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
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To understand the characteristics and sources of organic aerosols (OA) in North China, we studied diacids, oxoacids and <em>α</em>-dicarbonyls in summer- and winter-time fine aerosols (PM<sub>2.5</sub>) collected from Tianjin. Oxalic (C<sub>2</sub>) acid was found to be the most abundant diacid species, followed by succinic (C<sub>4</sub>), malonic (C<sub>3</sub>) and sebacic (C<sub>8</sub>) acids, respectively. Glyoxylic (wC<sub>2</sub>) was the most abundant oxoacids followed by pyruvic acid. Concentrations of total diacids, oxoacids and <em>α</em>-dicarbonyls in winter were 2~3 times higher than those in summer, but their mass fractions in PM<sub>2.5</sub> 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 (C<sub>3</sub>, C<sub>4</sub>, M, F C<sub>6</sub>, Ph and C<sub>9</sub>) species and the linear relations among the selected species (including åC<sub>2</sub>-C<sub>4</sub> and åC<sub>8</sub>-C<sub>12</sub>) and with inorganic markers (K<sup>+</sup> and SO<sub>4</sub><sup>2-</sup>) implied that the diacids and related compounds are mainly originated from coal combustion and biomass burning emissions and produced in the atmosphere by both <em>in-situ</em> 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 SO<sub>4</sub><sup>2-</sup> and they are produced in aqueous phase, implying the need of reduction in oxidants (NO<em><sub>x</sub></em> and SO<sub>2</sub>) emissions to control the water-soluble OA loading over North China.
