**4. Conclusions**

(1) Seven provinces and cities implemented the emission-reduction measures for air quality in a union. Emission-reduction measures, such as vehicle restrictions, enterprise suspension, production restriction, and construction site suspension, were put in place. As a result, the concentration of PM2.5 at 11 state-controlled air quality monitoring stations in Beijing dropped by an average of about 41 percent. Besides the average concentration of SO2, NO2 and PM10 reached the lowest levels in the history of monitoring, which made Beijing's air quality excellent in a short time. It suggested that the enhanced emissionreduction measures had made a significant contribution to the improvement of local air quality in Beijing.

(2) During the 2015 Military Parade, Beijing was in the front or lower part of the high-pressure system. Due to the strong effect of north or northeast winds, the weather situation was conducive to the diffusion of pollutants. Before the implementation, Beijing was affected by the southwest airflow, and the adverse atmospheric diffusion conditions had caused the accumulation of pollutants in Beijing. After the implementation, owing to the adverse temperature layer, the pollution diffusion conditions were poor, and the pollutant concentration rose slightly, in which PM10 was increased from 31 μg·m−<sup>3</sup> increased to 33 μg·m−<sup>3</sup> compared with yesterday, NO2 and PM2.5 increased to 33 μg·m−<sup>3</sup> and 53 μg·m<sup>−</sup>3, respectively. O3 decreased, and AQI increased from 36 to 73. The air quality changed from excellent to good. Thus, it can be seen that the weather situation had a great impact on air quality.

(3) During the implementation, PM2.5, PM10, NO2, and other pollutants decreased significantly, of which the concentration of PM10 decreased the most, from 109 μg·m−<sup>3</sup> down to 34 μg·m<sup>−</sup>3, and the concentration of PM2.5 decreased by 72.73%. According to the changes before and during the implementation and after the implementation, the concentration of PM10 and PM2.5 increased after the implementation of the emissionreduction measures, indicating that the enhanced control measures have made a great contribution to the emission reduction in particles.

(4) The number of good days before, during, and after the Military Parade in 2015 was 35, accounting for 77.78% of the effective days, higher than 66.67% and 68.89% in 2013 and 2014. The number of days with mild pollution was 9, less than 14 and 13 days in 2013 and 2014, respectively. There was one day of heavy pollution in 2013 and 2014,

respectively. In 2015, there were no pollution days. In addition, the annual average of AQI in the three years is 87.49. In order to achieve the objectives of class II standard and class I standard in the GB-3095-2012 ambient air quality standard, the annual average AQI needed to be reduced by 60.00% and 82.46%, respectively. The average value of the normal year was the average value of 2013 and 2014. The average value of the normal year during the military parade is 64.63, which was 70.40% lower than the average value of AQI during the military parade. The goal of reaching the secondary standard of GB-3095-2012 was achieved, and there is still a long way to go from the primary standard. In a few words, the enhanced control and emission-reduction measures had made a certain contribution to the improvement of air quality in Beijing. However, in order to achieve the goal of better air quality throughout the year, all parties still need to coordinate control and make joint efforts.

**Author Contributions:** Conceptualization, M.C. and B.H.; formal analysis, M.D.; investigation, B.H.; resources, Q.G.; data curation, B.H. and Z.M.; writing—original draft preparation, B.H.; writing review and editing, B.H.; funding acquisition, M.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (grant number 21976094, 22176100) and the National Key Research and Development Project 9 (grant number 2018YFC0213802).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This study was finally accomplished with the assistance of co-authors. All diagrams in the manuscript, whether drawn by yourself or another author, are permitted. In a word, thanks to the cooperation units for their help in data collection and processing, at the same time, also express sincere thanks to the co-authors for their careful guidance in the chart description and article written.

**Conflicts of Interest:** The authors declare no conflict of interest.
