Studies on the activity ratios of polonium-210 to lead-210 and their dry-deposition velocities at Bombay in India

Abstract

Atmospheric concentrations and dry fallout of 210Pb and 210Po have been measured at Bombay in India during 1985–1987 using air samplers and surrogate collectors respectively. The objective of these measurements was to compare the 210Po/ 210Pb activity ratios in surface air and dry fallout and to estimate the dry-deposition velocity, V g , of the atmospheric particulates carrying these two radionuclides. The location of Bombay is eminently suitable for such studies in view of its continuous dry period of about eight months. An analysis of the collected data shows that the average 210Po/ 210Pb activity ratio in dry fallout (0·47 ± 0·39) is considerably higher than that in surface air (0·12 ± 0·08). This has been attributed to the higher values for 210Po (0.76 ± 0.48 cm s −1) relative to 210Pb (0.14 ± 0.07 cm s −1). The higher V g values for 210Po could be due to complementary sources of 210Po, such as resuspension of soil particles, etc., in addition to atmospheric 222Rn. A source apportionment has also been carried out for atmospheric 210Po using the 210Po/ 210Pb activity ratios, the V g values for a range of radionuclides and their estimated atmospheric residence times. This calculation indicates that only about 17 ± 13% of 210Po may derive from atmospheric 222Rn, while 25 ± 10% is from soil and the rest from other sources. The V g values from surrogate collectors reported here are useful as indices of deposition to other surfaces and give an idea of the relative magnitudes of V g values for various species.