Origins and concentrations of 222Rn, 210Pb, 210Bi and 210Po in the surface air at Lisbon, Portugal, at the Atlantic edge of the European continental landmass

Abstract

Concentrations of 222Rn, 210Pb, 210Bi and 210Po were measured in surface air at Sacavem, near Lisbon, at the junction of the North Atlantic ocean and the European continent. 222Rn concentrations were strongly associated with the origin of the air masses, being very low, ∼0.1 Bq m −3, in maritime air and much higher, 2–10 Bq m −3, in continental air masses. Furthermore, different 222Rn: 210Pb activity ratios were found in those air masses with average values of 1 × 10 3 and 5 × 10 4, respectively. In the short time intervals of sampling, 210Pb concentrations in the atmosphere were positively correlated with aerosol concentrations, suggesting soil resuspension as 210Pb source. However, comparison of 210Pb: 238U activity ratios in aerosols and surface soil particles provided evidence that resuspensions give little contribution, averaging 10%, to 210Pb in surface air. A seasonal fluctuation in the concentration of 210Pb in surface air was observed being higher 210Pb concentrations consistently measured during fall and winter. Nevertheless, concentrations of 210Pb in surface air were not controlled by the seasonal rainfall. Instead, the seasonal fluctuation of 210Pb 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 210Pb at Sacavem, 181 μBq 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 210Pb, 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. 222Rn: 210Pb concentration ratios in those air masses are distinct and provide a means to identify their origin.