Three‐dimensional modeling of the concentration and deposition of 210Pb aerosols

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Abstract
Two aerosol‐borne species, 210Pb and 7Be, were simulated on‐line in a low‐resolution global climate model (ECHAM2). This paper mainly considered 210Pb, which basically has its source in the lower troposphere over continents and its main sink in wet deposition. A companion paper (this issue) discussed 7Be, 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 210Pb 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 7Be and reasonable agreement for 210Pb, increasing the rate of wet scavenging would usually improve the simulation of 210Pb but degrade that of 7Be. It is suggested that explicitly considering the cumulus updrafts and scavenging from the updrafts could preferentially scavenge 210Pb relative to 7Be and thus improve the 210Pb simulation without sacrificing the good 7Be 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.