Etude des variations des 7Be, 32P, 90Sr, 210Pb et 210Po dans la troposphère

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Marenco, Alain ; Fontan, Jacques (2011)

Radioactive elements of various origins (7Be, 32P, 90Sr, 210Pb, 210Po) have been measured at Toulouse in South-Western France, for several years (1965 to 1969) in ground-level air with samplings of short duration (1 to 4 days), as well as in precipitations and air in altitude. The study of results shows the very important influence of precipitation scavenging processes upon the variations of activity. As concerns 90Sr, the periodical variation connected with the exchanges between the stratosphere and the troposphere is confirmed, but it is shown that the increase recorded for 7Be and 32P in spring and summer, in ground-level air, is due to the variation in the precipitation scavenging processes and not to contributions from the stratosphere. In the case of 210Pb, moreover, the origin of the air mass must be taken into account. The 90Sr/7Be ratio, allowing to eliminate the influence of scavenging, is quite suitable to characterize the air coming from the stratosphere, but no evidence is found, in ground-level air, of any relation with the possibilities of downward influx from the stratosphere. As concerns the 7Be/32P ratio, the results show the existence of a seasonal variation (maximum in winter) likely due to vertical exchange processes. An important discrepancy is noticed between the theoretical and experimental values of this ratio. The results obtained at Toulouse are in good accordance with those found at the same latitude, but a comparison between the measurements obtained in various stations tends to prove that the vertical exchanges are decreasing in summer above the oceans. The vertical distribution of the 210Po/210Pb ratio, deduced from the measurements in precipitations, leads, for the vertical exchange coefficient Kz, to values of about 3 to 6 m2/s, and for the scavenging coefficient A1 (air-layer between 2 and 6 km), to values of about 1/30 to 1/40 d?1. Lastly, studies by simulation on models confirm the influence of the processes of precipitation scavenging and vertical exchanges.DOI: 10.1111/j.2153-3490.1974.tb01616.x
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