Tracing the Relative Significance of Primary versus Secondary Organic Aerosols from Biomass Burning Plumes over Coastal Ocean Using Sugar Compounds and Stable Carbon Isotopes

Biomass burning (BB), a pivotal source of both primary and secondary organic aerosols (POA and SOA, respectively), affects the regional and global climate. We have used stable carbon isotopic composition (delta C-13(TC)) of total carbon (TC) and BB tracers (anhydrosugars, sugars, and sugar alcohols) to elucidate the relative significance of POA and SOA over the Bay of Bengal, influenced by the long-range transport from the Indo-Gangetic Plain (IGP-outflow) and Southeast Asia (SEA-outflow) during a winter cruise. The molecular distributions of anhydrosugars (levoglucosan, Lev; galactosan, Gal; mannosan, Man) are different between IGP- (Lev > Gal > Man) and SEA-outflows (Lev > Man > Gal). The positive linear/nonlinear relationships of delta C-13(TC) with total sugar-C, K+, water-soluble organic carbon (WSOC), and TC in BBOA from the SEA-outflow are in sharp contrast to those from Mt. Tai, China and Rondonia, Brazil in summer; mainly because of the prevailing differences in ambient photochemical processing. The Keeling plots (delta C-13(TC) versus 1/TC, 1/WSOC, and 1/Lev) in the SEA-outflow revealed a mixing of "C-13-enriched POA" and C-13-depleted "fresh-SOA" of BB origin. Because the sugar compounds are mostly water-soluble and become bioavailable in the surface waters, we estimate the air-to-sea depositions of sugar-C and WSOC over the Bay of Bengal to contribute to no more than 0.1% and 13%, respectively, of their supply via peninsular rivers.