Decoding cold-season PM_2.5 and its chemical compositions in China's economic powerhouses after the Chinese COVID-19 pandemic lockdown: A national-scale analysis

This study utilized the China's National Aerosol Component Monitoring Network to conduct in situ measurements of PM_2.5 (particulate matter with an aerodynamic diameter ≤2.5 μm) and its chemical constituents during the cold season (November 15, 2023–March 15, 2024), coinciding with the inaugural cold season of China's post-pandemic economic recovery. The measured PM_2.5 concentrations exhibited obvious regional disparities, peaking in the Beijing-Tianjin-Hebei and its surrounding regions (BTHS, 66 ± 10 μg/m³) and the Fen-Wei Plain (FWP, 62 ± 11 μg/m³), while lowest in Southern coastal regions (SCR, 29 ± 3 μg/m³). Chemically, nitrate (NO₃⁻) dominated PM_2.5 composition in the Yangtze River Delta (YRD) and BTHS. The FWP showed elevated levels of organic matter (OM), sulfate (SO₄²⁻), and crustal matter (CMs), linked to coal combustion and dust. OM prevailed in the Cheng-Yu region (CY), SCR, and the Northeast regions (NEs) due to fuel combustion, while the Northwest regions (NWs) recorded the highest SO₄²⁻ proportions from coal burning. When air pollution occurred and worsened, OM, NO₃⁻, and SO₄²⁻ concentrations generally increased under stable meteorological conditions that favored pollutants accumulation and secondary formation, and their relative contributions shifted. NO₃⁻ and ammonium (NH₄⁺) rose proportionally, while OM and elemental carbon (EC) declined. Regional events like northern Spring Festival fireworks and northwestern dust storms further modulated composition. Conditional probability function analyses identified local PM_2.5 chemical component hotspots near emission sources like industrial zones and transport hubs at low wind speeds (<2–5 m/s), with directional patterns varying by city (southeast in Beijing and southwest in Shanghai). Potential source contribution function analyses revealed influences of regional transport in different economic zones, and Beijing (BTHS) and Shanghai (YRD) received obvious contributions from adjacent industrial areas like Hebei and Shandong provinces and Jiangsu and Zhejiang provinces, respectively. Chengdu's (CY) pollution was more locally driven, and Urumqi (NWs) experienced substantial inputs from western Xinjiang and cross-border sources of the central Asia. The results demonstrate a complex interplay of local emissions and regional transport, necessitating targeted local interventions for locally dominated components (EC and CMs) and regional control strategies for transported components (NO₃⁻, SO₄²⁻, OM and NH₄⁺) and their precursors.