ORIGINS AND CONCENTRATIONS OF RN-222, PB-210, BI-210 AND PO-210 IN THE SURFACE AIR AT LISBON, PORTUGAL, AT THE ATLANTIC EDGE OF THE EUROPEAN CONTINENTAL LANDMASS

Concentrations of Rn-222, Pb-210, Bi-210 and Po-210 were measured in surface air at Sacavem, near Lisbon, at the junction of the North Atlantic ocean and the European continent. Rn-222 concentrations were strongly associated with the origin of the air masses, being very low, similar to 0.1 Bq m(-3) in maritime air and much higher, 2-10 Bq m(-3), in continental air masses. Furthermore, different Rn-222:Pb-210 activity ratios were found in those air masses with average values of 1 x 10(3) and 5 x 10(4), respectively. In the short time intervals of sampling, Pb-210 concentrations in the atmosphere were positively correlated with aerosol concentrations, suggesting soil resuspension as Pb-210 source. However, comparison of Pb-210:U-238 activity ratios in aerosols and surface soil particles provided evidence that resuspensions give little contribution, averaging 10%, to Pb-210 in surface air. A seasonal fluctuation in the concentration of Pb-210 in surface air was observed being higher Pb-210 concentrations consistently measured during fall and winter. Nevertheless, concentrations of Pb-210 in surface air were not controlled by the seasonal rainfall. Instead, the seasonal fluctuation of Pb-210 corresponds to the seasonal influence on the region of air masses with different origins: predominantly, of continental origin in fall and winter and of maritime origin during spring and summer. The annual average of Pb-210 at Sacavem, 181 mu Bq m(-3), is substantially lower than values reported for inland locations, attesting for the strong influence of oceanic air masses. The mean residence time (MRT) of aerosols, 3.8 d, was found comparable to values reported for other regions, but data further suggest different MRTs with the seasons of the year. It is concluded that at Sacavem, the concentrations of radon and radon daughters, particularly those of Pb-210, are controlled mainly by large-scale meteorological conditions, especially through the alternate intrusion of either oceanic or continental air masses into the local surface air. Rn-222:Pb-210 concentration ratios in those air masses are distinct and provide a means to identify their origin.