**1. Introduction**

With the rapid economic growth and urbanization in China and the world, there is a growing concern in the international community and in China about the relationship between air pollution and public health [1]. Long-term exposure to PM2.5 and O3 can lead to cardiovascular disease [2], ophthalmic disease [3], and premature birth [4]. The combination of chronic and acute exposures leads to a high global and regional burden of mortality and morbidity [5–7]. The research by Edlund et al. [8] showed that air pollution is the biggest environmental contributor to disease, with a disproportionate impact on low and middle income countries. Furthermore, high levels of PM2.5 in the outdoor environment can also be exposure to indoors, affecting the health of people who are exposed to indoor rest and work for long periods of time [9]. Thus, both PM2.5 and O3 have attracted widespread public and scientific attention.

To improve air quality in China, in 2013, the Chinese State Council issued the Air Pollution Prevention and Control Action Plan (APPCAP) to reduce the number of PM2.5 and associated haze days. Through the implementation of the APPCAP measures, PM2.5 concentrations have decreased significantly across the country since 2013. The improvement in air quality was reported by Yue et al. [10] and was mainly associated with strengthening industrial emission standards, upgrading industrial boilers, phasing out obsolete industrial capacity, and promoting cleaner fuels in the residential sector. A decrease in NO*<sup>x</sup>*

**Citation:** Fang, C.; Gao, H.; Li, Z.; Wang, J. Regional Air Pollutant Characteristics and Health Risk Assessment of Large Cities in Northeast China. *Atmosphere* **2021**, *12*, 1519. https://doi.org/10.3390/ atmos12111519

Academic Editors: Duanyang Liu, Kai Qin and Honglei Wang

Received: 14 October 2021 Accepted: 16 November 2021 Published: 18 November 2021

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and SO2 accompanied the reduction in PM2.5. However, O3 concentrations showed an increasing trend. Fan et al. [11] found that the daily average maximum concentration of O3 (MDA8-O3) in summer increased from 91.6 μg/m<sup>3</sup> in 2015 to 103.1 μg/m<sup>3</sup> in 2018, with an annual growth rate of 4.4 μg/m3. The annual average PM2.5 concentrations in 2017 in key city groups such as Beijing–Tianjin–Hebei, Yangtze River Delta, and Pearl River Delta compared to 2013 decreased by 39.6%, 34.3%, and 27.7%, respectively, but the 90% quantile of the maximum ozone 8-h sliding average (MDA8-O3) instead increased by 19.1%, 10.4%, and 5.8%, respectively, compared to 2013 [12–15]. The increase in ozone has triggered a discussion among scholars about the synergistic management of O3 and PM2.5, and it is necessary and correct to study O3 and PM2.5 under the same perspective. In this study, for the 2016–2020 O3 and PM2.5 pollution phenomena in three provincial capitals in northeast China, we used time series, meteorological factor analysis, and correlation analysis to characterize the pollutants' pollution characteristics. Various methods based on backward trajectories including trajectory clustering, potential source contribution functions (PSCF), and concentration weight trajectories (CWT) were used to analyze the regional transport trends of pollutants. Health risks were assessed using methods recommended by the United States Environmental Protection Agency (USEPA).
