Impact of ambient relative humidity and acidity on chemical composition evolution for malonic acid/calcium nitrate mixed particles

Molecular characteristics and stable carbon isotope compositions of dicarboxylic acids and related compounds in wintertime aerosols of Northwest China

Dicarboxylic acids are one of the important water-soluble organic compounds in atmospheric aerosols, causing adverse effects to both climate and human health. More attention has therefore been paid to organic acids in aerosols. In this study, the molecular distribution and diurnal variations of wintertime dicarboxylic acids in a rural site of Guanzhong Plain, Northwest China, were explored. Oxalic acid (C₂, day: 438.9 ± 346.8 ng m⁻³, night: 398.8 ± 392.3 ng m⁻³) is the most abundant compound followed by methylglyoxal (mGly, day: 207.8 ± 281.1 ng m⁻³, night: 222.9 ± 231.0 ng m⁻³) and azelaic (C₉, day: 212.8 ± 269.1 ng m⁻³, night: 211.4 ± 136.7 ng m⁻³) acid. The ratios of C₉/C₆ and C₉/Ph indicating that atmospheric dicarboxylic acids in winter in the region mainly come from biomass burning. Furthermore, secondary inorganic ions (NO₃⁻, SO₄²⁻, and NH₄⁺), relative humidity, liquid water content, and in-situ pH of aerosols are highly linearly correlated with C₂, suggesting that liquid phase oxidation is an important pathway for the formation of dicarboxylic acids. The δ¹³C analysis of C₂ suggested that lighter carbon isotope compositions tend to be oxidized to form aqueous-phase secondary organic aerosols (aqSOA), leading to the decay of ¹³C in aqSOA products rather than aerosol aging. This study provides a theoretical basis for the mechanism of formation of dicarboxylic acid.