Dry deposition fluxes and mass size distributions of trace elements measured near southern Lake Michigan

Dry deposition is one of the most important processes controlling the atmospheric input of trace elements to large water bodies. As part of the multi-university Atmospheric Exchange Over Lakes and Oceans Study (AEOLOS) the dry deposition fluxes and atmospheric size distributions (SDs) of trace elements were measured over the southern basin of Lake Michigan, USA during summer and fall of 1994 and winter of 1995, concurrently, in Chicago, IL, over Lake Michigan, onboard the USEPA R/V Lake Guardian, and in South Haven, MI. Trace elements included in this study were Pb, Cu, and Zn which are primarily of anthropogenic origin and Al and Mg which are primarily of crustal origin. The flux of the crustal elements were at least an order of magnitude larger than anthropogenic elements. The average fluxes ranged from 0.003 mg/m(2)-day for Pb over Lake Michigan to 5.32 mg/m(2)-day for Mg in Chicago. When the wind was from Chicago over the lake, the fluxes measured over the lake were larger than when the wind was from other directions. Size distributions were also strongly influenced by wind direction, when the wind was from the north, the SDs had relatively flat profiles, and concentrations were smaller than when the wind was from over the land. Concentrations in the coarse fraction were high in Chicago compared to the SDs measured over the lake.