Elevated nitrogen isotope ratios of tropical Indian aerosols from Chennai : Implication for the origins of aerosol nitrogen in South and Southeast Asia

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To better understand the origins of aerosol nitrogen, we measured concentrations of total nitrogen (TN) and its isotope ratios (δ15N) in tropical Indian aerosols (PM10) collected from Chennai (13.04° N; 80.17° E) on day- and night-time basis in winter and summer 2007. We found high δ15N values (+15.7 to +31.2‰) of aerosol N (0.3-3.8 μg m-3), in which NH4+ is the major species (78%) with lesser contribution from NO3- (6%). Based on the comparison of δ15N in Chennai aerosols with those reported for atmospheric aerosols from mid-latitudes and for the particles emitted from point sources (including a laboratory study), as well as the δ15N ratios of cow-dung samples (this study), we found that the atmospheric aerosol N in Chennai has two major sources; animal excreta and bio-fuel/biomass burning from South and Southeast Asia. We demonstrate that a gas-to-particle conversion of NH3 to NH4HSO4 and (NH4)2SO4 and the subsequent exchange reaction between NH3 and NH4+ are responsible for the isotopic enrichment of 15N in aerosol nitrogen.