RADON DAUGHTER DISEQUILIBRIA AND LEAD SYSTEMATICS IN THE WESTERN NORTH-ATLANTIC

Concentrations of Rn-222 and (210)pb Were measured in the North Atlant ic troposphere in 1989 between April 12 and 28, during the Sulfide Exp eriment (SEX) Cruise I, and those of Rn-222, Pb-210 and Po-210, betwee n October 24 and November 9, during the SEX Cruise II. Concentrations of Pb-210 and Po-210 were also measured in the rain water, surface sea water, and marine microlayer collected during the SEX Cruise II. Other data used and published previously include stable lead and its isotop es [Veron et al., 1992; 1993] on parallel samples. Low R-222, contents , Of the order of 0.1 and 0.3 Bq m(-3), were found in the marine air: while continental air showed nearly 10 times higher concentrations of Rn-222. These results corroborate with the air mass trajectory analyse s and continental signatures of stable lead isotopes. Significant corr elation is found between Rn-222 and Pb-210 on the aerosol: indicative of excess continental Rn-222 Supporting the in growth of Pb-210 from t he atmosphere, in spite of its first-order removal by precipitation. C orrelation between Pb-210 and stable Pb on the aerosol and in the prec ipitation document the source of pollutant lead from the continental s urface. Mean residence times of marine aerosol based on Pb-210 is esti mated to be 5.4 +/- 1.8 days during the April cruise and 19.7+/-1.9 da ys during the October cruise. Corresponding deposition velocity for Pb -210 is estimated to be 1.9+/-1.9 cm s(-1)? a value that suggests the dominant role of precipitation scavenging, or aerosol scavenging by la rger host phases such as dust or sea salt. Excess Po-210 activities ar e found on the aerosol relative to what would be expected based on Pb- 210 and the aerosol residence times. In surface seawaters, deficiencie s of?Po-210 are observed. Mechanisms of Po-210 enrichment in atmospher ic aerosol may include enrichments from the organic components of mari ne microlayer, sea-salt aerosol, dust, or air-sea exchange of volatile organo-polonium species.