Depositional characteristics of 7Be and 210Pb in southeastern Michigan -: art. no. 4210

[1] The depositional fluxes in the bulk and dry fallout as well as the concentrations of Be-7 and Pb-210 in aerosols were measured for a period of 17 months at Detroit, Michigan. The bulk depositional fluxes of Be-7 and Pb-210 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 Be-7 and Pb-210 contributed 2.1-19.8% and 3.6-48.6% of the bulk depositional fluxes, respectively. The higher precipitation-normalized enrichment factor for Be-7 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 Pb-210 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 Be-7 and Pb-210 are the highest reported in literature so far. The concentration of Be-7 in the air showed a seasonal increase during the spring and summer months, and no seasonal variation was observed for Pb-210. The mean deposition velocities of aerosols calculated using concentrations of Be-7 and Pb-210 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 Be-7 and Pb-210 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.