3.2.2. Weather Background during the Military Parade

In view of the fine weather during the military parade, the weather background during the military parade period from 1 September to 4 September was analyzed by combining the weather chart three times a day and the one-hour wind field once a day. On this account, the influence of the weather situation on air quality in Beijing was investigated, for details, refered to Figures 3–6.

**Figure 3.** Maps of the weather and AQI situation at 07:00 on 1 September in 2015. Notes: (**a**,**c**,**d**) shows the map of the surface, 500 hPa height, 850 hPa height, respectively. (**b**) shows the automatic ground observation of 1 h wind field at 07:00 on 1 September in 2015.

**Figure 4.** Maps of the weather and AQI situation at 07:00 on 2 September in 2015. Notes: (**a**,**c**,**d**) shows the map of the surface, 500 hPa height, 850 hPa height, respectively. (**b**) shows the automatic ground observation of 1 h wind field at 07:00 on 2 September in 2015.

**Figure 5.** Maps of the weather and AQI situation at 07:00 on 3 September in 2015. Notes: (**a**,**c**,**d**) shows the map of the surface, 500 hPa height, 850 hPa height, respectively. (**b**) shows the automatic ground observation of 1 h wind field at 07:00 on 3 September in 2015.

**Figure 6.** Maps of the weather and AQI situation at 08:00 on 4 September in 2015. Notes: (**a**,**c**,**d**) shows the map of the surface, 500 hPa height, 850 hPa height, respectively. (**b**) shows the automatic ground observation of 1 h wind field at 08:00 on 4 September in 2015.

At 07:00 on 1 September in 2015, the ground situation chart which can be seen from Figure 3a showed that Beijing was in the lower part of the cold high pressure, which was influenced by the northeast airflow. At altitude, (d) resembled to (c), and Beijing was located on the north side of the North China vortex, which was influenced by the northwest airflow. After that, the vortex continued to move eastward, and the wind speed reached force 3–4 m·s−<sup>1</sup> (b), which was beneficial to the spread of pollutants. The main pollutant on that day was O3, with a concentration of 53 μg·m<sup>−</sup>3, while the concentrations of other pollutants, such as PM2.5, PM10, NO2, and SO2, were 10, 15, 23, and 2 μg·m<sup>−</sup>3, respectively, and the AQI was 21, which indicated the air quality was excellent.

At 07:00 on 2 September in 2015, Figure 4 provided the ground situation map (a), 500 hPa height map (b), 850 hPa height map (c) and automatic ground observation of 1 h wind field, respectively. Among of them, (a) showed that the North China Vortex continues to move eastward, the west was controlled by Hetao high pressure, the east was controlled by low pressure, and all were influenced by northwest airflow. (c) showed that the eastern part of mainland China was mainly influenced by the northwest airflow behind the cold trough, Beijing was located behind the trough, and the wind speed was 2–4 m·s−<sup>1</sup> according to (b). (d) showed that Beijing was in the front of the high-pressure trough and the rear of the low-pressure trough. Under the influence of the northwest airflow, the particulate matter that accumulated in the air could be removed, and the air quality in the whole of North China was improved. The main pollutant was O3, and other pollutants were lower. Finally, the AQI was 25. These all indicated the air quality was excellent, which also laid the foundation for the subsequent "Military Parade blue".

Figure 5 gived the ground situation map (a), 500 hPa height map (b), 850 hPa height map (c) and automatic ground observation of 1 h wind field at 07:00 on 3 September in 2015, respectively. (a) showed that the western part of Mainland China was controlled by Hetao high pressure, while the eastern part was controlled by low pressure. Beijing was in the front of the high center, affected by the northerly airflow, and the wind speed was maintained at force 1–2 m·s−<sup>1</sup> based on (b). The (d) showed that Beijing was in the front of the high-pressure trough and the back of the low-pressure trough, affected by the northerly airflow. (c) showed that North China was behind the cold trough, and "Military Parade blue" appeared in Beijing. Similar to the previous two days, O3 was still the main pollutant, and the concentration increased to 81 μg·m<sup>−</sup>3. Additionally, the concentration of PM10 increased slightly. The AQI was 36, and the air quality was still excellent, which may be related to the season at that time in Beijing.

Figure 6a showed that the western part of Northern China was controlled by Mongolian low pressure and the eastern part was by high pressure. As could be seen from (c), the entirety of mainland China lied behind the cold trough, mainly by the west airflow and a weak warm advection eastward. Due to the influence of southwest airflow, the water vapor in Beijing was increasing. In addition, on account of the existence of an inversion layer, the pollution diffusion condition became worse, and the pollutants' concentration increased slightly. Among them, the concentration of PM10 increased from 31 μg·m−<sup>3</sup> to 33 μg·m<sup>−</sup>3, which was the main pollutant on that day. NO2 and PM2.5 increased to 33 μg·m−<sup>3</sup> and 53 μg·m<sup>−</sup>3, respectively. AQI also increased from 36 to 73, while only O3 decreased slightly compared with yesterday. These all indicated that the weather situation had a great impact on air quality [16].

It could be seen from the above analysis that due to the influence of the strong cold high-pressure system during the Military Parade, Beijing was in the front or lower part of the high-pressure system. Additionally, the weather situation was conducive to the diffusion of pollutants under the influence of north or northeast winds.
