3-DIMENSIONAL MODELING OF THE CONCENTRATION AND DEPOSITION OF PB-210 AEROSOLS

Two aerosol-borne species, Pb-210 and Be-7, were simulated on-line in a low-resolution global climate model (ECHAM2). This paper mainly considered Pb-210, which basically has its source in the lower troposphere over continents and its main sink in wet deposition. A companion paper (this issue) discussed Be-7, whose source is in the upper troposphere and whose primary sink is also in wet deposition. In order to test a simple wet scavenging parameterization in which the depletion of aerosol in a grid cell depends on the condensation rate in the grid cell, we compared simulated and observed annual average values of Pb-210 concentration and deposition: the monthly average surface concentration is generally well simulated; the correlation coefficient is r = 0.79 for annual average concentration and r = 0.63 for annual average deposition, although globally the concentration averages 40% high and the deposition 18% low. Because the scavenging parameters were tuned to give good results for Be-7 and reasonable agreement for Pb-210, increasing the rate of wet scavenging would usually improve the simulation of Pb-210 but degrade that of Be-7. It is suggested that explicitly considering the cumulus updrafts and scavenging from the updrafts could preferentially scavenge Pb-210 relative to Be-7 and thus improve the Pb-210 simulation without sacrificing the good Be-7 agreement. On the other hand, this paper and its companion show that one can often describe the deposition and surface concentration to within about 20% for species with sources in the lower and upper troposphere with a simple "local" (in the vertical) wet scavenging parameterization. Poorer results should be expected, however, if we had tried to match simultaneously the observed concentration and deposition of a species with a true surface source and one with a true stratospheric source.