**1. Introduction**

The significant role of volatile organic compounds (VOCs) in the process of atmospheric photochemical reactions was first recognized by Haagen-Smit [1]. Since then, the formation of photochemical pollutants such as ozone (O3) and secondary organic aerosols (SOAs) by VOCs and NOx via solar radiation has been widely recognized [2–4]. Studying the impact of VOCs and their atmospheric chemical reactions on the formation of atmospheric secondary pollutants is of great significance, and is essential for the sake of not only controlling air pollution, but for revealing the mechanism of atmospheric photochemical reactions.

Factors determining the influence of VOCs on ozone formation in the atmosphere include the levels of hydroxyl radical and NOx, the proportions of various reaction processes and the efficiencies of NO transforming into NO2 during these processes, as well as the reaction rates of various VOCs and hydroxyl radicals. The complexity of VOC components results in an uncertain relationship between them and ozone formation. In the early 1990s, the United States Environmental Protection Agency produced a set of standard methods for the analysis of environmental atmospheric VOCs through research, with the intention of not only reducing NOx, but also of controlling the active VOC components in order to effectively solve the problem of ozone pollution [5,6]. Compared with foreign countries, the study of VOC–NOx–ozone in China started late, and the concentration limit and measurement specification of VOCs in the atmosphere have not been clearly defined until now

**Citation:** Liu, N.; Li, X.; Ren, W.; Li, L.; Su, C.; Wang, C. Concentration Characteristics and Photochemical Reactivities of VOCs in Shenyang, China. *Atmosphere* **2021**, *12*, 1240. https://doi.org/10.3390/ atmos12101240

Academic Editors: Duanyang Liu, Kai Qin and Honglei Wang

Received: 12 August 2021 Accepted: 20 September 2021 Published: 23 September 2021

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(Ambient Air Quality Standard (GB3095-2012) [7]). Furthermore, the research is mainly focused on the three major polluted areas in China, namely, the Beijing-Tianjin-Hebei region, Pearl River Delta, and Yangtze River Delta, where VOCs play very different roles in the process of ozone formation [8–16]. At present, studies of the VOC–NOx–ozone relationship and the interactions of VOCs in atmospheric chemical reactions are still urgently needed.

In the formation of urban haze pollution in China, the secondary generated aerosol components account for 51%–77%, of which organic aerosol is the most significant component [17,18]. Observations from 37 sites around the world show that SOA accounts for more than 90% in remote areas and more than 60% in urban areas, despite a relatively high proportion of primary aerosol [16]. In recent years, a set of aerosol formation coefficients (fractional aerosol coefficients, or FACs) were proposed by Grosjean and Seinfeld [19] based on a large number of smoke chamber experimental data and atmospheric chemical kinetics data. Several studies conducted by scholars inside and outside China on SOA formation potential, combining VOCs' observation data and FACs, have consistently shown that aromatic hydrocarbons contribute the most to SOA formation potential [19–22]. Wang et al. [23] studied the SOA formation potential of all aromatic hydrocarbons and isoprene in Shenzhen using the FAC method, and found that toluene contributed the most to SOA formation. In addition, the reactions between VOCs and hydroxyl radicals are the main chemical processes for the transformation of organic compounds, in which the consumption rate of hydroxyl radical is used to evaluate the chemical activity of various VOC species [24].

As the largest city in northeast China, Shenyang's atmospheric environmental problems have hardly been solved in the recent two decades. Despite strong governance, the urbanization trend, leading to a current population of over 9.1 million and an urban area that has rapidly grown to 3500 km2 (within the total area of Shenyang of 13,000 km2), makes air pollution a continuing and significant problem in this metropolis. Since 2007, most of the original heavy industry enterprises in downtown Shenyang have been relocated to a new economic development zone approximately 20 km away to the west of the urban center; thus, it is not industrial emissions but traffic emissions that directly affect the ambient air quality over the Shenyang urban area with a total of 2.65 million automobiles, which increase at the rate of 800 per day. Although a number of studies have analyzed the mechanism of pollution formation in Shenyang and its ambient areas [25], few of them have focused on VOCs. In this study, using the whole-year hourly data of 52 types of VOC at each of the three sites over the year 2019, photochemical reactivities of VOCs were revealed, and the prior VOC species were further selected to provide theoretical support for local pollution management.
