<p>The northern slope of the Tianshan Mountains, a representative region in Xinjiang, China, experiences severe fine particulate matter (PM_2.5) pollution in winter; however, the underlying causes have not been fully elucidated. This study investigates PM_2.5 pollution episodes in the Kui-Du region on the northern slope of the Tianshan Mountains in Xinjiang spanning January 25–February 7, 2022 by analyzing its mass concentrations, chemical composition, and optical properties (light absorption and fluorescence spectra). The results revealed persistently high PM_2.5 levels during the study period, with mean concentrations of 240.95±82.88 µg/m³ in Kuitun and 142.40±76.18 µg/m³ in Dushanzi. Water-soluble ions (WSIs) constituted the dominant components, and a NO<Stack> ₃ ⁻ </Stack>/SO<Stack> ₄ ²⁻ </Stack> ratio &lt;1 indicated that stationary sources were the primary source contributors. A comparative analysis of samples before and after acid fumigation revealed that dust interference can inflate organic carbon (OC) measurements by 16.3±7.2% in Kuitun and 20.8±14.4% in Dushanzi, highlighting the necessity of sample pretreatment in dust-laden environments. Carbonaceous component ratios further indicated significant influences from coal combustion and secondary aerosol formation. The mass absorption efficiency of water-soluble organic carbon (WSOC, 0.62±0.24 m²/g in Kuitun and 0.53±0.20 m²/g in Dushanzi) in the study region was markedly lower than the winter values in other Chinese cities, which was attributable to dust mixing and secondary processes. Fluorescence spectroscopy revealed humic-like substances as the primary chromophores in WSOC, consistent with the predominance of primary emissions (e.g., coal combustion) and dust. These findings advance the mechanistic understanding of air pollution in Xinjiang and can support evidence-based policy-making for regional air quality management.</p>