Measurements are reported of 210Pb activities in aerosols collected during the winter of 1985--1986 at Davis, located in California's Central Valley, and of 210Pb and 137Cs inventories in soils collected from the nearby Sierra Nevada mountains and Sierra Nevada-Central Valley foothills. The results show that Centra Valley surface air 210Pb activities increase (up to 98 disintegrations per min (dpm) 1000 m-3 STP) while high-pressure systems are stationed over the western United States. During these periods the processes that control 210Pb activities in the boundary layer are the emanation of 222Rn from soils and, most likely, the exchange of air between the Central Valley boundary layer and the free troposphere.

Model calculations indicate that the mean residence time of air in the Central Valley boundary layer during these periods ranges from 1 day to 4.5 days, depending upon the intensity of the regional high. Furthermore, because 210Pb is not removed efficiently from the boundary layer by other processes, these results indicate that as much as 90% of the 210Pb produced as a result of the decay of 222Rn emanating from Central Valley soils eventually escapes from the Central Valley to be deposited elsewhere, even though the boundary layer is relatively stagnant. Thus it is expected that a similar percentage of other species characterized by gas-to-particle conversion rates of about 3 days or longer (e.g., SO4=from SO2) will also eventually escape from the Central Valley to be deposited elsewhere. During the passage of cold fronts, surface air 210Pb activities decrease dramatically (to as low as 6 dpm 1000 m-3 STP). The study of 210Pb and 137Cs inventories in Central Valley-Sierra Nevada foothill soils and in coastal soils indicates that this decrease is a result of marine air displacing the stagnant air in the Central Valley boundary layer and not a result of local washout of 210Pb. In the Sierra Nevada the ratio of the soil inventories of atmospherically derived 210Pb to 137Cs increases with elevation. This increase indicates that 210Pb derived recently from the continental boundary layer is preferentially deposited at higher elevations in the Sierra Nevada. A simple mass balance calculation indicates that between 28 and 53% of the 210Pb deposited in the Sierra Nevada is derived recently from the continental boundary layer. Together, the surface air and soil studies suggest that a large fractio of this 210Pb was transported from the Central Valley boundary layer during the passage of precipitating fronts. ¿ American Geophysical Union 1989