MODELING THE ATMOSPHERIC TRANSPORT OF TRACE-METALS INCLUDING THE ROLE OF PRECIPITATING CLOUDS

A three-dimensional mesoscale model with parameterized microphysics of clouds and precipitation has been extended to include wet scavenging and deposition of trace metals from the atmosphere. As a sample case the wet removal of atmospheric lead has been chosen. Simulations were carried out in a domain of 200 x 200 km2 in the southeastern part of the North Sea including many clouds in various stages of development. The basic assumption is that the particles acting as host for the lead particles are cloud condensation nuclei (CCN) calculated in the model as part of aerosol particles in the air. The variation of the average concentration of aerosol particles with height as a function of size is based on data given in Pruppacher and Klett (1978, Microphysics of Clouds and Precipitation, Reidel, Dordrecht). Both calculated (long range transport model) and measured atmospheric monthly mean concentration of lead at the German coast of North Sea and Baltic Sea given by Petersen et al. (1989, NATO) and StoBel (1987, Ext. Rep. GKSS 87/E/34) are used as an input pollution level by calculation of the vertical profile of the initial mean mass lead concentration in the air. The lead is assumed to be distributed uniformly on the aerosol particles, and the vertical profile of the initial mass concentration of lead in the air is assumed to be proportional to the vertical profile of number of aerosol particles as a function of size with height. In other words, in the model we deal with that fraction of lead, which can enter cloud water mainly through nucleation scavenging (in-cloud scavenging, 'rainout'). The calculated concentration of lead in rainwater at the first model layer (50 m above the sea surface) is in good agreement with measured weekly averages of lead deposition fluxes during precipitation events on the island of Pellworm in the vicinity of the model domain selected for validation of the numerical model.