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 (δ ¹³ C _TC ) were measured. The average concentrations of total WSI, OC and EC were 7.91 ± 3.44 μg/m ³ , 2.04 ± 1.25 μg/m ³ and 0.30 ± 0.22 μg/m ³ , respectively. Among the investigated WSI, sulfate (SO ₄ ^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 δ ¹³ C _TC ranged from -26.6‰ to -24.4‰ with an average of -25.3 ± 0.7‰. Based on the literature data of δ ¹³ C _TC , back-trajectory analysis and satellite fire spots, we propose that C ₃ plant burning in Southeast Asia significantly contributes to carbonaceous aerosols over the western SCS. This is also supported by a good correlation between δ ¹³ C _TC 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 SO ₄ ^2- by ammonium (NH ₄ ⁺ ), excess nss-SO ₄ ^2- and oxalate (C ₂ O ₄ ^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.
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