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

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