Excess 210Po in the coastal atmosphere

Abstract

Many researchers have reported the widespread occurrence of excess 210Po in the global atmosphere and suggested probable sources such as resuspension of top soils, stratospheric aerosols, sea spray of the surface micro-layer, volcanic emission, and bio-volatile 210Po species from the productive ocean. We have observed excess 210Po on aerosols in the coastal atmosphere of the Chesapeake and Delaware Bays. On-board measurements in the Chesapeake Bay atmosphere show that the increase of this excess 210Po is dependent upon wind speed. Simultaneously measured activity ratios of 7Be/210Pb and 210Pb/222Rn argue against either higher altitude air or continental soils as the source of this excess. We hypothesize that the excess 210Po originates mainly from surface waters either by the sea-spray of the surface microlayer, or more likely, by gas exchange. We conclude gas exchange as the mechanism since the polonium excess increases linearly with wind speed over a threshold of 3 m s-1 (mean) similar to other gases (i.e., CO2, SF6, and DMS). In addition, higher 210Po excess with lower 222Rn is observed in on-shore marine air at Lewes, DE. This suggests sea-air exchange of volatile Po along with other bio-volatile species (i.e., DMS, DMSe, and MMHg) in the coastal productive ocean during high wind speeds.