**5. Factors Affecting Atmospheric Ozone Level**

#### *5.1. Precursors*

Photochemical reactions are the main source of tropospheric O3, whose concentration is closely related to the concentrations of NOX and VOCs. Generally, the ozone concentration in urban and suburban areas is mainly affected by photochemistry. The transmission of the "aging" urban plume has resulted in extremely high O3 levels (up to 286 ppbv) in rural sites downwind of Beijing, which are most affected by local photochemistry. In the suburbs of Shanghai, Guangzhou, and Lanzhou, strong in situ photochemical production is the main focus [25]. The characteristics of VOCs pollution and its contribution to atmospheric O3 formation in Wuhan city was studied [65]. The results showed that the local pollution source was the main source of VOCs pollution, and olefins had the highest chemical activity and the biggest contribution to atmospheric O3. Based on the observed data of atmospheric O3 and its precursors in Beijing in autumn 2004, the O3 generation efficiency in the region near the main traffic lines was calculated, and the results suggested that the reduction in VOCs emission was beneficial to the reduction in atmospheric O3 concentration [66]. Carbonyl compounds are important members of the VOCs family and are important precursors of secondary organic aerosols (SOA); alkenes, aromatics, and isoprene are primarily secondary products of carbonyl compounds; carbonyl groups are usually dominant in the formation of atmospheric O3 in rural areas [67]. Biovolatile organic compounds (BVOCs) played an important role in the formation of tropospheric O3, especially in urban areas [68]. The effects of BVOCs emission on the formation of tropospheric O3 and SOA were studied by using a WRF-CMAQ simulation system, and the results showed that the biogenic emission peaked in summer and decreased gradually from south China to north China [69]. High BVOCs emissions in eastern and southwestern China increased the ground-level ozone, particularly in the Beijing–Tianjin–Hebei region, Sichuan Basin, Yangtze River Delta and the central Pearl River Delta. The ozone isolines in the summer of 2013 showed that the O3 concentrations were controlled by NOx in most areas of China, and the effect of VOCs reduction on O3 concentration was less, except in the urban areas of Shanghai and Guangzhou [70]. An investigation into the O3 exposure indices and the source contributions in the forests of China throughout the entire year of 2013 suggested that the O3 production was much more due to NOx than due to VOCs [71].
