**2. Materials and Methods**

*2.1. Data*


**Figure 1.** Location of monitoring sites of the Beijing Olympic Sports Center. Notes: The Chinese characters in the left picture is the meaning of Chinese Geographic Names, which also represents its geographical location. The green circle (a), yellow circle (b), blue circle (c), orange circle (d) in the left picture is the meaning of Urban environmental assessment point, Urban cleanliness control point, Area background transfer point, Traffic pollution monitoring point, respectively. And which the number of this points are 23, 1, 6, 5, respectively.

#### *2.2. Methods*

The research method adopted in this study was time series analysis in statistics. Firstly, the enhanced control measures such as traffic restriction and emission reduction adopted in Beijing and its surrounding areas during the Military Parade were summarized and sorted out. Secondly, the air quality before, during, and after the implementation of the enhanced control measures was analyzed. At the same time, the air quality in 2013 and the air quality in 2014 were compared.

Taking into account the calculation of ambient air quality in China, the 1-h concentration air quality sub-index (IAQI) classification concentration limit of particulate matter (PM2.5 and PM10) is based on the 24-h concentration AQI classification concentration limit. The study consulted AQI standards and technical regulations from Data (2), and an AQI greater than 150 (corresponding to the daily average concentration limit of PM2.5 greater than 115 μg·m<sup>−</sup>3) is defined as a pollution event, i.e., an event with continuous occurrence of moderate pollution or more is defined as a pollution event. At the same time, it is stipulated that a pollution event is composed of two parts: the pollution accumulation process and the pollution disappearance process. The pollution accumulation process refers to the process in which pollutants rise from the trough (minimum value) to the peak (maximum value). One pollution disappearance process refers to the process in which the pollution drops from the peak value to another valley value. Generally speaking, the accumulation time of the pollution accumulation process will be longer than that of the pollution disappearance process.

Supplemented by sky photos from 08:00 to 08:30 every morning as a reference, it could visually compare the evolution of air quality around the Beijing Olympic Sports Center. The photos were taken from the top floor of the Chinese Academy of Environmental Sciences to the Southwest (the tree of life sightseeing tower). Although the photos were static and instantaneous, they also reflected the actual situation of air quality in Beijing to some extent.

In short, in order to simplify the structure of the article, the implementation of enhanced control and emission-reduction measure was abbreviated as IECERM. The time of

IECERM (during the Military Parade) was from 20 August to 3 September. Among them, 15 days from 5 August to 19 August were chosen as before IECERM (before the Military Parade), and 15 days from 4 September to 18 September were selected as after IECERM (after the Military Parade).
