Characteristics, sources and formation of aerosol oxalate in an Eastern Asia megacity and its implication to haze pollution

Abstract. A total of 238 samples of PM_2.5 and TSP were analyzed to study the characteristics, sources, and formation pathways of aerosol oxalate in Shanghai in four seasons of 2007. The concentrations of oxalate were 0.07–0.41 μg m⁻³ in PM_2.5 and 0.10–0.48 μg m⁻³ in TSP, respectively. Oxalate displayed a seasonal variation of autumn>summer>winter>spring in both PM_2.5 and TSP and was dominantly present in PM_2.5 in all samples. Correlation between oxalate and K⁺ and high ratio of oxalate/K⁺ suggested that biomass burning was a secondary source of aerosol oxalate in Shanghai, in addition to urban VOCs sources (vehicular and industrial emissions), especially in autumn. Secondary formation accounted for the majority of aerosol oxalate in Shanghai, which was supported by the high correlation of oxalate with nss-SO₄²⁻, K⁺ and NO₃⁻, proceeding from different mechanisms. Relatively high ambient RH together with high cloud cover was found benefiting the secondary formation of aerosol oxalate. The in-cloud process (aqueous-phase oxidation) was proposed to be likely the major formation pathway of aerosol oxalate in Shanghai, which was supported by the high correlation of oxalate with nss-SO₄²⁻ and K⁺ , dominant residence of oxalate in droplet mode and result of favorable meteorological condition analysis. High correlation of oxalate and NO₃⁻ reflected the OH radical involved oxidation chemistry of the two species in the atmosphere and also suggested that gas-particle surface reactions and evaporation-condensation process were both possible secondary formation pathways of aerosol oxalate in coarser particle mode (>1.0 μm). Aerosol oxalate contributed to the haze pollution and visibility degradation of the local environment. As a major water-soluble organic compound in aerosols, concentration of oxalate showed a distinct negative correlation to the atmospheric visibility, which implied that aerosol organic compounds could play an important role in air quality in Shanghai.