Atmospheric <SUP>210</SUP>Pb and <SUP>7</SUP>Be in ambient aerosols over low- and high-altitude sites in semiarid region: temporal variability and transport processes

Time series measurements of 210Pb and 7Be made on ambient aerosols, collected from a low-altitude urban site (Ahmedabad, 23.0&#176;N, 72.6&#176;E, 49 m asl) and a high-altitude station (Mt. Abu, 24.6#176;N, 72.7#176;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 (&gt;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 &#177; 0.1 to 6.0 &#177; 0.3; 210Pb, 0.32 &#177; 0.03 to 1.9 &#177; 0.2) and Mt. Abu (7Be, 3.8 &#177; 0.2 to 7.6 &#177; 0.3; 210Pb, 0.39 &#177; 0.05 to 1.8 &#177; 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