*4.1. SOA Formation Potential in VOCs*

Figure 7 shows the chemical reactivities of various types of VOCs. Contributions to the formation potential of SOA from large to small are aromatic hydrocarbons (94%), alkanes (4%), and alkenes (2%). Despite the lower level of aromatic hydrocarbons compared with other types of VOCs, they play an absolutely dominant role in the contribution to the formation of SOA, which is demonstrated in the results of the northern suburbs of Nanjing [8,22], which show a similar percentage, indicating that the formation of SOA is significantly influenced by aromatic hydrocarbons. The dominant contributions of aromatic hydrocarbons to the formation potential of SOA appear in various seasons, with the percentage from high to low in autumn, spring, winter, and summer (Table 2).

**Figure 7.** Photochemical reactivities for various types of VOCs in Shenyang.


**Table 2.** Seasonal variations of the contribution of VOCs to ozone and SOA formation potential and hydroxyl radical consumption rate.

The total formation potential of SOA in each VOC component is around 8.04 ppbv, while the 10 VOC components with the highest contributions in 2019 were toluene (39.21%), benzene (31.07%), o-xylene (5.93%), ethylbenzene (5.52%), isoprene (2.33%), 1,3,5-trimethylbenzene (2.12%), m-ethyltoluene (2.09%), methylcyclohexane (1.80%), p-diethylbenzene (1.51%), and 1,2,3-trimethylbenzene (1.47%) (Table 3). Similar to the results in the northern suburbs of Nanjing [8], the contribution percentages of toluene appear the highest in Shenyang as well, where the percentage is higher than that in the northern suburbs of Nanjing (27.28%). The contribution of benzene is next, and, along with the contribution of toluene, the total percentage add up to over 70%. Benzene and toluene, including other benzene series which contribute significantly to the formation of SOA, are both key raw materials of the organic chemical industry that can be used as solvents [34]. Furthermore, they are among the important products of flow sources [35]. Therefore, VOC components that contribute more to SOA formation in Shenyang come significantly from solvents and vehicle emissions, meaning that control of the use of a large number of solvents and vehicle emissions is an effective method to suppress SOA formation.


**Table 3.** 10 VOC components with the highest contributions to SOA formation potential.
