Post-clean air action plan challenges in China: A prolonged winter haze in the Yangtze River Delta from 2023 to 2024

Despite improvements in air quality following China's 2013 Clean Air Action Plan, severe haze persists as a critical challenge. This study investigates an intermittent winter heavy haze episode in the Yangtze River Delta. Jiaxing's winter air quality improved overall from 2013 to 2023. Compared to the 2013 winter, the number of excellent and good air quality days in the 2022 winter (81 days) increased by 38 days, reflecting an average annual growth rate of 9.8 %. However, polluted days increased significantly during the winter of 2023 to 2024. This heavy haze episode had three distinct pollution phases (Pollution I, II, III) and three clean intervals (Clean I, II, III). Nitrate was the main component of PM_2.5 across all pollution phases, making up 43.1 %-47.9 % of the total, significantly higher than sulfate. Among other ions, Cl^- had the biggest increase during pollution, followed by K⁺. Ca²⁺ and Mg²⁺ levels were lowest in clean Ⅱ, with slight changes in other phases. Secondary aerosols made a significant contribution to pollution, with nitrogen (NOR > 30 %) and sulfur (SOR > 30 %) oxidation ratios, and secondary organic carbon (SOC) comprised over 50 % of organic carbon (OC). Primary industrial sources were the main contributor in pollution Ⅰ (48.62 %), while secondary sources were most important in pollution Ⅱ (62.28 %). Pollution Ⅲ was a mix of primary industrial (39.94 %) and secondary (41.08 %) sources. Pollution Ⅰ stemmed from local emissions and short-distance transport from Suzhou, whereas Pollution Ⅱ involved long-range contributions from multiple cities (Nanjing, Suzhou, Xuancheng, Wuhu, Hangzhou). Pollution III resulted from a combination of regional transport in central/southern Jiangsu and local emissions.