Atmospheric fluxes of 7Be and 210Pb on monthly time-scales and during rainfall events at Stony Brook, New York (USA)

Abstract

The particle-reactive radionuclides 7Be and 210Pb have been employed extensively as tracers and chronometers for a variety of aquatic and terrestrial processes. Both radionuclides are delivered to the Earth's surface from the atmosphere, and in order to use them effectively as natural tracers, an understanding of variations in atmospheric fluxes of these radionuclides due to latitudinal differences and storm events is required. The monthly atmospheric fluxes of 7Be and 210Pb, measured from April-2008 to December-2009 at Stony Brook, NY, ranged from 67 to 385 Bq m−2 and 6.7 to 16.7 Bq m−2, respectively. Composite annual atmospheric fluxes over the sampling period were 3110 ± 1200 Bq m−2 y−1 for 7Be and 146 ± 50 Bq m−2 y−1 for 210Pb and were similar to geographically comparable sites. The monthly atmospheric fluxes of 7Be and 210Pb were significantly correlated with rainfall. The 7Be/210Pb ratio in the monthly samples varied seasonally, with values of ∼10–11 during the winter months and ∼20–28 during the spring – fall. The pattern of seasonal variation in 7Be fluxes and 7Be/210Pb ratios is most consistent with that observed in surface air at continental sites in which more frequent deep convective storms occur during the summer and therefore result in an increased transport of 7Be from the upper troposphere to the Earth's surface. An additional factor may be that the winds at Stony Brook were dominantly from the northwest during the winter of 2009 and so were characterized by low 7Be/210Pb ratios while in the spring, winds from the southwest brought marine air with higher 7Be/210Pb ratios to the sampling site. Fluxes of 7Be and 210Pb also were measured over two long (16–24 h) and two short (∼1 h) intense periods of rainfall in June and July 2009. Fluxes of 7Be and 210Pb continued throughout the short events, but the two radionuclides showed different patterns during the long events. While the entirety of the 210Pb flux accumulated during the first ∼1 h, 7Be continued to be accumulated over the course of both long events. The 7Be/210Pb ratio ranged from 20 to ∼300 during the events and, in general, was considerably greater than the ratios measured in the monthly samples. Radar image snapshots taken during the events show cloud heights of 7–10.5 km, and the 7Be/210Pb ratios measured in the precipitation are consistent with previous model estimates of 7Be/210Pb ratios in aerosols at altitudes of 5–10 km (latitude 40°N). The data suggest that 210Pb can be effectively stripped from the lower troposphere early in a rainfall event, but intense convective mixing and scavenging of 7Be from the upper troposphere results in a continuous flux of 7Be and elevated 7Be/210Pb ratios as the event progresses.