Assessment of atmospheric photochemical reactivity in the Yangtze River Delta using a photochemical box model

We investigated variations of ozone (O-3), its precursors, and their influence on photochemical reactivity by applying a photochemical box model to field data from three cities (Nantong, Xuzhou, and Yancheng) in the Yangtze River Delta. The photochemical box model was coupled with the near-explicit mechanism (Master Chemical Mechanism, MCM version 3.2) for the degradation of volatile organic compounds (VOCs), which provided detailed descriptions of different pathways for photochemical O-3 formation and related atmospheric photochemical reactivity based on observed data. Alkanes were the most important VOC group overall, while the contributions of aromatics and alkenes varied by city. The highest levels of O-3 and VOCs (indicating more severe photochemical pollution) were found in Nantong. This induced a higher contribution of O-3 photolysis to HOx production and promoted OH-HO2 propagation, resulting in maximum HOx (OH + HO2) and higher O-3 formation rates from RO2 + NO. In Yangchen, the highest occurrence of alkenes resulted in the maximum contribution of ozonolysis to HOx production. In Nantong, the highest levels of NO2 and HO2 led to the greatest contributions of OH + NO2 and HO2 + HO2 to HOx destruction. However, the longest OH chain length was found in Xuzhou, suggesting that O-3 production was most efficient there. Although vehicular emissions were the dominant sources of VOCs overall, industrial emissions in Nantong, solvent usage in Xuzhou and vehicular emissions in Yancheng were the most important VOC sources for O-3 formation. These results show that appropriate photochemical pollution control measures should consider both the emission sources and photochemical reactivities of VOCs.