Atmospheric deposition of contaminants to the Great Lakes and coastal waters : proceedings from a session at SETAC's 15th annual Meeting, 30 October-3 November 1994, Denver, Colorado
The condensation of gaseous water and its precipitation from the atmosphere in the form of hydrometeors (e.g., fog, rain, snow, sleet) are important processes not only for the global water budget, but also because they bring to terrestrial and aquatic ecosystems other atmospheric species. Not only compounds that have a high affinity toward atmospheric water droplets or soluble aerosol particles are transported, but so are those that are hydrophobic or attracted toward insoluble particulate matter [1]. Nutrients (P, N, C, S), anthropogenically mobilized acids (H2SO4, HNO3), trace elements (Pb, Cd, Cu, As), carbonaceous particulate matter (soot components, secondary organic aerosol), and industrially synthesized and combustion-derived organic contaminants (polychlorinated biphenyls, chlorinated agrochemicals, polycyclic aromatic hydrocarbons) are several examples of species that can be removed from the atmosphere during precipitation events and deposited to the ground or water surfaces. Sources of these compounds include point sources (i.e., fossil fuel combustion products from power generation plants, non-ferrous smelting), area sources (i.e., volatilization from land fills and soils, soil erosion, drift from agrichemical applications, biomass burning), and mobile sources (i.e., motor vehicles). Once released into the atmosphere, contaminants may be partitioned between gaseous and particulate phases. Cloud and precipitation droplets may incorporate both gases and aerosol particles. In this chapter, the uptake and removal mechanisms are discussed together with the speciation of contaminants that they control. Wet deposition is the result of dynamic processes linked with microphysical and chemical processes. While dynamic processes are responsible for the removal of water, gases, and aerosol particles from the troposphere, microphysical processes that occur prior to or during cloud and rain droplet formation, such as gas-particle and gas-water partitioning, condensation, and coalescence, largely control the ground-level concentrations of atmospheric species in precipitation. The incorporation of aerosol particles into precipitation droplets, and the role which particle size plays, is relatively poorly understood yet plays an especially important role in the persistence and removal of airborne contaminants.