*4.3. Influence of VOCs on the Consumption Rate of Hydroxyl Radicals*

Contributions to the consumption rate of hydroxyl radicals from large to small are alkenes (73%), alkanes (15%), and aromatic hydrocarbons (12%). Here, the contribution of alkenes is far more significant (Figure 7). The contribution of alkenes to the consumption rate of hydroxyl radicals in Shenyang, which is a bit higher than that in Guangzhou (64%) [15], appears dominant throughout the year with the percentages from high to low in winter, spring, autumn, and summer (Table 2).

As is shown in Table 5, the total consumption rate of VOCs on the consumption rate of hydroxyl radicals is around 99.49 ppbv, and the 10 VOC components with the highest contributions are ethylene (24.64%), propylene (16.16%), 1-hexene (9.93%), 1-butene (5.17%), trans-2-butene (4.98%), cis-2-butene (3.62%), propane (3.47%), toluene (3.32%), cis-2-pentene (3.16%), and styrene (3.15%). It is significantly different from the results in Guangzhou, where the contribution of trans-2-pentene appears the largest with a percentage of 19.14% [15].


**Table 5.** 10 VOC components with the highest contributions to the consumption rate of hydroxyl radical.

## **5. Conclusions**

In Shenyang, the concentrations of TVOCs from high to low appear in winter, autumn, spring, and summer. For various types of VOCs, the concentrations of alkanes, alkenes, and alkynes appear generally the highest in winter and the lowest in summer, whereas the concentrations of aromatic hydrocarbons appear the highest in autumn and the lowest in summer. Alkanes appear with the highest concentration, followed by alkynes, alkenes, and aromatic hydrocarbons. Diurnal variations in each type of VOC experience peaks mainly in the morning and evening, among which alkanes and alkenes reach the maximum at 7:00, whereas alkyne and aromatic hydrocarbons reach a maximum in traffic peak hours of 22:00 and 21:00, respectively. The concentration of each type of VOC reaches a minimum in the afternoon (13:00–14:00).

As they are the most active type of VOC in atmospheric chemical reactions, aromatic hydrocarbons are the dominant contributor to the formation of both ozone and SOA. Contributions to the formation potential of SOA from large to small are aromatic hydrocarbons (94%), alkanes, and alkenes, which is similar with the results of the northern suburbs of Nanjing. 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. The 10 VOC components with the highest contributions are toluene, benzene, o-xylene, ethylbenzene, isoprene, 1,3,5-trimethylbenzene, m-ethyltoluene, methylcyclohexane, p-diethylbenzene, and 1,2,3-trimethylbenzene, while the contributions of toluene and benzene add up to over 70%, meaning that control of the use of a large number of solvents and vehicle emissions would be an effective method to suppress SOA formation in the Shenyang area.

Contributions to the formation potential of ozone from large to small are aromatic hydrocarbons (78%), alkene, and alkane. The contribution of aromatic hydrocarbons to the potential of ozone formation is much more significant than that of the three other types of VOCs, in accordance with the results of the urban area of Guangzhou. The discrepancy is that the contribution percentage of alkenes to ozone formation in Shenyang appears twice as high. 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. In contrast, most likely due to the different estimation method, the results of the northern suburbs of Nanjing show that alkenes contribute the most to ozone formation. The 10 VOC components with the highest contributions are toluene, isoprene, o-xylene, 1,3,5-trimethylbenzene, benzene, 1,2,4-trimethylbenzene, 1,2,3 -trimethylbenzene, ethylbenzene, methylcyclopentane, and methylcyclohexane. Similar to SOA, control of the use of a large number of solvents and vehicle emissions would be an effective way to control ozone pollution in Shenyang. Moreover, exploring solutions to control isoprene emission from broad-leaved forests and shrubs also might be factors worth considering.

Contributions to the consumption rate of hydroxyl radicals from large to small are alkenes (73%), alkanes, and aromatic hydrocarbons, while the contribution of alkenes is

far more significant, and is a bit higher than that in Guangzhou. The 10 VOC components with the highest contributions are ethylene, propylene, 1-hexene, 1-butene, trans-2-butene, cis-2-butene, propane, toluene, cis-2-pentene, and styrene.

**Author Contributions:** Conceptualization, N.L.; methodology, N.L. and X.L.; software, N.L.; validation, L.L. and C.W.; formal analysis, N.L. and L.L.; investigation, N.L.; resources, N.L.; data curation, W.R. and C.S.; writing—original draft preparation, N.L.; writing—review and editing, N.L. and X.L.; visualization, N.L.; project administration, N.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Basic Research Funds of Central Public Welfare Research Institutes, grant number 2020SYIAEZD1, the Science & Technology Project of Liaoning Province, grant number 2019JH8/10300095, and the Key program of science foundation of Liao-ning Meteorological Office, grant number D202101.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Informed consent was obtained from all subjects involved in the study.

**Conflicts of Interest:** The authors declare no conflict of interest.
