Abstract: The content of organic matter in atmospheric fine particles is about 20%-80%. In addition to strong toxicity, some organic matter also has strong hygroscopicity, which affects the quality of the atmospheric environment. In order to fully understand the source of monocarboxylic acids in fine particles and their impact on the atmospheric environment, fine particles were collected in Qinhuangdao urban area (seaport site) and industrial park (Changli site) in 2020. After the pretreated samples were derivatized with boron trifluoride-methanol (BF₃-CH₃OH) derivatization reagent, the monocarboxylic acid in PM_2.5 was determined by gas chromatography-mass spectrometry (GC-MS). A total of 17 monocarboxylic acids (C₁₀-C₂₄) were detected. The results showed that: (1) The seasonal change of PM_2.5 concentration showed the characteristics of winter > autumn > spring > summer, and the PM_2.5 concentration of in the industrial park (Changli site) was 21.40-112.41 μg/m³, which was higher than the concentration in the urban area (seaport site) (9.01-104.88 μg/m³). (2) The seasonal variation of the concentration of monocarboxylic acids at the two sampling points was not obvious. The average annual concentration of monocarboxylic acids at the seaport site and Changli site was 873.91 and 895.22 ng/m³, respectively. (3) The concentrations of monocarboxylic acids with carbon numbers less than 22 at the two sites showed obvious even-numbered carbon advantages. The monocarboxylic acid with the highest concentration at the seaport site and the Changli site was palmitic acid (C₁₆), with annual average concentrations of 512.86 and 514.34 ng/m³; followed by stearic acid (C₁₈), with annual average concentrations of 270.06 and 268.17 ng/m³, respectively. C₁₆ and C₁₈ accounted for about 60% and 30% of the annual average concentration of monocarboxylic acids, respectively. The carbon dominance index (CPI) of acid was negatively correlated with the total amount of monocarboxylic acids (carbon number ≥ 22) contributed by plants. (4) The source of monocarboxylic acids and its influence on the atmospheric environment was preliminarily determined based on the carbon number distribution, the ratio of the concentration of monocarboxylic acids with carbon number greater than or equal to 22 to the concentration of monocarboxylic acids with carbon number less than 22 (∑C _{≥ 22}/∑C _{< 22}), the ratio of the concentration of stearic acid and palmitic acid (C₁₈/C₁₆), the CPI value and the ratio of the concentration of oleic acid to stearic acid (C_18:1/C₁₈). The results showed that the two sites in Qinhuangdao City had the most oxidizing atmosphere in summer (the atmospheric oxidation in urban areas was stronger than in industrial park), and the atmospheric oxidizing power was weak in spring and autumn. It was the weakest in spring in the industrial site. The monocarboxylic acids in PM_2.5 in Qinhuangdao City mainly came from local sources, such as coal burning, motor vehicle exhaust emissions, road dust and meat cooking, while plant sources contributed less. The study showed that seasonal changes in the concentration of monocarboxylic acids were not significant at two sampling sites. Coal burning, motor vehicle emissions, road dust and meat cooking contributed more to monocarboxylic acids.