Variation in PM_2.5 Chemical Composition During the Heating Seasons from 2018–2019 to 2020–2021 in Harbin: Response to Mitigation Strategies

To better understand the changes in chemical components and potential source regions of PM_2.5 in cold region of Northeast China, a field study was conducted during 2018–2019 and 2020–2021 heating seasons at an urban site in Harbin. On average, PM_2.5 concentration decreased from 55.57 ± 44.45 μg/m³ in 2018–2019 to 49.70 ± 32.14 μg/m³ in 2020–2021, reflecting a notable improvement in air quality. Overall, a reduction in the fraction of organic matter (OM) and an increase in the fraction of secondary inorganic aerosol (SNA) in PM_2.5 were observed. Indeed, the decrease in OM was primarily attributed to the reduction in emissions from open straw burning. However, during the heavily polluted period of 2020–2021, the influence of biomass burning significantly increased. This indicated that, although the impact of biomass burning emissions was mitigated by stricter straw burning prohibition policies compared to 2018–2019, it still played a crucial role in haze pollution. Additionally, the proportion of SNA increased due to the rise in the sulfur oxidation ratio (SOR), nitrogen oxidation ratio (NOR) and a slight increase in NO₂ concentration. The study revealed that liquid-phase reactions played a significant role in sulfate formation, while photochemical processes were more critical for nitrate formation. Meanwhile, higher SOR and NOR levels were observed during pollution episodes, highlighting the enhanced secondary formation on polluted days. Moreover, backward trajectory cluster, potential source contribution function (PSCF) and weighted concentration weighted trajectory (CWT) analyses identified the northwest and central-western regions of Jilin Province, including cities such as Changchun and Songyuan, as main potential source regions influencing air quality in Harbin.