Analysis of coordinated relationship between PM<sub>2.5</sub> and ozone and its affecting factors on different timescales

Abstract

<p indent="0mm">The North China Plain (NCP) and the Yangtze River Delta (YRD) regions have recently been suffering from complex air pollution, which is characterized by higher concentrations of fine particulate matter (PM_2.5) in fall and winter, higher ground-level ozone (O₃) concentrations during spring and summer, as well as double high PM_2.5 and O₃ (P-O) during the transition between cold and warm seasons. As a way of improving the air quality in the NCP and YRD, the coordinated control and comprehensive understanding of P-O coordinated relationship becomes very important towards developing effective control strategies. In this study, hourly and daily observations of P-O concentrations of two regions and four representative cities with chemical compositions and meteorological data in 2019 were used. The Pearson correlation coefficient (COR) and partial correlation coefficients (PCOR) were used to analyze the coordinated relationship between P-O and its influencing factors on different timescales. The results showed that the P-O correlation was positive in all seasons except in autumn and winter in the NCP on the daily time scale, and the P-O correlation was strongly positive in the YRD than NCP in summer. On the hourly time scale, the correlation was only positive in summer, and the P-O negative correlation was higher in the NCP compared to YRD in winter. In addition, on two different timescales, the P-O relationships in the southern area (coastal region) were strongly positive in comparison to those in the northern area (inland region). On the daily time scale, the positive synergistic relationship of P-O was jointly influenced by the major chemical components in summer. In other seasons, however, sulfate (SO₄²⁻) and primary organic carbon (POC) were the dominant components in Beijing and Shijiazhuang, while nitrate (NO₃⁻) and secondary organic carbon (SOC) dominated in Nanjing and Shanghai. Furthermore, SO₄²⁻ and SOC dominated the positive synergistic relationship in summer, while the strong negative correlation between NO₃⁻ and O₃ led to the negative synergistic relationship in spring and autumn on the hourly time scale. On different timescales, the positive synergistic relationship was greatly influenced by chemical reaction, while the meteorological effect was less significant in summer. In other seasons, however, the meteorological effects could weaken the P-O positive synergistic relationship (or enhance the negative synergistic relationship) on different timescales. The meteorological effects weakened the positive synergistic relationship, which was mainly due to the adverse effects of relative humidity on O₃ and PM_2.5. In winter, wind speed and planet boundary layer height also contributed to the meteorological effects, which enhanced the negative relationship.