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

To better understand the origins of aerosol nitrogen, we measured concentrations of total nitrogen (TN) and its isotope ratios (δ 15N) in tropical Indian aerosols (PM 10) collected from Chennai (13.04°N; 80.17°E) on day- and night-time basis in winter and summer 2007. We found high δ 15N values (+15.7 to +31.2‰) of aerosol N (0.3–3.8 μg m −3), in which NH 4 + is the major species (78%) with lesser contribution from NO 3 − (6%). Based on the comparison of δ 15N 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 δ 15N 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 NH 3 to NH 4HSO 4 and (NH 4) 2SO 4 and the subsequent exchange reaction between NH 3 and NH 4 + are responsible for the isotopic enrichment of 15N in aerosol nitrogen.