Atmospheric 210Pb and 7Be in ambient aerosols over low- and high-altitude sites in semiarid region: Temporal variability and transport processes

摘要

Time series measurements of 210Pb and 7Be made on ambient aerosols, collected from a low-altitude urban site (Ahmedabad, 23.0°N, 72.6°E, 49 m asl) and a high-altitude station (Mt. Abu, 24.6°N, 72.7°E, 1680 m asl) located in a high-dust semiarid region of western India, reveal characteristic pattern of temporal variability consistent over three years (2000–2002). The relatively high concentrations of 210Pb (>1 mBq m?3) during wintertime (November–February) are dominated by continental air masses from the northeast; whereas lower values during the summer months (April–May) and southwest monsoon season (June–August) are associated with (southwesterly) maritime air. The concentration of 210Pb in individual rain events at Ahmedabad, collected during the southwest monsoon for the same three-year period, also exhibits large variability (range of 3–367 mBq L?1; volume-weighted mean of 74 mBq L?1). This first set of data on the simultaneous measurements in rain and aerosols from a semiarid region is useful in deriving scavenging ratio (SR = 290) of 210Pb. On the basis of data for the three consecutive years (2000–2002), we find that both dry and wet deposition fluxes of 210Pb center around 2–4 mBq cm?2 a?1, suggesting relative dominance of dry deposition in a semiarid region. When used in conjunction with 210Pb, 7Be provides information on the vertical mixing of air masses. During the dry season (January–May and September–December), abundances of 7Be and 210Pb (mBq m?3) in ambient aerosols over Ahmedabad (7Be, 1.9 ± 0.1 to 6.0 ± 0.3; 210Pb, 0.32 ± 0.03 to 1.9 ± 0.2) and Mt. Abu (7Be, 3.8 ± 0.2 to 7.6 ± 0.3; 210Pb, 0.39 ± 0.05 to 1.8 ± 0.2) do not show any covariance, suggesting their usefulness as independent tracers of air masses and pollutants transport. Assuming literature-based constant 222Rn flux and measured abundance of 210Pb in aerosols, a model-based approach has provided a simple way to ascertain residence time of tropospheric aerosols, varying from ～5 days during the dry season and ～2 days in the wet season. These results attempt to fill a major existing gap for the south Asian region under Global Atmospheric Watch program.