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

Abstract

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#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 (>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.