Unexpectedly persistent PM _2.5 pollution in the Pearl River Delta, South China, in the 2015–2017 cold seasons: the dominant role of meteorological changes during the El Niño-to-La Niña transition over emission reduction

Abstract. Effective air quality management requires a comprehensive understanding of how meteorological variability and emission changes shape multiannual changes in regional PM2.5 pollution. During the cold seasons of 2015–2017, persistent PM2.5 pollution occurred in the Pearl River Delta (PRD), South China, despite rapid emission reductions. This study systematically investigated the interconnections between climate variability, meteorology, PM2.5 levels, source contributions and budgets during these periods, aiming to uncover the detailed impacts of meteorological and emission changes on PM2.5 pollution. We found that drastic meteorological changes, closely linked to a transition from strong El Niño (2015) to weak/moderate La Niña (2017), were the main drivers of the three-year PM2.5 changes. Strengthened northerly winds and reduced humidity enhanced cross-regional PM2.5 transport into the PRD while concurrently suppressing local PM2.5 production and accumulation. WRF/CMAQ simulations indicate that transport (non-local) contributions to PM2.5 in the PRD increased from 70 % in 2015 to 74 % in 2016 and 78 % in 2017. While the transport of secondary inorganic PM2.5 components overall intensified, their responses to meteorological and emission changes varied: Variations in sulfate were more sensitive to emission reductions outside the PRD, whereas those for nitrate were primarily driven by meteorological shifts. Simulated PM2.5 mass budgets further support the increasing dominance of transport, especially via advections. Our findings underscore the potentially crucial role of meteorological variability in driving multiannual PM2.5 pollution changes in the PRD and other regions strongly impacted by cross-regional transport, emphasizing the necessity for regionally coordinated emission control strategies to effectively mitigate PM2.5 pollution.