Residence Times of Fine Tropospheric Aerosols as Determined by 210Pb Progeny

Fine tropospheric aerosols can play important roles in the radiative balance of the troposphere. The fine aerosols can act directly by scattering and light absorption (particularly in the case of carbonaceous soots), as well as indirectly by serving as cloud condensation nuclei. In addition, aerosols smaller than 2.5 mm have been implicated in health effects of air pollution. Recently, we reported a simple technique for isolation and monitoring of 210Pb (22.6-year half-life) and its progeny 210Bi (5.01-day half-life) and 210Po (138.38-day half-life) using low-level alpha- and beta-counting techniques. This method uses an anion exchange resin filter for separation and direct surface counting. By using a cascade impactor system with Teflon filters, we have been able to collect samples from a number of sites (including Phoenix, AZ, Argonne, IL, Mexico City, Mexico, and Socorro, NM) for intercomparison. The samples can also be counted for 7Be (53.28-day half-life) by gamma counting. 7Be is formed by cosmic ray interactions in the upper troposphere and lower stratosphere and can be useful in further identification of fine aerosols transported over long distances. Our data indicate that many of the fine submicron aerosols have residence times in the range of 20-80 days. This result indicates that long-range transport of fine aerosols is important and that many fine aerosols will be encountered on regional and global scales, particularly fine aerosols that are not water soluble, such as carbonaceous soots. The potential implications of this work for environmental impacts of aerosols will be discussed.