Measurements of Natural Radioactivity in Submicron Aerosols in the Pittsburgh Area.

Natural radionuclides can be useful in evaluating the transport of ozone and aerosols in the troposphere. We have used natural radioactivity to estimate apparent residence times for submicron aerosols in the troposphere by looking at the disequilibrium of lead-210 with its daughters bismuth-210 (5-day half-life) and polonium-210 (138-day half-life). We have also measured the activity of beryllium-7 on fine aerosols to examine potential upper-air transport into the tropospheric boundary layer. Two sites in the Pittsburgh area during the summer of 2001 were sampled for beryllium-7, lead-210, bismuth-210, and polonium-210 on fine aerosols by using a Sierra Impactor (Stage 4) that allowed a nominal 1-micrometer cutoff diameter. One site was located approximately 5 km east of downtown Pittsburgh, on a rooftop next to Schenley Park (40.4395o N latitude and 79.9405o W longitude, elevation 310 m). A second site was located at the U.S. Department of Energy National Energy Technology Laboratory (NETL) Ambient Air Monitoring Station, approximately 15 km south of downtown Pittsburgh (40.30655 deg N latitude and 79.9794 deg W longitude, elevation 325 m). At both sites, 24-hour samples were collected from July 22 to July 30, 2001. Nine samples were taken at the NETL site, and six were taken at the Schenley Park site. The method for determining the lead-210 and its daughters will be described briefly. Apparent residence times ranged from 10-46 days, with an average of 23 days. Data indicate that little wind-blown soil or dust affected the area during the study. The importance of fine aerosol transport will be emphasized. The authors wish to thank Donald Martello of the NETL and Natalie J. Anderson of Carnegie Mellon University for sample collection. This work was supported by the United States Department of Energy, Atmospheric Chemistry Program.