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

Bikkina, Srinivas  ,  Kawamura, Kimitaka  ,  Sarin, Manmohan

This study reports on stable carbon (delta C-13(TC)) and nitrogen (delta N-15(TN)) 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 delta C-13(TC) (-25.0 to -22.8 parts per thousand; -23.8 +/- 0.6 parts per thousand) than those from the SEA-outflow (-27.4 to -24.7 parts per thousand; -25.3 +/- 0.9 parts per thousand). Similarly, delta N-15(TN) varied from +11.8 to +30.6 parts per thousand (+20.4 +/- 5.4 parts per thousand) and +10.4 to +31.7 parts per thousand (+19.4 +/- 6.1 parts per thousand) for IGP-and SEA-outflows, respectively. Based on the literature data, MODIS-derived fire hotspots and back trajectories, we infer that higher delta C-13(TC) in the IGP-outflow is predominantly associated with fossil fuel and biofuel combustion. In contrast, contribution of primary organic aerosols from the combustion of C-3 plants or secondary organic aerosol (SOA) formation from biomass/biofuel-burning emissions (BBEs) can explain the lower delta C-13(TC) values in the SEA-outflow. This inference is based on the significant linear correlations among delta C-13(TC), water-soluble organic carbon and non-sea-salt potassium (nss-K+, a proxy for BBEs) in the SEA-outflow. A significant linear relationship of delta N-15 with NH4+and equivalent mass ratio of NH4+/SO42-is evident in both the continental outflows. Since NH4+abundance dominates the TN over the BoB (>90 %), atmospheric processes affecting its concentration in fine mode aerosols can explain the observed large variability of delta N-15(TN).

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