Influence of vertical transport on chemical evolution of dicarboxylic acids and related secondary organic aerosol from surface emission to the top of Mount Hua, Northwest China

Dicarboxylic acids are strong hygroscopic organic compounds in the atmosphere, and thus significantly affect the cloud formation process and radiative forcing on a regional scale. So far, the evolution of dicarboxylic acids during vertical transport from the surface to the mountaintop has yet to be explicitly understood. In this study, the molecular distribution and stable carbon isotopic (δ13C) compositions of dicarboxylic acids and related organic compounds (DCRCs) in PM2.5 were measured simultaneously at the top (c. 2060 m a.s.l.) and foot (c. 400 m a.s.l.) of Mount (Mt.) Hua during the summer of 2020. Due to the strong anthropogenic emissions at ground level, the concentrations of DCRCs at foot of Mt. Hua were generally higher than those at the top. Oxalic acid (C2) was the predominant diacid in both sites, whose concentrations at foot and top of Mt. Hua were 87–852 and 40–398 ng m−3, respectively. Ratios of adipic acid to azelaic acid (C6/C9), phthalic aid to azelaic acid (pH/C9), glyoxal to methylglyoxal (Gly/mGly), and lower δ13C values (−21.0 ± 2.3 ‰ and − 21.9 ± 2.7 ‰) of C2 indicated that the contributions of anthropogenic sources to DCRCs in PM2.5 in the mountain region are more significant than biogenic sources. Aerosols from the foot of Mt. Hua could affect the atmosphere on the top of the mountain via vertical transport under the influence of daytime valley wind, even though the altitude of Mt. Hua is beyond the boundary layer most of time. The value δ13C of C2 is linearly correlated with C2/mGly, C2/pyruvic acid (Pyr), C2/glyoxylic acid (ωC2) at the top of the mountain, and C2/Gly, C2/ωC2 at the foot of the mountain, indicating that the formation pathway of C2 is mGly-Pyr-ωC2-C2 at the top of Mt. Hua and Gly-ωC2-C2 at the foot of Mt. Hua.