| Summary: | Beijing, the capital city of China, has achieved remarkable progress in terms of an improvement in air quality under strict control policies in the past 10 years from various sources. In this paper, the characteristics of fine particulate matter (PM<sub>2.5</sub>) and O<sub>3</sub> in January 2013 and 2018 in Beijing are discussed on the basis of daily sample analysis and hourly monitoring data. It was found that the PM<sub>2.5</sub> pollution for the month of January in Beijing has been greatly curbed. The SO<sub>4</sub><sup>2−</sup> concentration and proportion of PM<sub>2.5</sub> decreased, while the proportions of NO<sub>3</sub><sup>−</sup> and NH<sub>4</sub><sup>+</sup> increased. Organic matter represented the major component during the two periods with the proportions of 31.7% ± 8.2% and 31.4% ± 9.8%. The results of the Hybrid Single Particle Lagrangian Integrated Trajectory (Hysplit) model and Potential Source Contribution Function (PSCF) method showed that air mass from southern nearby regions accounted for 34% and 10% in 2013 and 2018, respectively, which was closely related to the pollution period. Thus, the input direction of air mass in January 2018 was more conducive to the diffusion of pollutants. Modeling results of the Weather Research and Forecasting model (WRF) coupled with Comprehensive Air Quality Model Extensions (CAMx) indicated that the contribution of industry sources to PM<sub>2.5</sub> and O<sub>3</sub> decreased from 2013 to 2018, while mobile sources increased. This was mainly due to the different control policies on various emission sources. In terms of O<sub>3</sub> sources, more control measurements should be taken on volatile organic compounds (VOCs) due to its prominent effect on O<sub>3</sub> concentration in both periods. The reduction in emissions and the meteorological conditions both contributed effectively to the sharp decrease in PM<sub>2.5</sub> concentration. However, the change in weather conditions had the greater impact on the decrease in PM<sub>2.5</sub> concentration, while the reduction in emissions was weakened as a function of this change.
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