WET DEPOSITION FLUXES OF COSMOGENIC P-32 AND P-33 AND VARIATIONS IN THE P-33 P-32 RATIOS AT BERMUDA/

A study was undertaken to quantify the supply of cosmogenic P-32 (half life 14.3 days) and P-33 (halflife 25.3 days) on the island of Bermuda in order to use the natural radioisotopes as tracers of the phosphoru s biodynamic in the ocean, The activities of P-32 and P-33 were measur ed in individual precipitation events over a 1 yr period in 1991-1992 at the Bermuda Biological Station for Research (32 degrees 30'N, 64 de grees 40'W). For the first time, wet deposition rates were determined at a marine site and averaged 8.6 +/- 1.5 X 10(-2) dpm cm(-2) yr(-1) f or P-32 and 8.2 +/- 2.0 X 10(-2) dpm cm(-2) yr(-1) for P-33. The (33)p /P-32 activity ratios in individual precipitation events were found to be remarkably constant, ranging from 0.66 to 1.22. The monthly means of the activity ratio in rainwater showed variations about the annual mean of 0.96. Higher ratios were observed during the combined period M arch-April 1991 and January-March 1992, contrasting with the lower rat ios found during July-November 1991. The individual rain samples of th e two contrasting periods belonged to two different populations with m ean activity ratios of 1.02 and 0.89 for the high and low ratio period , respectively. The activity ratios in individual precipitation events allowed determinations of the mean irradiation period based on a simp le non-steady-state model. The deduced irradiation periods averaged 27 days in the July-November 1991 period and 50 days in the combined Mar ch-June 1991 and January-March 1992 period, assuming a well-mixed trop hosphere, efficient scavenging of the aerosols and a P-33/P-32 activit y ratio of 0.7 for the ratio of the tropospheric production rates. The annual average irradiation time was estimated at 40 days, consistent with the calculation of the aerosol residence time in the upper tropos phere. Furthermore, the tropospheric production rates of P-32 and P-33 were determined from the ratios, wet deposition rates, and aerosol re sidence time, and were consistent within the range of previously publi shed estimates.