A global three‐dimensional model is used to investigate the transport and tropospheric residence time of 210Pb, an aerosol tracer produced in the atmosphere by radioactive decay of 222Rn 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 210Pb 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 210Pb 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 210Pb 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 210Pb produced in the lowest 0.5 km of atmosphere is on average four times shorter than that of 210Pb produced in the upper atmosphere. Both model and observations indicate a weaker decrease of 210Pb concentrations between the continental mixed layer and the free troposphere than is observed for total aerosol concentrations; an explanation is that 222Rn is transported to high altitudes in wet convective updrafts, while aerosols and soluble precursors of aerosols are scavenged by precipitation in the updrafts. Thus 210Pb 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 210Pb and nitrate, whose precursor NOx is sparingly soluble, and explain in this manner the strong correlation observed between nitrate and 210Pb concentrations over the oceans.

A three‐dimensional model is used to simulate the global tropospheric distributions of dimethylsulfide (DMS), SO2, SO42−, and methanesulfonic acid (MSA). The model uses meteorological input from a general circulation model (GCM) developed at the Goddard Institute of Space Studies (GISS) with 4° × 5° horizontal resolution, nine layers in the vertical, and a time resolution of 4 hours. Model results are compared with observations from surface sites, ships, and aircraft. The model reproduces generally to within 30% the observed SO2 and SO42− concentrations over the United States and Europe; these concentrations are highly sensitive to the supply of H2O2 as an in‐cloud SO2 oxidant. Sulfate concentrations and wet deposition fluxes observed at remote marine sites can be accounted for using a global DMS source of 22 Tg S yr−1 in the model. However, this source overestimates DMS air concentrations by a factor of 2 unless we assume the presence of another DMS oxidant besides OH and NO3. Inclusion of another DMS oxidant in our model also improves the simulation of the MSA to SO42− concentration ratio in marine air. Simulated SO42− concentrations in the northern hemispheric free troposphere are much lower than in previous global models and are more consistent with the few observations available. The difference reflects in part our accounting of efficient scavenging of SO2 and SO42− in wet convective updrafts. Global mean tropospheric lifetimes computed in our model are 1.0 days for DMS, 1.2 days for SO2, 3.9 days for SO42−, and 6.2 days for MSA. Fossil fuel combustion and industrial activities represent 68% of global non‐sea‐salt sulfur emissions. About 50% of SO2 globally is converted to SO42− aerosol (principally by in‐cloud oxidation) while the remainder is removed by deposition (30% by dry, 20% by wet). In‐cloud oxidation of SO2 represents 85% of the global SO42− source.

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The factors regulating summertime O3 over the United States and its export to the global atmosphere are examined with a 3‐month simulation using a continental scale, three‐dimensional photochemical model. It is found that reducing NOx emissions by 50% from 1985 levels would decrease rural O3 concentrations over the eastern United States by about 15% under almost all meteorological conditions, while reducing anthropogenic hydrocarbon emissions by 50% would have less than a 4% effect except in the largest urban plumes. The strongly NOx‐limited conditions in the model reflect the dominance of rural areas as sources of O3 on the regional scale. The correlation between O3 concentrations and temperature observed at eastern U.S. sites is attributed in part to the association of high temperatures with regional stagnation, and in part to an actual dependence of O3 production on temperature driven primarily by conversion of NOx to peroxyacetylnitrate (PAN). The net number of O3 molecules produced per molecule of NOx consumed (net O3 production efficiency, accounting for both chemical production and chemical loss of O3) has a mean value of 6.3 in the U.S. boundary layer; it is 3 times higher in the western United States than in the east because of lower NOx concentrations in the west. Approximately 70% of the net chemical production of O3 in the U.S. boundary layer is exported (the rest is deposited). Only 6% of the NOx emitted in the United States is exported out of the U.S. boundary layer as NOx or PAN, but this export contributes disproportionately to total U.S. influence on global tropospheric O3 because of the high O3 production efficiency per unit NOx in the remote troposphere. It is estimated that export of U.S. pollution supplies 8 Gmol O3 d−1 to the global troposphere in summer, including 4 Gmol d−1 from direct export of O3 out of the U.S. boundary layer and 4 Gmol d−1 from production of O3 downwind of the United States due to exported NOx. This U.S. pollution source can be compared to estimates of 18–28 Gmol d−1 for the cross‐tropopause transport of O3 over the entire northern hemisphere in summer.