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

We present and discuss results from a 1 year (1991) global simulation of the transport and deposition of Pb-210 With a new size-resolved aerosol transport model. The model accounts for aerosol size distribution and its evolution during transport. Our wet deposition scheme is size-dependent and distinguishes between scavenging by deep and shallow convective rains. It treats separately below-and in-cloud scavenging by synoptic rains. Although the model is formulated to treat all aerosol sizes, the validation was done for the Pb-210 submicronic aerosol for which the main sink is wet deposition. We assess the model transport and deposition of submicron aerosols by a comparison of model results with available surface measurements. Annual mean surface concentrations are compared at 117 stations throughout the globe; seasonal variations are examined for 35 of these sites. The mean bias between simulated and measured yearly averaged surface concentrations is -2.7%, and the correlation coefficient is 0.80. The observed seasonal cycle and the annual mean concentrations are particularly well reproduced, although the model's poor vertical resolution does not capture the strong winter peak at some continental stations, nor the transport to Indian Ocean stations. Using the observed precipitation at or near the sites studied, we were able to explain a large part of the bias in model annual deposition. Deposition at coastal sites deserves also a special treatment since influenced by the land-ocean partition inherent to the model. When we represent correctly these coastal stations, we reduce the mean bias between observed and predicted annual deposition fluxes from 7.7% to 1.2% at 147 stations, and the correlation coefficient improves from 0.70 to 0.77.