt the step linking gas-phase Hg with rain The refined budget indicate s ca. 24% of total deposition is from summer particle dry deposition, and that this deposition also contributes ca 24% of all reactive Hg de position. Additionally, almost all (86%) deposition (wet and dry) occu rs during the summer months.