Depositional characteristics of 7Be and 210Pb in southeastern Michigan

Abstract

The depositional fluxes in the bulk and dry fallout as well as the concentrations of 7Be and 210Pb in aerosols were measured for a period of 17 months at Detroit, Michigan. The bulk depositional fluxes of 7Be and 210Pb varied between 3.11 and 63.0 dpm cm−2 yr−1 (mean of 11.4 dpm cm−2 yr−1) and 0.35 and 10.3 dpm cm−2 yr−1 (mean of 1.41 dpm cm−2 yr−1), respectively, and this variability in the depositional fluxes is attributed to the frequency and amount of precipitation and seasonal variations in the depositional fluxes. The dry depositional fluxes of 7Be and 210Pb contributed 2.1–19.8% and 3.6–48.6% of the bulk depositional fluxes, respectively. The higher precipitation‐normalized enrichment factor for 7Be during spring is attributed to the stratosphere‐troposphere exchange of air masses during spring and large‐scale atmospheric mixing during summer. We report that snow is more efficient than rain in removing 210Pb from the atmosphere, and this is likely due to higher surface area of snow flurries compared to the rain droplets. The volume‐weighted concentrations of 7Be and 210Pb are the highest reported in literature so far. The concentration of 7Be in the air showed a seasonal increase during the spring and summer months, and no seasonal variation was observed for 210Pb. The mean deposition velocities of aerosols calculated using concentrations of 7Be and 210Pb in air and precipitation are 1.6 and 1.1 cm s−1, respectively, and the corresponding washout ratios are 948 and 637. There is no significant correlation between the aerosol mass and depositional velocities of 7Be and 210Pb in the air masses. We propose that a relatively minor portion of the aerosols actively participate in the removal of these nuclides from the air masses.