*3.1. Seasonal Variations*

In Shenyang, the total concentration of the 52 types of VOCs (TVOC) from high to low appears in winter, autumn, spring, and summer, whereas the concentration of ozone from high to low appears in summer, spring, autumn, and winter. The TVOC reaches a maximum in January, with a value of a little more than 33 ppbv, and reaches a minimum in June. The concentration of ozone reaches a maximum in May and July, with a value of near 50 ppbv in July, and a minimum in December (Figure 2). For various types of VOCs, the concentrations of alkanes, alkenes, and alkynes generally appear highest in winter and lowest in summer, but the concentrations of aromatic hydrocarbons were highest in autumn and lowest in summer. Alkanes appear with the highest concentration, followed by alkynes, alkenes, and aromatic hydrocarbons (Figure 3). In general, seasonal variations of VOC concentrations are mainly influenced by three factors, namely, the variations of VOC source strength, the various degrees of photochemical reaction due to the seasonal levels of hydroxyl radical and the variations of the atmospheric mixing state. The higher values of TVOC and its components in autumn and winter over Shenyang are mainly due to the large amount of emission of primary pollutants from heating in autumn and winter, which directly or indirectly influences the source strength of VOCs. In addition, the atmospheric photochemical reaction is hardly active enough to remove VOCs due to the lower temperatures in autumn and winter. Moreover, the relatively stable atmospheric and temperature inversion at night are not conducive to the diffusion and dilution of

pollutants [31]. The fact that aromatic hydrocarbons appear higher in autumn than in winter was also found in the study of Baoji, Shanxi, with the process here related to the combustion emissions of petrochemical, pharmaceutical, pesticide, paint, and other industries and fossil fuels [32].

**Figure 2.** Monthly variation of concentrations for total VOCs and ozone in Shenyang.

**Figure 3.** Monthly variation of concentrations for various types of VOCs in Shenyang.

The highest concentration component is acetylene, with a mean annual concentration of a little more than 5 ppbv. Ethane has the highest concentration among alkanes, with a mean annual concentration of just less than 5 ppbv. Ethylene has the highest concentration among alkenes, with a mean annual concentration of somewhat below 3 ppbv. Benzene has the highest concentration among aromatic hydrocarbons, with a mean annual concentration of around 1 ppbv. Compared with the northern suburbs of Nanjing, the concentrations of ethane, ethylene, and benzene are all lower, but the concentration of acetylene is slightly higher in Shenyang [8], which is significantly related to the emissions from traffic sources and industrial sources in both places. During 2019 in Shenyang, industrial emissions of NOx and VOCs are 31,603.05 tons and 6475.59 tons, respectively, whereas town emissions of NOx and VOCs are 3916.15 tons and 1172.63 tons, respectively [33].

For the three VOC environmental monitoring sites in Shenyang, the annual mean VOC concentrations from high to low appear at HGY, DNS, and TZSR, whereas the annual mean ozone concentration from high to low appears at DNS, TZSR (with the same level as DNS), and HGY. The concentration of each VOC component is the highest in HGY, which is located in an industrial area, whereas the concentration of ozone is lower than those at DNS and TZSR, which are located to its east and northeast (Figure 4). The reason is that the dominant wind direction in Shenyang is southwesterly on average throughout the year, while the wind speed is over 3 m/s, which is faster than those of other directions (Figure 5). Driven by the dominant wind, ozone formed locally and VOCs at the site located in the upwind direction (e.g., HGY) are transported to the east and northeast, with the further formation of ozone due to photochemical reaction during this process [25]. Elevated ozone then appears in the downwind direction (e.g., at DNS and TZSR sites).

**Figure 4.** Concentrations for various types of VOCs and ozone at each site in Shenyang.

**Figure 5.** Wind frequency (%) and wind speed (m/s) roses in Shenyang.
