COMPUTING HEAVY-METALS IN EUROPE ATMOSPHERE .1. MODEL DEVELOPMENT AND TESTING

This paper presents the development of the TRACE model (TRace toxic Air Concentrations in Europe) which computes the air concentration and deposition of various heavy metals (As, Cd, Pb, Zn) on a European scale. TRACE is an improved climatological-type model in that (1) travel time is computed from an empirical function rather than from an assumed constant velocity, (2) the model tends to conserve mass, (3) the irregularity of spatial deposition patterns is captured, and (4) parameters are objectively determined. The dry deposition velocity is spatially varying, and is computed with a dry deposition model as a function of ''local'' u*, z(o), together with an assumed characteristic particle size distribution. The model has been used to compute levels of heavy metals for 1978-1985 throughout Europe. Calculations agree with As and Pb observations within a factor of two, and underestimate Cd and Zn observations. Using the model it was estimated that wet deposition exceeds dry deposition in most of central Europe. The mean residence time of the mass of As, Cd, Pb and Zn in Europe's lower atmosphere is estimated to be about 64 h and for Pb, 96 h.