Influence of rainfall on atmospheric deposition fluxes of ~7Be and ~(210)Pb in Mangaluru (Mangalore) at the Southwest Coast of India

摘要

The depositional fluxes of Be-7 and Pb-210 were measured at Mangalore (12.82 degrees N, 74.92 degrees E) on the South West Coast region, which is one of the highest rainfall regions in India. This region receives pristine air masses from the Southern Indian Ocean (oceanic) and polluted air from Asia (continental). The activity concentration for individual rain events collected during the monsoon season varied between 0.15 and 4.3 Bq L-1 (with geometric mean, GM = 1.1 Bq L-1, geometric standard deviation, GSD = 2.6) and 0.01-0.71 Bq L-1 (GM = 0.12 Bq L-1, GSD = 2.8) for Be-7 and Pb-210, respectively. The GM value for monthly (dry period) Be-7 and Pb-210 concentrations in air were 6.9 mBq m(-3) (GSD = 1.3) and 1.2 mBq m(-3) (GSD = 2.1), respectively. The dry deposition flux of Be-7 varied in the range of 0.081-0.184 Bq m(-2) d(-1) (GM = 0.12 Bq m(-2) d(-1), GSD = 1.3) and that of Pb-210 varied in the range of 0.026-0.293 Bq m(-2) d(-1) (GM = 0.11 Bq M-2 d(-1), GSD = 1.8). The GM value of the wet deposition flux of Be-7 was higher by two orders of magnitude when compared with that of the dry deposition flux, whereas, that in the case of Pb-210 it was higher by an order of magnitude. The mean annual deposition fluxes of Be-7 and Pb-210 were 2026 Bq m(-2) y(-1) and 221 Bq m(-2) y(-1), respectively. The large variability observed in the wet deposition fluxes was attributed to the large variations in the intensity of the rainfall. The wet deposition velocities of the aerosols, estimated based on the concentrations of these radionuclides in air and their corresponding deposition fluxes, were found to be 6.8 cm s(-1) for Be-7 and 6.5 cm s(-1) for Pb-210, Although the two radionuclides originate from distinct sources, a strong positive correlation was observed between the temporal variability of their deposition fluxes. In addition to this, a high degree of correlation was observed between the deposition fluxes of both Be-7 and Pb-210 and rainfall. This points at the dominating effect of scavenging by rainfall in controlling their fluxes.