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

Mohan, M.P., D'Souza, Renita Shiny, Nayak, S. Rashmi, Kamath, Srinivas S., Shetty, Trilochana, Kumara, K. Sudeep, Mayya, Y.S., Karunakara, N.
Atmospheric environment 2019 v.202 pp. 281-295
aerosols, air, air pollution, coasts, dry deposition, geometry, lead, monsoon season, radionuclides, rain, standard deviation, temporal variation, wet deposition, India, Indian Ocean
The depositional fluxes of 7Be and 210Pb were measured at Mangalore (12.82° N, 74.92° 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 7Be and 210Pb, respectively. The GM value for monthly (dry period) 7Be and 210Pb 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 7Be 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 210Pb 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 7Be was higher by two orders of magnitude when compared with that of the dry deposition flux, whereas, that in the case of 210Pb it was higher by an order of magnitude. The mean annual deposition fluxes of 7Be and 210Pb 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 7Be and 6.5 cm s−1 for 210Pb. 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 7Be and 210Pb and rainfall. This points at the dominating effect of scavenging by rainfall in controlling their fluxes.