Measurement report: Observational insights into the impact of dust transport on atmospheric dicarboxylic acids in ground region and free troposphere

This study investigated the vertical distribution of PM2.5 and size-segregated aerosols at the foot and top of Mount Hua in Northwest China, focusing on C2 formation and its δ13C characteristics influenced by dust transport. Under non-dust conditions, PM2.5 and diacids concentrations at the foot were 4.5 and 2.1 times higher than those at the top, with stronger local anthropogenic signals (C9, 5.67 times higher) and pronounced diurnal diacids variation. Higher C2/C4 (5.84 vs. 4.74), C3/C4 (1.04 vs. 0.56) ratios and δ13C values (-21.5 ‰ vs. -27.6 ‰) at the top indicated photochemical aging during aerosol transport. C2 concentration was positively correlated with aerosol liquid water content and its size distribution pattern matched with precursors, confirming aqueous-phase oxidation as the dominant formation pathway. During dust events, PM2.5 concentrations at the foot and top reached 457 μg m−3 and 165 μg m−3, but C2 concentrations in PM2.5 decreased by 59 % (foot) and 25 % (top), while δ13C values of C2 exhibited a significant positive shift (foot: -27.6 ‰ to -23.9 ‰; top: -21.5 ‰ to -13.2 ‰), attributed to alkaline dust catalyzing 13C-enriched oxalate formation. mGly became the second most abundant acid due to enrichment on dust-particle surfaces. Size-segregated data revealed decreased fine-particle C2 but increased coarse-particle C2, elevating the coarse-to-fine ratio from 0.3~0.4 to 0.6~1.1. These findings offer valuable insights into the altitude-dependent transformation of SOA affected by dust transport, enhancing our understanding of mountain atmospheric chemistry and regional air quality.