Molecular distributions of diacids, oxoacids and -dicarbonyls in summer- and winter-time fine aerosols from Tianjin, North China: Emissions from combustion sources and aqueous phase secondary formation

To understand the characteristics and sources of organic aerosols (OA) in North China, we studied diacids, oxoacids and α-dicarbonyls in summer- and winter-time fine aerosols (PM_2.5) collected from Tianjin. Oxalic (C₂) acid was found to be the most abundant diacid species, followed by succinic (C₄), malonic (C₃) and sebacic (C₈) acids, respectively. Glyoxylic (wC₂) was the most abundant oxoacids followed by pyruvic acid. Concentrations of total diacids, oxoacids and α-dicarbonyls in winter were 2~3 times higher than those in summer, but their mass fractions in PM_2.5 were exactly the opposite. On average, total diacids carbon accounted for 2.9% in total carbon and 3.3% in organic carbon (OC) in summer and 1.8% and 2.0%, respectively, in winter. Their contributions to water-soluble OC (WSOC) was almost the same in both seasons (5.5% and 5.3%, respectively). Molecular distributions, mass ratios of selected diacid (C₃, C₄, M, F C₆, Ph and C₉) species and the linear relations among the selected species (including åC₂-C₄ and åC₈-C₁₂) and with inorganic markers (K⁺ and SO₄^2-) implied that the diacids and related compounds are mainly originated from coal combustion and biomass burning emissions and produced in the atmosphere by both in-situ photochemical reactions at local scale and aging during long-range transport in both summer and winter. This study revealed that diacids and related compounds and WSOC are increased with increasing SO₄^2- and they are produced in aqueous phase, implying the need of reduction in oxidants (NO_x and SO₂) emissions to control the water-soluble OA loading over North China.