Abstract. Despite the growing interest in understanding haze formation in Chinese
megacities, air pollution has been largely overlooked for the
Harbin–Changchun (HC) metropolitan area, located in the severe cold climate
region in northeast China. In this study, we unfolded significant variations
of fine particulate matter (PM2.5) in HC's central city (Harbin) during
two sequential heating seasons of 2018–2019 and 2019–2020, and we explored
major drivers for the observed variations. The two campaigns showed
comparable organic carbon (OC) levels but quite different OC sources. The
biomass burning (BB) to OC contribution decreased substantially for
2019–2020, which was attributed primarily to the transition of local
policies on agricultural fires, i.e., from the “legitimate burning” policy
released 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 to
OCsec, the major secondary inorganic ions, i.e., sulfate, nitrate and
ammonium (SNA), also exhibited RH-dependent increases. Given the
considerable aerosol water contents predicted for the high-RH conditions,
heterogeneous reactions were likely at play in secondary aerosol formation
even in the frigid atmosphere in Harbin (e.g., with daily average
temperatures down to below −20 ∘C). In brief, compared to
2018–2019, the 2019–2020 measurement period was characterized by a
policy-driven decrease of biomass burning OC, a RH-related increase of
OCsec and a RH-related increase of SNA, with the first two factors
generally offsetting each other. In addition, we found that open burning
activities were actually not eliminated by the strict prohibition policy
released in 2019, based on a synthesis of air quality data and fire count
results. Although not occurring during the 2019–2020 measurement period,
agricultural fires broke out within a short period before crop planting in
spring of 2020, and this resulted in off-the-charts air pollution for Harbin, with
1 h and 24 h PM2.5 concentrations peaking at ∼ 2350 and
900 µg/m3, respectively. This study indicates that sustainable use
of crop residues remains a difficult challenge for the massive agricultural
sector in northeast China.