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. Despite the growing interest in understanding haze formation in Chinesemegacities, air pollution has been largely overlooked for theHarbin–Changchun (HC) metropolitan area, located in the severe cold climateregion in northeast China. In this study, we unfolded significant variationsof fine particulate matter (PM2.5) in HC's central city (Harbin) duringtwo sequential heating seasons of 2018–2019 and 2019–2020, and we exploredmajor drivers for the observed variations. The two campaigns showedcomparable organic carbon (OC) levels but quite different OC sources. Thebiomass burning (BB) to OC contribution decreased substantially for2019–2020, which was attributed primarily to the transition of localpolicies on agricultural fires, i.e., from the “legitimate burning” policyreleased in 2018 to the “strict prohibition” policy in 2019. Meanwhile,the contribution of secondary OC (OCsec) increased significantly,associated with the much more frequent occurrences of high relative humidity(RH) conditions during the 2019–2020 measurement period. Similar toOCsec, the major secondary inorganic ions, i.e., sulfate, nitrate andammonium (SNA), also exhibited RH-dependent increases. Given theconsiderable aerosol water contents predicted for the high-RH conditions,heterogeneous reactions were likely at play in secondary aerosol formationeven in the frigid atmosphere in Harbin (e.g., with daily averagetemperatures down to below −20 ∘C). In brief, compared to2018–2019, the 2019–2020 measurement period was characterized by apolicy-driven decrease of biomass burning OC, a RH-related increase ofOCsec and a RH-related increase of SNA, with the first two factorsgenerally offsetting each other. In addition, we found that open burningactivities were actually not eliminated by the strict prohibition policyreleased in 2019, based on a synthesis of air quality data and fire countresults. Although not occurring during the 2019–2020 measurement period,agricultural fires broke out within a short period before crop planting inspring of 2020, and this resulted in off-the-charts air pollution for Harbin, with1 h and 24 h PM2.5 concentrations peaking at ∼ 2350 and900 µg/m3, respectively. This study indicates that sustainable useof crop residues remains a difficult challenge for the massive agriculturalsector in northeast China.

Dramatic changes in Harbin aerosol during 2018–2020: the roles of open burning policy and secondary aerosol formation