Characteristics, Seasonality, and Secondary Formation Processes of Diacids and Related Compounds in Fine Aerosols During Warm and Cold Periods: Year‐Round Observations at Tianjin, North China

<jats:title>Abstract</jats:title><jats:p>To better understand the characteristics and sources of water‐soluble organic aerosols (OA) under different weather conditions, we collected fine aerosols (PM<jats:sub>2.5</jats:sub>) at an urban (ND) and a suburban (HEP) sites in Tianjin, North China over a 1‐year period and studied for diacids, oxoacids, and α‐dicarbonyls. Oxalic acid was found to be the most abundant (55.8 %± 10.7% at ND and 60.7% ± 13.0%) species during the campaign, followed by succinic and malonic acids, except for winter, in which phthalic acid was the third most abundant species at both the sites. Glyoxylic acid was found to be the most abundant (∼42%) oxoacids. Concentrations of diacids (range: 96.0–2137 ng m<jats:sup>−3</jats:sup> at ND and 185–1227 ng m<jats:sup>−3</jats:sup> at HEP) and related compounds were much higher in winter and lower in summer. Whereas the mass fractions of total diacids in PM<jats:sub>2.5</jats:sub> and their carbon contents in TC, OC, and WSOC were 2–3 times higher in summer than that in winter. Based on seasonal variations in mass ratios, relative abundances and linear relations of selected marker species including nss‐K<jats:sup>+</jats:sup>, we found that water‐soluble OA over the Tianjin region were mainly derived from fossil fuel including coal combustion and biomass burning emissions and subsequent intensive photochemical processing in the atmosphere. The linear relations of diacids with ambient temperature, RH, SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> and oxidants (O<jats:sub>3</jats:sub> and NO<jats:sub>2</jats:sub>) and SO<jats:sub>2</jats:sub> inferred that secondary formation and transformations of water‐soluble OA, particularly diacids and related compounds, in aqueous phase were more effective in cold period (October‐February), whereas in gas phase in warm period (March–September).</jats:p>