Influence of biomass burning on atmospheric aerosols over the western South China Sea: Insights from ions, carbonaceous fractions and stable carbon isotope ratios.

Total suspended particle (TSP) samples were collected during a cruise campaign over the western South China Sea (SCS) from August to September 2014. Ten water-soluble ions (WSI), organic carbon (OC), elemental carbon (EC) and stable carbon isotope ratios of total carbon (δ13 CTC ) were measured. The average concentrations of total WSI, OC and EC were 7.91 ± 3.44 μg/m3 , 2.04 ± 1.25 μg/m3 and 0.30 ± 0.22 μg/m3 , respectively. Among the investigated WSI, sulfate (SO4 2- ), sodium (Na+ ) and chloride (Cl- ) were the most abundant species, accounting for 39.2%, 24.5% and 14.3% of the total mass of the WSI, respectively. Significantly positive correlations of OC and EC with non-sea-salt potassium (nss-K+ ), a tracer for biomass burning, suggest that biomass burning is the major source of carbonaceous aerosols. The values of δ13 CTC ranged from -26.6‰ to -24.4‰ with an average of -25.3 ± 0.7‰. Based on the literature data of δ13 CTC , back-trajectory analysis and satellite fire spots, we propose that C3 plant burning in Southeast Asia significantly contributes to carbonaceous aerosols over the western SCS. This is also supported by a good correlation between δ13 CTC and the mass ratios of nss-K+ /TC. Furthermore, high Cl- depletion (73 ± 23%) was observed in the aerosols over the western SCS. Given the neutralization of SO4 2- by ammonium (NH4 + ), excess nss-SO4 2- and oxalate (C2 O4 2- ) made major contributions to Cl- depletion in the samples strongly influenced by biomass burning. This study provides useful information to better understand the influence of biomass burning on atmospheric aerosols over the SCS.