Detecting causal relationships between fine particles and ozone based on observations in four typical cities of China

Abstract

As the concentration of fine particles (PM2.5) is declining, ozone (O3) concentration has been increasing in China in recent years. To collaboratively control PM2.5 and O3, it is critical to understand the relationship between the two and identify major controlling factors. We use a convergent cross-mapping method to detect the causal relationship between daily PM2.5 and maximum daily 8 h average (MDA8) O3 concentrations in Beijing, Taizhou, Shenzhen and Chengdu, China, in the four seasons in 2015–2021. In addition, we also examined causal effects of atmospheric oxidation capacity, precursors and meteorological elements on PM2.5 and MDA8 O3 in the four cities. PM2.5 and MDA8 O3 are strongly positively correlated and show bidirectional causal relationships during the Beijing and Taizhou summer and in the four seasons in Shenzhen, due mainly to the strong photochemical reactions in the daytime. During the Beijing winter, PM2.5 and MDA8 O3 show bidirectional causal relationships, but the two are significantly negatively correlated, driven by NO2 and relative humidity. Weak bidirectional, unidirectional and no causal effects between PM2.5 and MDA8 O3 are detected in other seasons in the four cities. In these seasons and cities, the top three causal factors of PM2.5 differ from those of MDA8 O3. Season-, city- and pollutant-specific control measures of PM2.5 and MDA8 O3 are required.