Constraints from 210Pb and 7Be on wet deposition and transport in a global three‐dimensional chemical tracer model driven by assimilated meteorological fields

Abstract

The atmospheric distributions of the aerosol tracers 210Pb and 7Be are simulated with a global three‐dimensional model driven by assimilated meteorological observations for 1991–1996 from the NASA Goddard Earth Observing System (GEOS1). The combination of terrigenic 210Pb and cosmogenic 7Be provides a sensitive test of wet deposition and vertical transport in the model. Our simulation of moist transport and removal includes scavenging in wet convective updrafts (40% scavenging efficiency per kilometer of updraft), midlevel entrainment and detrainment, first‐order rainout and washout from both convective anvils and large‐scale precipitation, and cirrus precipitation. Observations from surface sites in specific years are compared to model results for the corresponding meteorological years, and observations from aircraft missions over the Pacific are compared to model results for the days of the flights. Initial simulation of 7Be showed that cross‐tropopause transport in the GEOS1 meteorological fields is too fast by a factor of 3–4. We adjusted the stratospheric 7Be source to correct the tropospheric simulation. Including this correction, we find that the model gives a good simulation of observed 210Pb and 7Be concentrations and deposition fluxes at surface sites worldwide, with no significant global bias and with significant success in reproducing the observed latitudinal and seasonal distributions. We achieve several improvements over previous models; in particular, we reproduce the observed 7Be minimum in the tropics and show that its simulation is sensitive to rainout from convective anvils. Comparisons with aircraft observations up to 12‐km altitude suggest that cirrus precipitation could be important for explaining the low concentrations in the middle and upper troposphere.