3.2.1. Hourly Concentration of PM2.5

For evaluating the improvement effect of enhanced control and emission-reduction measures on air quality in Beijing, Figure 2 showed the hourly concentration of PM2.5 at Beijing Olympic Sports Center monitoring sites combined with photos of the sky from 8:00 a.m. to 08:30 a.m from 5 August to 19 September in 2015. By comparing the sky photos before, during, and after the traffic restriction, it could be seen that a blue sky appeared for 7 days during the traffic restriction and only appeared for 5 days before and after the traffic restriction. Before the restrictions, there were three pollution processes, the longest of which was four days. During the three pollution accumulation processes, the hourly concentration of PM2.5 exceeded 100 μg·m<sup>−</sup>3, and the maximum concentration of PM2.5 reached 153 μg·m−<sup>3</sup> during the pollution accumulation process. There were no pollution processes during the traffic restriction. The maximum concentration of PM2.5 reached 58 μg·m−<sup>3</sup> in the pollution accumulation process, and the concentration of PM2.5 remained below 100 μg·m<sup>−</sup>3, indicating that fine weather was dominant during the period of traffic restriction, but there were only 7 days with blue skies, which may be related to the weather situation at that time. After the traffic restriction, there was a pollution process. Compared with the traffic restriction, the pollution accumulation time and pollution degree increased, especially from 14 September to 18 September. The pollution accumulation time was most serious for 5 days, and the concentration of PM2.5 reached a maximum of 252 μg·m−<sup>3</sup> during the pollution process. As can be seen from the maximum concentration of PM2.5 in the pollution accumulation process, the air quality during the traffic restriction was better than that before or after the traffic restriction. In addition, the average concentration of PM2.5 for 15 days before, during, and after the traffic restriction was 66, 18, and 52, respectively. The concentration of PM2.5 during the traffic restriction period was significantly lower than the concentration at other times, which showed that the enhanced control and emissionreduction measures significantly improved Beijing's air quality.

**Figure 2.** Hourly concentration of PM2.5 in Monitoring sites of Beijing Olympic Sports Center. Notes: The yellow line represents the trends of hourly concentration of PM2.5 when before, during, and after the restriction (from 5 August to 19 September). The *y*-axis is the concentration of PM2.5, among the unit is μg·m<sup>−</sup>3. The *x*-axis is the date when from 5 August to 19 September. The picture shows the daily weather conditions at 07:00 of the monitoring site from 5 August to 19 September.
