Comparison of PM_2.5 Pollution during Heating and Non-Heating Periods in Northwest China: Chemical Characteristics, Secondary Contribution, and Regional Transport

Atmospheric pollution in heating season remains a persistent and acute challenge in Xinjiang Uygur Zizhiqu (hereafter Xinjiang), Northwest China, where a prolonged winter heating season prevails. In this study, continuous collection and chemical composition analysis of PM_2.5 were conducted from May 2021 to April 2022 in Changji, Xinjiang. Separated by typical non-heating period (May to October) and heating period (November to April of the next year), the concentration of PM_2.5 exhibited a substantial increase, rising from 23.3 ± 9.29 µg m⁻³ during the non-heating period to 98.4 ± 75.2 µg m⁻³ during the heating period. The concentrations of most water-soluble ion components in PM_2.5 during the heating period significantly exceeded those during the non-heating period, especially for secondary inorganic aerosols, such as sulfate ( ${\mathrm{S}\mathrm{O}}_4^{2-}$), nitrate ( ${\mathrm{N}\mathrm{O}}_3^{-}$), and ammonium ( ${\mathrm{N}\mathrm{H}}_4^{+}$), which increased by 8.40, 7.47, and 6.55 times, respectively. In contrast, the concentrations of Mg²⁺ and Ca²⁺ remained relatively stable across the two periods. During the heating period, the concentration of liquid water content (LWC) was averaged at 4.28 ± 4.07 µg m⁻³,8.23 times higher than those during the non-heating period (0.52 ± 0.48 µg m⁻³). This enhanced LWC during the heating period provides a reactive medium for the oxidation ( ${\mathrm{S}\mathrm{O}}_4^{2-}$) of SO₂, with 2–3 times reaction rates higher than that during the non-heating period. Notably, the sulfur oxidation ratio (SOR) likely follows a segmented linear growth pattern during the heating season. Specifically, when LWC exceeds the threshold of 10 μg m⁻³, the formation rate of ${\mathrm{S}\mathrm{O}}_4^{2-}$ shifts to a slower linear growth trend. Moreover, regional transport dominates local primary component (e.g., elemental carbon) variations, while surrounding areas of Changji contributed more significantly to secondary components (notably ${\mathrm{N}\mathrm{O}}_3^{-}$) during the heating period.