Wet deposition in a global size-dependent aerosol transport model 2. Influence of the scavenging scheme on 210Pb vertical profiles, surface concentrations, and deposition

Abstract

The main atmospheric sink for submicron aerosols is wet removal. Lead 210, the radioactive decay product of 222Rn, attaches immediately after being formed to submicron particles. Here we compare the effects of three different wet-scavenging schemes used in global aerosol simulations on the 210Pb aerosol distribution using an off-line, size-resolved, global atmospheric transport model. We highlight the merits and shortcomings of each scavenging scheme at reproducing available measurements, which include concentrations in surface air and deposition, as well as vertical profiles observed over North America and western and central North Pacific. We show that model-measurement comparison of total deposition does not allow to distinguish between scavenging schemes because compensation effects can hide the differences in their respective scavenging efficiencies. Differences in scavenging parameterization affect the aerosol vertical distribution to a much greater extent than the surface concentration. Zonally averaged concentrations at different altitudes derived from the model vary by more than a factor of 3 according to the scavenging formulation, and only one scheme enables us to reproduce reliably the individual profiles observed. This study shows that ground measurements alone are insufficient to validate a global aerosol transport model. Copyright 1998 by the American Geophysical Union.