DRY DEPOSITION MEASUREMENTS USING WATER AS A RECEPTOR - A CHEMICAL APPROACH

The field measurement of dry deposition still represents a difficult t ask. In our approach, a 1 to 2 cm thick layer of water in a petri dish with a diameter of 22 cm, serves as a surrogate surface. The atmosphe ric constituents taken up by the water can be analyzed chemically by t he same procedure as for the wet deposition samples. In contrast to so lid surrogate surfaces, water exhibits the following advantageous prop erties: low and constant surface resistance, high sticking coefficient for aerosols, and predictable sorption behavior for gases. Consequent ly, the deposition rates measured to the wet surface are generally hig her, by up to a factor of 4 for NH4+, Cl-, NO3- and SO42-, than those to a dry surface, but still smaller than the concurrent wet deposition rates. We observed the following average dry deposition rates in mumo l m-2 d-1: NH4+ 48.3, Ca2+ 40.7, Na+ 15.8, Mg2+ 8.4, K+ 4.2, H-Aci 36. 4; SO42- 57.2, Cl- 39.2, NO3- 34.5, HSO3- 5.7, formate 4.0; acid solub le metals: Fe 2.8, Zn 0.60, Cu 0.11, Pb 0.073, Cd 0.0022. The soluble fraction of Zn, Cd, Cu, Pb and Fe in the dry deposition varied with th e pH of the water phase corresponding to the adsorption tendency of th ese metals to oxide surfaces. The sampling method also allows tracing of regionally and locally emitted atmospheric pollutants. In our study the local pollution sources included road salting, construction work and a refuse incinerator. Finally, chemical reactions occurring in the atmosphere, such as the conversion of Cl- to HCl by HNO3 or the oxida tion of SO2, can be identified by evaluating the data. The method prop osed is relevant to measure reproducibly the dry deposition of a varie ty of compounds to water bodies and moist vegetation.