Atmospheric gaseous organic acids in winter in a rural site of the North China Plain

Organic acids are important contributors to the acidity of atmospheric precipitation, but their existence in the Chinese atmosphere is largely unclear. In this study, twelve atmospheric gaseous organic acids, including C₁-C₉ alkanoic acids, methacrylic acid, pyruvic acid, and benzoic acid, were observed in the suburb of Wangdu, Hebei Province, a typical rural site in the northern China plain from 16^th December, 2018 to 22^nd January, 2019, using a Vocus® Proton-Transfer-Reaction time-of-flight mass spectrometer (Vocus PTR-ToF). The quantification of C₂-C₄ alkanoic acids by the Vocus PTR-ToF was calibrated according to the titration of a NaOH solution by C₂-C₄ alkanoic acids from home-made permeation sources, and the other organic acids except for formic acid were quantified based on the k_cap-sensitivity linearity in the Vocus PTR-ToF, whereas formic acid was not quantified because our instrument setting led to a significant underestimation in its concentration. The average total concentration of eleven gaseous organic acids was 6.96 ± 5.20 ppbv (parts per billion by volume). The average concentration of acetic acid was the highest (3.86 ± 3.00 ppbv), followed by propanoic acid, butyric acid, and methacrylic acid. Domestic straw burning was likely the dominant source of the observed gaseous organic acids according to the good correlations between acetonitrile and organic acids and between particulate K⁺ and organic acids, and traffic emissions could also have contributed. During episodes with continuously high concentrations of organic acids, short-distance transport dominated in Wangdu according to the backward trajectory analysis. Baoding, Shijiazhuang, and Hengshui areas were the main source areas based on potential source contribution function and concentration weighing track analysis.