Transport and residence times of tropospheric aerosols inferred from a global three-dimensional simulation of 210_Pb

Abstract

A global three‐dimensional model is used to investigate the transport and tropospheric residence time of Pb, an aerosol tracer produced in the atmosphere by radioactive decay of Rn emitted from soils. The model uses meteorological input with 4°×5° horizontal resolution and 4‐hour temporal resolution from the Goddard Institute for Space Studies general circulation model (GCM). It computes aerosol scavenging by convective precipitation as part of the wet convective mass transport operator in order to capture the coupling between vertical transport and rainout. Scavenging in convective precipitation accounts for 74% of the global Pb sink in the model; scavenging in large‐scale precipitation accounts for 12%, and scavenging in dry deposition accounts for 14%. The model captures 63% of the variance of yearly mean Pb concentrations measured at 85 sites around the world with negligible mean bias, lending support to the computation of aerosol scavenging. There are, however, a number of regional and seasonal discrepancies that reflect in part anomalies in GCM precipitation. Computed residence times with respect to deposition for Pb aerosol in the tropospheric column are about 5 days at southern midlatitudes and 10–15 days in the tropics; values at northern midlatitudes vary from about 5 days in winter to 10 days in summer. The residence time of Pb produced in the lowest 0.5 km of atmosphere is on average four times shorter than that of Pb produced in the upper atmosphere. Both model and observations indicate a weaker decrease of Pb concentrations between the continental mixed layer and the free troposphere than is observed for total aerosol concentrations; an explanation is that Rn is transported to high altitudes in wet convective updrafts, while aerosols and soluble precursors of aerosols are scavenged by precipitation in the updrafts. Thus Pb is not simply a tracer of aerosols produced in the continental boundary layer, but also of aerosols derived from insoluble precursors emitted from the surface of continents. One may draw an analogy between Pb and nitrate, whose precursor NO is sparingly soluble, and explain in this manner the strong correlation observed between nitrate and Pb concentrations over the oceans.