Atmospheric residence time of 210Pb determined from the activity ratios with its daughter radionuclides 210Bi and 210Po

Abstract

The residence time of 210Pb created in the atmosphere by the decay of gaseous 222Rn is a key parameter controlling its distribution and fallout onto the landscape. These in turn are key parameters governing the use of this natural radionuclide for dating and interpreting environmental records stored in natural archives such as lake sediments. One of the principal methods for estimating the atmospheric residence time is through measurements of the activities of the daughter radionuclides 210Bi and 210Po, and in particular the 210Bi/210Pb and 210Po/210Pb activity ratios. Calculations used in early empirical studies assumed that these were governed by a simple series of equilibrium equations. This approach does however have two failings; it takes no account of the effect of global circulation on spatial variations in the activity ratios, and no allowance is made for the impact of transport processes across the tropopause. This paper presents a simple model for calculating the distributions of 210Pb, 210Bi and 210Po at northern mid-latitudes (30°–65°N), a region containing almost all the available empirical data. By comparing modelled 210Bi/210Pb activity ratios with empirical data a best estimate for the tropospheric residence time of around 10 days is obtained. This is significantly longer than earlier estimates of between 4 and 7 days. The process whereby 210Pb is transported into the stratosphere when tropospheric concentrations are high and returned from it when they are low, significantly increases the effective residence time in the atmosphere as a whole. The effect of this is to significantly enhance the long range transport of 210Pb from its source locations. The impact is illustrated by calculations showing the distribution of 210Pb fallout versus longitude at northern mid-latitudes.