The Sensitivity of Heterogeneous Atmospheric Mercury Processes to Climate Change

Andrew P. Rutter
USDA Forest Service,
Southern Global Change Program
Raleigh, NC USA

The sensitivity of heterogeneous atmospheric mercury physicochemical processes to variance in environmental conditions is only partially understood, but is central to understanding the atmospheric transport and fate of natural and anthropogenic emissions of mercury under future climate scenarios. The importance of this sensitivity lies in the fact that although gaseous elemental mercury consists of greater than 99% of atmospheric mercury away from the influences of urban areas and point sources, reactive mercury (the remaining fraction) is the predominant species that deposits from the atmosphere. Therefore only slight variations in the mass conversion of GEM to RM can have significant impacts on mercury removal from the atmosphere. In order to successfully predict modern and future atmospheric mercury source receptor relationships, the dependence of these physicochemical processes on environmental conditions must be parameterized for use in models.

Under support from the US EPA STAR Program, researchers at UW-Madison and the University of New Hampshire are addressing these needs with a project that is divided into four modules: 1) climate sensitivity measurements of gaseous elemental mercury and reactive dry deposition to a variety of environmental surfaces; 2) measurements of the dependence of aqueous mercury speciation in surrogate and synthetic fog and cloud water on environmental conditions; 3) smog chamber studies of modifications in GEM oxidation by heterogeneous VOC/O3/OH/NOx photochemical reaction systems; and, 4) a comparison between a year of rural and urban hourly measurements of atmospheric GEM and RM concentrations, and corresponding results from CMAQ-Hg updated with the findings from project modules 1-3 and recent peer-reviewed publications. An overview of all four modules will be presented with particular attention given to preliminary results from the dry deposition module.

GEM dry deposition sensitivity to climate variance was evaluated by exposing a variety of environmental surfaces and plants to isotopically enriched GEM in a controlled environment room at the UW-Madison Biotron Facility. Uptake of isotopically enriched GEM from deciduous and conifer trees, grass turf, 3 types of soil, sand, concrete, asphalt, adsorbent coated quartz fiber filter deposition coupons, and a water surface sampler were quantified over several days of exposure that were conducted over a range of irradiances, temperatures and relative humidities. After exposure the mercury was recovered from the samples using thermal and acidic treatments, and then analyzed for the content of the isotopically enriched GEM by high resolution ICPMS. We will explore the performance of these new experimental methods, and present initial findings.

Andrew Rutter, Environmental Chemistry and Technology Program, 660 N. Park St, Madison, WI 53705