Key Takeaways & Executive Findings
- •• • PM2.5 concentrations in Nanjing, Shanghai, Hangzhou, and Hefei decreased significantly from 2017 to 2023, with the most pronounced improvement in 2020 during COVID-19 lockdowns, demonstrating the efficacy of stringent emission controls; O3 levels remained high or increased, indicating a control bottleneck. • • Correlation analysis revealed that PM2.5 and O3 are positively correlated during O3 pollution episodes but negatively correlated during PM2.5 episodes, implying that simultaneous control of both pollutants requires season-specific strategies. • • Compound pollution days occurred predominantly from February to October, with peak frequency (20 days) in April–June, driven by high temperature, stagnant air, and weak diffusion conditions that favor both PM2.5 accumulation and photochemical O3 production. • • The reduction in PM2.5 weakened the aerosol 'umbrella effect', enhancing surface radiation and promoting O3 formation; combined with altered NOx/VOCs ratios and climate warming, this led to increased O3 accumulation in the cold season, raising compound pollution frequency during colder months.
Abstract
In the context of accelerated urbanization, regional air composite pollution in medium and large urban agglomerations is primarily characterized by PM2.5-O3 compound pollution. To elucidate the meteorological causes of PM2.5-O3 compound pollution in the Yangtze River Delta (YRD) region over the recent seven years (2017–2023), this study analyzed monitoring data from typical cities (Nanjing, Shanghai, Hangzhou, and Hefei) using Pearson and partial correlation coefficients. Results indicate: (1) PM2.5 pollution exhibited a significant downward trend across all four cities, with notable improvement during the COVID-19 pandemic in 2020, underscoring the effectiveness of air pollution control measures. Conversely, O3 pollution remained elevated or increased in some cities, indicating persistent challenges in O3 control. (2) During O3 pollution episodes, PM2.5 and O3 concentrations were positively correlated, whereas during PM2.5 pollution episodes, they were negatively correlated. (3) Compound pollution days were predominantly observed from February to October, with the highest frequency (20 days) occurring from April to June. (4) The significant reduction in PM2.5 weakened the aerosol 'umbrella effect', enhancing surface radiation and promoting near-surface O3 formation. Concurrently, changes in the NOx/VOCs ratio weakened O3 titration, and climate warming accelerated O3 precursor generation and potentially altered boundary layer structure, collectively contributing to O3 accumulation in the cold season and an increasing frequency of compound pollution during that period. (5) The formation mechanisms of PM2.5 and O3 are driven by distinct meteorological conditions, with low overall concentration correlation; however, under compound meteorological conditions such as high temperature, stagnant air, and weak diffusion, both pollutants tend to rise synchronously, indicating that compound pollution events are typically driven by multiple adverse meteorological factors. This study demonstrates that from 2017 to 2023, PM2.5 pollution significantly decreased while O3 pollution showed an increasing trend. Compound pollution was concentrated in April–June and influenced by high temperature, stagnant air, and weak diffusion. With effective PM2.5 control, enhanced surface radiation and changes in O3 precursors led to O3 accumulation in the cold season, increasing compound pollution frequency. Overall, compound pollution is driven by multiple meteorological factors, posing complex challenges for control.
1. Introduction
The Yangtze River Delta (YRD) region, one of China's most urbanized and industrialized areas, has experienced severe air pollution episodes characterized by simultaneous high concentrations of fine particulate matter (PM2.5) and ozone (O3). While PM2.5 concentrations have declined due to effective emission controls, O3 pollution has worsened, leading to complex PM2.5-O3 compound pollution that poses significant risks to public health and ecosystem productivity. Existing air quality management strategies often target individual pollutants, but the coupled formation mechanisms and meteorological drivers of compound pollution remain inadequately understood, hindering the development of synergistic control policies.
This study addresses this gap by systematically analyzing seven years (2017–2023) of monitoring data from four representative YRD cities—Nanjing, Shanghai, Hangzhou, and Hefei. Employing Pearson and partial correlation analyses, we quantify the temporal trends of PM2.5 and O3, identify the seasonal and meteorological conditions conducive to compound pollution, and elucidate the underlying mechanisms. Our findings reveal that compound pollution events are predominantly driven by high temperature, stagnant air, and weak diffusion, and that the reduction in PM2.5 has inadvertently enhanced surface radiation, promoting O3 formation. These insights are critical for designing integrated air pollution control strategies that address both PM2.5 and O3 simultaneously, particularly in the context of climate change.
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ZHANG Leran, KANG Na, JI Haonan, CHEN Yue, HE Yijing, LI Mingxuan (2026). Characteristics and Meteorological Causes of PM2.5-O3 Compound Pollution in Typical Cities of the Yangtze River Delta Region. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025030602
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Frequently Asked Questions
What are the key meteorological factors that trigger PM2.5-O3 compound pollution events in the Yangtze River Delta?
The study identifies high temperature, stagnant air, and weak diffusion as the primary meteorological conditions that simultaneously elevate PM2.5 and O3 concentrations. These conditions favor both the accumulation of primary and secondary aerosols and the photochemical production of O3. The analysis shows that compound pollution days are most frequent from April to June, when such conditions are prevalent.
How does the reduction in PM2.5 affect O3 formation, and what are the implications for air quality management?
The reduction in PM2.5 weakens the aerosol 'umbrella effect' (aerosol-induced cooling and shading), which enhances surface solar radiation and promotes photochemical O3 formation. Additionally, changes in the NOx/VOCs ratio reduce O3 titration. This leads to increased O3 accumulation, particularly in the cold season, and a rising frequency of compound pollution. This implies that PM2.5 control alone may exacerbate O3 pollution, necessitating coordinated control of PM2.5 and O3 precursors.
What statistical methods were used to analyze the relationship between PM2.5 and O3, and what were the key findings?
Pearson and partial correlation coefficients were used to analyze the relationship. The results show that during O3 pollution episodes, PM2.5 and O3 are positively correlated, while during PM2.5 pollution episodes, they are negatively correlated. This suggests that the interaction between PM2.5 and O3 depends on the dominant pollution type, and that controlling one pollutant may have unintended effects on the other.
What are the seasonal patterns of PM2.5-O3 compound pollution, and how have they changed from 2017 to 2023?
Compound pollution days were predominantly observed from February to October, with the highest frequency (20 days) occurring from April to June. Over the seven-year period, the frequency of compound pollution in the cold season (November–January) has shown an increasing trend, attributed to enhanced O3 formation due to reduced PM2.5 and climate warming.
What are the implications of this study for designing effective air pollution control policies in the Yangtze River Delta?
The study underscores the need for integrated control strategies that simultaneously address PM2.5 and O3, rather than focusing on a single pollutant. Given the complex interactions and meteorological drivers, policies should consider seasonal and regional variations, and incorporate measures to reduce both primary PM2.5 emissions and O3 precursors (NOx and VOCs) in a coordinated manner. Additionally, climate change mitigation may be necessary to alleviate the meteorological conditions that exacerbate compound pollution.
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