S. Anand

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

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, Vg, 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 cms−1) relative to 210Pb (0.14 ± 0.07 cm s−1). The higher Vg values for 210Po could be due to complementary sources of 210Po, such as resuspension of soil particles, etc., in addition to atmospheric 222Rn. n nA source apportionment has also been carried out for atmospheric 210Po using the 210Po/210Pb activity ratios, the Vg 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 Vg values from surrogate collectors reported here are useful as indices of deposition to other surfaces and give an idea of the relative magnitudes of Vg values for various species.

Coagulation effect on the activity size distributions of long lived radon progeny aerosols and its application to atmospheric residence time estimation techniques

The long lived naturally occurring radon progeny species in the atmosphere, namely (210)Pb, (210)Bi and (210)Po, have been used as important tracers for understanding the atmospheric mixing processes and estimating aerosol residence times. Several observations in the past have shown that the activity size distribution of these species peaks at larger particle sizes as compared to the short lived radon progeny species - an effect that has been attributed to the process of coagulation of the background aerosols to which they are attached. To address this issue, a mathematical equation is derived for the activity-size distribution of tracer species by formulating a generalized distribution function for the number of tracer atoms present in coagulating background particles in the presence of radioactive decay and removal. A set of these equations is numerically solved for the progeny chain using Fuchs coagulation kernel combined with a realistic steady-state aerosol size spectrum that includes nucleation, accumulation and coarse mode components. The important findings are: (i) larger shifts in the modal sizes of (210)Pb and (210)Po at higher aerosol concentrations such as that found in certain Asian urban regions (ii) enrichment of tracer specific activity on particles as compared to that predicted by pure attachment laws (iii) sharp decline of daughter-to-parent activity ratios for decreasing particle sizes. The implication of the results to size-fractionated residence time estimation techniques is highlighted. A coagulation corrected graphical approach is presented for estimating the residence times from the size-segregated activity ratios of (210)Bi and (210)Po with respect to (210)Pb. The discrepancy between the residence times predicted by conventional formula and the coagulation corrected approach for specified activity ratios increases at higher atmospheric aerosol number concentrations (>10(10) #/m(3)) for smaller sizes (<1 μm). The results are further discussed.

Ventilation dependence of concentration metrics of Ultra-fine Particles in a coagulating household smoke

Abstract Role of Ultra-fine Particles (UFPs) in causing adverse health effects among large population across the world, attributable to household smoke, is being increasingly recognized. However, there is very little theoretical perspective available on the complex behavior of the UFP metrics with respect to controlling factors, such as ventilation rate and particle emission rate from the combustion sources. This numerical study examines through coagulation dynamics, the dependence of UFP metrics, viz., number (PN), mass (PM0.1) and surface area (PA0.1) concentrations below 0.1u2009μm diameter, on ventilation and the number emission rate from household smoke. For strong sources, the steady-state concentrations of these metrics are found to increase initially with increasing Air Exchange Rate (AER), reach a peak value and then decrease. Counter correlations are seen between UFP metric and PM2.5 concentrations. The concepts of Critical Air Exchange Rate (CAER) and Half-Value Air Exchange Rate (HaVAER) have been introduced which indicate a feasibility of mitigation of PM0.1 and PA0.1, unlike PN, by ventilation techniques. The study clearly brings forth complex differential behavior of the three UFP metrics. The results are further discussed.