## 3 Results and Discussion
### Continuous Observation Experiment
#### 3.4.2 Pollution Characteristics of VOCs at Different Functional Areas  
To further understand the pollution characteristics of VOCs at different functional areas of Hefei, VOC data were collected from three sampling sites using PTR-TOF-MS. The VOCs from the three different functional areas were broadly classified into three major categories: aromatic hydrocarbons (AHs), aliphatic hydrocarbons (ALHs), and oxygenated volatile organic compounds (OVOCs). Figure 21 shows the average concentration of different groups of VOCs observed throughout the navigation observation period. OVOCs accounted for a large proportion of total VOCs at all three sampling sites, followed by ALHs and AHs. The RA site had a relatively low concentration of total VOCs with a mean value of 92.8 ppb. The CD site was second with a mean value of 125.3 ppb. The IP site had a relatively high concentration of total VOCs with a mean value of 178.1 ppb, with OVOCs accounting for 65% of all VOCs, while AHs and ALHs accounted for 21% and 14%, respectively. The high proportion of OVOCs was mainly from industrial emissions at the IP site. At the RA and CD sites, OVOCs are likely emitted from vegetation and major human activities [44; 45]. Individual analyses of the data from the three sampling sites were carried out to determine the chemical compositions of the VOCs further.  
Figure 22 shows the percentages of individual VOCs. The compounds shown in red font in the legend are O\({}_{3}\) precursors.  
Case 1: On 4 August 2021, at approximately 5:30 p.m., navigation monitoring was carried out about 80 m south of the kindergarten on Wangxi Road in Shushan District, Hefei (see Figure 22a). The concentrations of this region's top three O\({}_{3}\) precursors were 19.82 ppb, 15.84 ppb, and 15.33 ppb, respectively. The area reflects the characteristics of vehicle emissions and chemical solvents, with relatively high levels of aromatic hydrocarbons in VOCs [46]. In particular, the region exceeded the evening peak hour limit for methyl mercaptan, ethyl mercaptan, and butyl mercaptan; methyl mercaptan is an ozone precursor with a detected concentration of 0.49 ppb.  
Case 2: On 5 August 2021, at around 11:00 a.m., navigation monitoring of the Hefei Economic Development Zone was carried out (see Figure 22b). Toluene is a component of the VOCs in the area, accounting for 51.47% of the total and detected at concentrations up to 48.7 ppb. There are many sources of toluene, which may be emitted from industrial activities, vehicle exhaust, and chemical solvents. The proportion of xylene/ethylbenzene, butene, and aniline also cannot be ignored.  
Case 3: On 5 August 2021, at around 2:00 p.m., navigation monitoring was conducted downstream of the IP of Shushan District, Hefei. Among the VOCs (Figure 22c), xylene/ethylbenzene was the most abundant substance at the IP site, accounting for 78.97%  
Figure 21: Concentrations of the different groups of VOCs were collected at the three sampling sites.  
of the total top ten. This can be attributed to the presence of rubber and paint factories, where xylene/ethylbenzene is used as an essential chemical material in the production of rubber and paint. Meanwhile, VOCs such as aldehydes and ketones, which are products of the oxidation of hydrocarbons, can be detected in high quantities downstream of the contaminated area. The three case studies above show that the concentrations and compositions of VOCs have regional characteristics.