*3.2. Diurnal Variation*

Diurnal variations in each type of VOC show rather similar bimodal patterns, with the peaks mainly appearing in the morning and evening. Alkanes and alkenes reach their maximum at 7:00 (Beijing time, the same below), whereas alkyne and aromatic hydrocarbons reach their maximum at 22:00 and 21:00, respectively, which is basically consistent with the peak hours of traffic. The concentration of each type of VOC reaches a minimum in the afternoon (13:00–14:00) (Figure 6). In the morning rush hours, human activities, such as traffic and industry, become frequent, rapidly increasing the concentration of VOCs to reach the first peak. Then, with the gradual significance of solar radiation and turbulence, VOC levels keep decreasing until they reach a minimum in the afternoon. At night, the suspension of photochemical reactions due to solar radiation decreases, along with the

accumulated contribution of the evening traffic peak, results in the second peak of VOCs. The concentration of each type of VOC is higher at night and lower in the daytime, which is significantly related to the lack of photochemical reaction consumption at night, and the lower atmospheric boundary layer height hardly being conducive to pollutant diffusion.

**Figure 6.** Diurnal variations of concentrations for various types of VOCs in Shenyang.
