Photochemical smog, characterized by high concentrations of ozone (O3) and fine particles (PM2.5) in the atmosphere, has become one of the top environmental concerns in China. Volatile organic compounds (VOCs), one of the key precursors of O3 and secondary organic aerosol (SOA) (an important component of PM2.5), have a critical influence on atmospheric chemistry and subsequently affect regional and global climate. Thus, VOCs have been extensively studied in many cities and regions in China, especially in the North China Plain, the Yangtze River Delta and the Pearl River Delta regions where photochemical smog pollution has become increasingly worse over recent decades. This paper reviews the main studies conducted in China on the characteristics and sources of VOCs, their relationship with O3 and SOA, and their removal technology. This paper also provides an integrated literature review on the formulation and implementation of effective control strategies of VOCs and photochemical smog, as well as suggestions for future directions of VOCs study in China.

Ozone (O₃), a main component in photochemical smog, is a secondary pollutant formed through complex photochemical reactions involving nitrogen oxides (NO_x) and volatile organic compounds (VOCs).

Over the past 10 years (2005–2014), ground-level O₃ in Hong Kong has consistently increased in all seasons except winter, despite the yearly reduction of its precursors, i.e. nitrogen oxides (NO_x = NO + NO₂), total volatile organic compounds (TVOCs), and carbon monoxide (CO). To explain the contradictory phenomena, an observation-based box model (OBM) coupled with CB05 mechanism was applied in order to understand the influence of both locally produced O₃ and regional transport. The simulation of locally produced O₃ showed an increasing trend in spring, a decreasing trend in autumn, and no changes in summer and winter. The O₃ increase in spring was caused by the net effect of more rapid decrease in NO titration and unchanged TVOC reactivity despite decreased TVOC mixing ratios, while the decreased local O₃ formation in autumn was mainly due to the reduction of aromatic VOC mixing ratios and the TVOC reactivity and much slower decrease in NO titration. However, the decreased in situ O₃ formation in autumn was overridden by the regional contribution, resulting in elevated O₃ observations. Furthermore, the OBM-derived relative incremental reactivity indicated that the O₃ formation was VOC-limited in all seasons, and that the long-term O₃ formation was more sensitive to VOCs and less to NO_x and CO in the past 10 years. In addition, the OBM results found that the contributions of aromatics to O₃ formation decreased in all seasons of these years, particularly in autumn, probably due to the effective control of solvent-related sources. In contrast, the contributions of alkenes increased, suggesting a continuing need to reduce traffic emissions. The findings provide updated information on photochemical pollution and its impact in Hong Kong.

In the summer of 2017, measurements of ozone (O₃) and its precursors were carried out at an urban site in Jinan, a central city in the North China Plain (NCP). A continuous O₃ pollution event was captured during 4–11 August, with the maximum hourly O₃ mixing ratio reaching 154.1 ppbv. Model simulation indicated that local photochemical formation and regional transport contributed 14.0±2.3 and 18.7±4.0 ppbv h⁻¹, respectively, to the increase in O₃ during 09:00–15:00 LT (local time) in this event. For local O₃ formation, the calculated OH reactivities of volatile organic compounds (VOCs) and carbon monoxide (CO) were comparable between O₃ episodes and non-episodes (p>0.05), so was the OH reactivity of nitrogen oxides (NO_x). However, the ratio of OH reactivity of VOCs and CO to that of NO_x increased from 2.0±0.4 s⁻¹ s¹ during non-episodes to 3.7±0.7 s⁻¹ s¹ during O₃ episodes, which resulted in the change in the O₃ formation mechanism from the VOC-limited regime before the O₃ pollution event to the transitional regime during the event. Correspondingly, the simulated local O₃ production rate during the event (maximum: 21.3 ppbv h⁻¹) was markedly higher than that before the event (p<0.05) (maximum: 16.9 ppbv h⁻¹). Given that gasoline and diesel exhaust made large contributions to the abundance of O₃ precursors and the O₃ production rate, constraint on vehicular emissions is the most effective strategy to control O₃ pollution in Jinan. The NCP has been confirmed as a source region of tropospheric O₃, where the shift in regimes controlling O₃ formation like the case presented in this study can be expected across the entire region, due to the substantial reductions of NO_x emissions in recent years.