Atmospheric abundances of nitrogen species in rain and aerosols over a semi-arid region: sources and deposition fluxes

Abstract

The temporal variation for a three-year period (2000-2002) in the atmospheric abundances of principal nitrogen species (NH₄⁺ and NO₃^-) has been studied in rain and aerosols from an urban city (Ahmedabad, 23.0°N, 72.6°E) located in a semi-arid region of western India. Their concentrations in ambient aerosols over the annual seasonal cycle exhibit large variation [NH₄⁺: < 0.001 to 1.3 (GM = 0.25 µ g/m³); NO₃-: 0.09 to 4.4 (GM = 1.3 µ g/m³)]; with systematically higher concentrations during Nov-Feb (drier period) and relatively low during Jun-Aug (wet season). In comparison, abundances of NH₄+ and NO₃- in individual precipitation events (n = 91) collected during the southwest monsoon (Jun-Aug) for three years varied as NH4+: <1.0 to 220 (VWM: 30 µ eq/L) and NO3-: 1.3 to 115 (VWM: 13 µ eq/L). Using corresponding rainfall data, the wet-deposition fluxes of NH4+ during 2000, 2001 and 2002 have been derived: 364, 327 and 297 mg/m²/y, respectively; which are considerably higher than the dry-deposition fluxes (16, 11 and 16 mg/m²/y). The wet deposition of NO₃- over the three years varied as 392, 569 and 487 mg/m2/y in comparison to dry-deposition fluxes averaging as 201, 134 and 137 mg /m²/y. These results also indicate that both dry and wet deposition of NO₃- accounts for as much as ~65% of the total inorganic-N species (NH₄+ + NO₃-), possibly mediated via in-situ chemical reactions of acidic species (NO₃- and its precursors) with mineral aerosols. Such regional scale studies are important for quantitative assessment of atmospheric deposition of biogeochemically important trace species and bring to focus the role of semi-arid regions where current knowledge is limited.