[Evaluation of Air Pollution Characteristics and Air Quality Improvement Effect in Beijing and Chengdu]

In recent years, China's air quality has been improving, and the concentration of atmospheric particulate matter has decreased significantly. In this study, the pollution characteristics and trends of two typical representative cities (Beijing and Chengdu) were analyzed. The geographical locations, pollution emissions, and meteorological diffusion conditions of the two cities were compared, to evaluate the relative contribution of meteorological conditions and pollution reduction regulations in decreasing fine particulate matter (PM_2.5) concentrations. The results showed that the number of heavily polluted days and pollution episodes in Beijing and Chengdu decreased significantly from 2013 to 2018, and the concentration of SO₂ and PM_2.5 decreased substantially. Compared to 2013, SO₂ concentration in Beijing and Chengdu has decreased by 77.8% and 70.9%, whereas PM_2.5 concentration has decreased by 42.7% and 48.5%, respectively. The largest reduction appeared in winter, when PM_2.5 decreased at an annual rate of 13.5 μg ·m^-3 for Beijing and 14.1 μg ·m^-3 for Chengdu. During the study period, the wind speed in Chengdu was less than that in Beijing, temperature was approximately 3℃ higher, and static wind in winter was more frequent. A significantly lower mixed-layer height, atmospheric capacity index, and ventilation coefficient in Chengdu resulted in more unfavorable atmospheric diffusion conditions. The static and stable weather index and the environmental meteorological index (EMI) also showed that the atmospheric diffusion conditions were better in Beijing than in Chengdu. The EMI of the two cities showed a decreasing trend during the study period, and the decline in EMI in Chengdu was the most significant in 2018, indicating an evident improvement in meteorological conditions. In 2018, emission reductions are estimated to have contributed 33.5% and 24% to the decrease in PM_2.5 in Beijing and Chengdu, respectively, and meteorological conditions contributed 7.2% and 11.1% to the reduction in these two cities. In winter, emission reductions respectively contributed 31.7% and 32.5% to reduction in Beijing and Chengdu, while meteorological conditions made a larger contribution.

[Analysis of Transport Pathways and Potential Sources of Atmospheric Particulate Matter in Zigong, in South of Sichuan Province]

Atmospheric particulate matter pollution in Zigong City in southern Sichuan is quite severe. The average concentrations of PM₁₀ and PM_2.5 from 2015 to 2018 were (95.42±9.53) μg·m^-3 and (65.95±6.98) μg·m^-3, with an obvious trend of decline. The concentrations of PM₁₀ and PM_2.5 in winter were much higher than in other seasons, with the highest average concentrations being(138.08±52.29) μg·m^-3 and (108.50±18.05) μg·m^-3 in January, respectively, whereas in summer, the average concentrations were lowest. The average ratio of PM_2.5 to PM₁₀ is 69.12%, and the ratio in winter is about 1.17 times that in summer; thus, PM_2.5 is mainly responsible for the air pollution. To explore the potential sources of fine particulate matter (PM_2.5) in Zigong City and the pollution contributions of different sources in different seasons, the concentration of PM_2.5 in Zigong and the daily trajectory after 72 h were calculated and clustered by the combined use of a variety of potential source analysis methods and data. These methods and data included the hybrid single particle lagrangian integrated trajectory (HYSPLIT) model, global data assimilation system (GDAS) meteorological data, potential source contribution analysis (PSCF), and concentration of weight trajectory analysis (CWT). The results showed that the area near Zigong is mostly controlled by southeasterly, westerly, and northwesterly winds in all seasons, and the high PM_2.5 concentration is mostly located in the low-wind-speed zone of 0-2 m·s^-1. The influence of different seasons and transport routes on PM_2.5 pollution in Zigong is significant. In spring, it is mainly affected by short-distance air flow from the west and north; in summer, the pollution mainly comes from the southeast air flow of short-distance transportation; in autumn, it is mainly affected by short-distance transportation air flow from Ziyang, Suining, Chongqing, and Neijiang; and in winter, it is not only affected by the surrounding cities such as Ziyang, Suining, and Neijiang but also by the long-distance transportation air flow from central Tibet. In general, the potential source area of particulate matter in Zigong City is mainly located in the border area between the west of Chongqing and the south of Sichuan. In winter, the main contribution area is at its widest, while in summer, the potential source area is smallest.