Stable carbon and nitrogen isotopic composition of fine mode aerosols (PM2.5) over the Bay of Bengal: impact of continental sources

Abstract

This study reports on stable carbon (δ13CTC) and nitrogen (δ15NTN) isotopic composition of total carbon and nitrogen (TC and TN) in the fine mode aerosols (PM2.5; N=31) collected over the Bay of Bengal (BoB). The samples represent two distinct wind regimes during the cruise (27 December 2008–28 January 2009); one from the Indo-Gangetic Plain (referred as IGP-outflow) and another from Southeast Asia (SEA-outflow). The PM2.5 samples from the IGP-outflow show higher δ13CTC (−25.0 to −22.8 ‰; −23.8±0.6 ‰) than those from the SEA-outflow (−27.4 to −24.7 ‰; −25.3±0.9 ‰). Similarly, δ15NTN varied from +11.8 to +30.6 ‰ (+20.4±5.4 ‰) and +10.4 to +31.7 ‰ (+19.4±6.1 ‰) for IGP- and SEA-outflows, respectively. Based on the literature data, MODIS-derived fire hotspots and back trajectories, we infer that higher δ13CTC in the IGP-outflow is predominantly associated with fossil fuel and biofuel combustion. In contrast, contribution of primary organic aerosols from the combustion of C3 plants or secondary organic aerosol (SOA) formation from biomass/biofuel-burning emissions (BBEs) can explain the lower δ13CTC values in the SEA-outflow. This inference is based on the significant linear correlations among δ13CTC, water-soluble organic carbon and non-sea-salt potassium (nss-K+, a proxy for BBEs) in the SEA-outflow. A significant linear relationship of δ15N with and equivalent mass ratio of / is evident in both the continental outflows. Since abundance dominates the TN over the BoB (>90 %), atmospheric processes affecting its concentration in fine mode aerosols can explain the observed large variability of δ15NTN.

Keywords

• stable C- and N-isotopes
• Bay of Bengal
• aerosols
• biomass burning
• fossil-fuel combustion
• South Asia
• South-east Asia
• Indo-Gangetic Plain