Residence times and temporal variations of 210Po in aerosols and precipitation from southeastern Michigan, United States

Daughter products of 222Rn (such as 210Pb, half‐life = 22.1 years, and 210Po, half‐life = 138 days) have been widely used as tracers to determine the removal rates of aerosols as well to obtain analog information on the behavior of other chemical species in the lower atmosphere. To establish the first data set on the depositional fluxes of 210Po in bulk precipitation and concentrations in air samples from the midwestern United States, we measured the depositional fluxes of 210Pb and 210Po in the bulk (wet plus dry) and dry fallout by deploying a rain collector (dry and bulk) for a period of 17 months. We also collected 30 aerosol samples during this period to assess the temporal variations of these nuclides in the surface air in Detroit, Michigan. The activity of 210Po in the surface air ranged from below detection limit to 0.118 mBq m−3 (mean of 0.072 mBq m−3). The specific activities of 210Po and 210Pb in aerosols ranged between 0.28 and 4.48 Bq g−1 (mean of 0.87 Bq g−1) and 2.7 and 30 Bq g−1 (mean of 13.6 Bq g−1), respectively; these values are about 1–2 orders of magnitude higher than the surface soil. The mean activity ratios of 210Po/210Pb in the bulk precipitation and dry fallout were found to be 0.049 (n = 27) and 0.25 (n = 8), corresponding to residence times of 15 and 75 days, respectively. Higher values in the dry fallout are attributed to resuspension of very fine older material from the ground. The 210Po/210Pb activity ratios in the aerosols varied between 0 and 0.210 (mean of 0.075), with corresponding residence times of 0–61 days (mean of 22 days). The mean deposition velocity and bulk washout ratio obtained using 210Po is compared with those obtained for 210Pb. From the measured aerosol mass concentrations and calculated values of the depositional velocity, the calculated depositional fluxes of aerosols are reported.