Assessment of sulfate and nitrate variations in China during 19902020: Insights into source contributions and formation pathways

Sulfate (SO₄^2-) and nitrate (NO₃^-) are two critical constituents of fine particulate matter (PM_2.5), significantly impacting air quality and public health. This study provides a comprehensive assessment of spatial and temporal variations in SO₄^2- and NO₃^- levels across China from 1990 to 2020 by using a revised Community Multiscale Air Quality Modeling System (CMAQ), focusing on elucidating insights into source contributions and formation pathways. The results reveal that NO₃^- pollution has become increasingly significant, with its concentrations surpassing those of SO₄^2- in most regions of China since 2017. Industrial emissions were identified as the primary contributors to SO₄^2- levels, accounting for 40.2 %57.5 % in Beijing–Tianjin–Hebei region (BTH), 51.0 %76.6 % in Yangtze River Delta (YRD), 46.8 %68.2 % in Pearl River Delta (PRD), 38.9 %62.8 % in Sichuan Basin (SCB), and 36.1 %58.8 % in Fenwei Plain (FWP). For NO₃^-, industrial emissions were predominant in BTH (27.7 %33.1 %) and YRD (31.0 %33.6 %), while transportation emissions were the major source in PRD (26.4 %36.3 %), in SCB and FWP, contributions from these sectors were comparable. The formation pathways of SO₄^2- exhibited distinct regional variations: overall, primary emission of SO₄^2- was the predominant pathway in BTH (23.7 %47.3 %) and YRD (24.1 %30.5 %), heterogeneous reaction dominated in SCB (23.2 %55.5 %) and FWP (20.5 %48.9 %), and aqueous-phase oxidation reaction of SO₂ by H₂O₂ was the leading formation pathway in PRD (23.6 %32.8 %). In contrast, the formation pathways of NO₃^- demonstrated consistency across all regions, with combined gas-phase oxidation reaction of NO₂ by OH and the heterogeneous reaction of N₂O₅ accounting for over 90 % of its production. This study highlights the need for targeted air quality management strategies that account for regional variations in source contributions and prioritize the reduction of key oxidants driving secondary aerosol formation. The findings could offer valuable insights for policymakers developing effective measures to mitigate air pollution.