Figure 2 Averaged (a) f44 of organic aerosols and sulfur oxidation ratios; (b) NR-PM2.5 mass concentrations and contributions from different aerosol species of each aircraft experiment.
3.2 Air pollution in free troposphere
On July 30, the aircraft ascended from the airport in Baicheng and made a clockwise flight in the North of the province at an altitude of around 2.5 km corresponding to the flight route in Fig. 1b and Fig. 3a. To capture the vertical distributions of air pollutants, the aircraft descended to about 1 km over the two nearby cities, Daan and Songyuan, as shown in Fig. 3a. The flight was carried out from 12:30 to 16:50 LT on 30 July and covered about 500 km in total distance with the horizontal speed of nearly 200 km per hour.
Fig. 3b shows the time series of SO2,O3, Bsp, and NR-PM2.5and the flight altitude during the aircraft experiment on July 30. Bsp and NR-PM2.5 tracked each other well and both varied substantially between different altitudes. Organic aerosols dominated NR-PM2.5 compositions within the PBL, the height of which can be derived from the vertical profile of temperature, which shows an obvious inversion layer at 1 900 m altitude (Fig. 4a). Once into the free troposphere, both ozone and NR-PM2.5 mass concentrations showed sharp increases. Four pollution hot spots were observed during the flight, marked as A, B, C and D in Fig. 3a. Three of the plumes distributed around 1 ~ 2 km (defined as the Lower Free Troposphere (LFT)) above the cities of Baicheng, Daan and Songyuan, indicating the existence of pollution layer near the top of the PBL. Elevated pollution layers above PBL were also reported in previous aircraft studies (Ding et al., 2009; Liu et al., 2018; Sarangi et al., 2016), attributed to the regional transport. The fourth pollution hot spot was captured in the Higher Free Troposphere (HFT), e.g., ~ 3 km above Baicheng city, at 16:15 LT, marked as ā€œDā€ in Fig. 3. Air pollutant levels were relatively low above 3.3 km.