Excess ²¹⁰Po in the coastal atmosphere

Abstract

Many researchers have reported the widespread occurrence of excess ²¹⁰ Po 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.