[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.