Stable (C and N) isotopic composition of bulk aerosol particles over India and northern Indian Ocean

Atmospheric carbonaceous aerosols of South Asian origin have received immense concerns in the Anthropocene owing to their plausible role in the observed regional to inter-continental scale climate anomalies. Tracking plausible sources and alterations during their transport (secondary processes) are keys to understanding their net influence on regional climate. Here, we report elemental concentrations of C and N (TC and TN), their isotopes (δC, δN) and TC/TN ratios of bulk carbonaceous aerosol particles over northern Indian Ocean [Bay of Bengal (BOB) and Arabian Sea (AS)] collected as a part of integrated campaign of Aerosol Radiation Budget (ICARB) during March-May, 2006. In order to understand typical sources, we also measured same parameters of (i) aerosols emitted from typical bio-fuels conventionally burnt in north and northeastern India and (ii) aerosols particles present in ambient air over selected Indian cities during pre-monsoon season. Bulk aerosols over AS are characterized by significantly higher TC/TN ratios (~50±10) compared to aerosol over Indian cities (5.6±2.6) as well as over BOB (6.8±12.5), most likely due to having significant inorganic carbon contributed by mineral dust. δC of aerosols over AS and BOB do not show significant variation (−25.6‰±0.6, −26.5‰±0.8; n=24 and 21 respectively), however δN values showed a conspicuous difference between the two braches of northern Indian Ocean (10.6‰±2.7 over BOB and 1.4‰±3.3 over the AS). Depleted δN of aerosols over AS can be interpreted in terms of significant mixing of isotopically depleted nitrogenous compounds (NH3 and NOxs) emitted from the underlying (denitrifying) waters.