214Bi/214Pb radioactivity ratio in rainwater for residence time estimation of cloud droplets and raindrops

Abstract

Continuous observation of radioactivity ratio of (214)Bi to (214)Pb as tracers in rainwater was carried out with 15-min cyclic measurement at Nagoya, Japan, from August 2011 to July 2012 to characterise wet deposition. The ratio ranged from 0.55 to 3.82. Based on the observation and a simplified time evolution model of (214)Pb and (214)Bi radioactivities during rainwater formation, the 'age' of raindrops ranged from sub-10 to several 100 min. The age would have negative correlation to rainfall intensity, but its variation for lower rainfall intensities was quite large. Secular equilibrium of (214)Pb and (214)Bi radioactivities in cloud water with airborne radon did not hold for some rainfall events.