*4.2. Ozone Formation Potential in VOCs*

Contributions to the formation potential of ozone from large to small are aromatic hydrocarbons (78%), alkene (15%), and alkane (7%). Contributions of aromatic hydrocarbons to the potential of ozone formation are much more significant than those of the three other types of VOCs (Figure 7), meaning that it is the most active type of VOC in atmospheric chemical reactions, in accord with the results from an examination of the urban area of Guangzhou [15]. The main discrepancy between the two regions is that the percentage

of alkenes contributing to ozone formation in Shenyang appears two times higher than that in Guangzhou (38%). Thus, it is shown that aromatic hydrocarbons are the dominant contributor to the formation of ozone and SOA, rather than alkanes, although alkanes are the chief VOC component in the atmospheric environment of Shenyang. The contributions of aromatic hydrocarbons to the potential of ozone formation appear the most significant throughout the year, with the percentages from high to low in autumn, winter, spring, and summer (Table 2). In contrast, the results of the northern suburbs of Nanjing [8] show that alkenes contribute the most to ozone formation, which is probably due to the different estimation method.

As is shown in Table 4, the total formation potential of ozone in each VOC component is about 5.74 ppbv, and the 10 VOC components with the highest contributions in 2019 were toluene (29.64%), isoprene (14.83%), o-xylene (10.79%), 1,3,5-trimethylbenzene (10.36%), benzene (9.13%), 1,2,4-trimethylbenzene (8.32%), 1,2,3-trimethylbenzene (5.09%), ethylbenzene (3.87%), methylcyclopentane (2.06%), and methylcyclohexane (1.68%). In accordance with the observations in Guanzhou [15,36], toluene contributes the most to ozone formation, while the contribution of isoprene is not ignorable. In general, isoprene originates not only from biological emissions throughout the year at times other than in winter [37], but also from anthropogenic emissions, especially traffic emissions [38], meaning that control of the use of a large number of solvents and vehicle emissions is also an effective way to control ozone pollution in Shenyang. Moreover, exploring solutions to control isoprene emission from broad-leaved forests and shrubs [37] might also be factors worth considering.


**Table 4.** 10 VOC components with the highest contributions to ozone formation potential.
