<p><strong>Abstract.</strong> Abstract: Molecular distributions and stable carbon isotopic (δ<sup>13</sup>C values) compositions of dicarboxylic acids and related SOA in PM<sub>2.5</sub> aerosols collected on a day/night basis at the summit of Mt. Tai (1534 m  a.s.l.) in the summer of 2016 were analyzed to investigate the sources and photochemical aging process of organic aerosols in the forested highland region of North China Plain. The molecular distributions of dicarboxylic acids and related SOA are characterized by the dominance of oxalic acid (C<sub>2</sub>), followed by malonic (C<sub>3</sub>), succinic (C<sub>4</sub>) and azelaic (C<sub>9</sub>) acids. The concentration ratios of C<sub>2</sub>/C<sub>4</sub>, diacid-C/OC and C<sub>2</sub>/total diacids are larger in daytime than in nighttime, suggesting that the daytime aerosols are more photochemically aged than those in nighttime due to the higher temperatures and stronger solar radiation. Both ratios of C<sub>2</sub>/C<sub>4</sub> (<i>R</i><sup>2</sup> > 0.5) and C<sub>3</sub>/C<sub>4</sub> (<i>R</i><sup>2</sup> > 0.5) correlated strongly with the ambient temperature, indicating that SOA in the mountaintop atmosphere are mainly derived from the photochemical oxidation of local emissions rather than long-range transport. The mass ratios of C<sub>9</sub>/C<sub>6</sub>, C<sub>9</sub>/Ph, Gly/mGly and the strong linear correlation of major dicarboxylic acids and related SOA with biogenic precursors further suggest that aerosols in this region are mainly originated from biogenic sources (i.e., tree emissions). <br><br> C<sub>2</sub> concentrations correlated well with aerosol pH, indicating that particle acidity favors the organic acid formation. The stable carbon isotopic compositions (δ<sup>13</sup>C) of the dicarboxylic acids are higher in daytime than in nighttime with the highest value (−16.5 ± 1.9 ‰) found for C<sub>2</sub> and the lowest value (−25.2 ± 2.7 ‰) found for C<sub>9</sub>. An increase in δ<sup>13</sup>C values of C<sub>2</sub> along with increases in C<sub>2</sub>/Gly and C<sub>2</sub>/mGly ratios was observed, largely due to the isotopic fractionation during photochemical degradation of the precursors.</p>

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