Ozone production sensitivity analysis for the Chengdu Plain Urban Agglomeration based on a muti-site and two-episode observation

Chengdu Plain Urban Agglomeration (CPUA) is one of the most serious areas of ozone pollution in China. A comprehensive field experiment focused on the ozone episode characteristics, and temporal and spatial variations of ozone production rate was conducted at CPUA in the summer of 2019. Six sampling sites were set and two ozone pollution episodes were recognized. The daily maximum 8-h average (MDA8) O₃ concentration reached 137.9 ppbv in the urban sites during the ozone episode. The high concentration of O₃ was closely related to intense solar radiation, high temperatures, and precursor emissions. Based on the calculation of OBM, the OH-HO₂-RO₂ radical chemistry and ozone production rate (P(O₃)) was analyzed. The OH daily maximum is in the range of 3-13×10⁶ molecules cm⁻³, and HO₂ and RO₂ are in the range of 2–14×10⁸ molecules cm⁻³ during ozone episodes, varying by the location of sites. During ozone episode, the average maximum of P(O₃) in suburb sites (about 30 ppb h^-1.) were compared with urban sites, while the maximum of P(O₃) was 18 ppb h^-1 in rural sites. The relative incremental reactivity (RIR) and empirical kinetic modeling approach (EKMA) results demonstrate that centered on the urban area of Chengdu, where it was a VOC-limited regime, the northern and southern suburban area was transition region. In the remote rural area of the southern CPUA, it was highly NOx-limited. Local ozone production driven by the photochemical process is important for CPUA. The geographically differentiated recognition of the ozone regime found by this study can help to tailor control strategies for local conditions and avoid the negative effects of a one-size-fits-all approach.