Wet deposition in a global size-dependent aerosol transport model: 1. Comparison of a 1 year 210 Pb simulation with ground measurements (original) (raw)

1998, Journal of Geophysical Research

Wet deposition in a global size-dependent aerosol transport model 2. Influence of the scavenging scheme on vertical profiles, surface concentrations, and deposition W. Guelle, 1 y. j. Balkanski, 1 j. E. Dibb, 2 M. Schulz, 3 and F. Dulac • 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 2•øPb 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. 1. Introduction Very little has been done to evaluate tropospheric aerosol vertical distribution predicted from models against actual measurements. This represents a clear gap in our ability to adequately represent aerosol mass and number concentrations, to assess their direct radiative effect, and to estimate the heterogeneous reactions that take place at their surfaces. A description of the processes that affect aerosol number concentration necessitates a good representation of the fate of the aerosol in and below clouds. Boucher [1995, p. 87] highlighted the differences that arise in sulfate distributions between the models MO-GUNTIA and IMAGE. Not only are the amplitude of the resulting radiative forcings different, but the tim-1Laboratoire des Sciences du Climat et de l'Environnement, Laboratoire mixte Commissariat k l'Energie Atom