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

Chengdu Plain Urban Agglomeration (CPUA) is one of the most serious areas suffering from ozone pollution in China. A comprehensive field observation focused on the ozone production rate and its sensitivity 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 pollution episode. Peak concentration of O₃ was closely related to intense solar radiation, high temperatures, and precursor emissions. The OH-HO₂-RO₂ radical chemistry and ozone production rate (P(O₃)) were calculated using an observation-based model (OBM). The daily peak OH concentration varied in the range of 3-13 × 10⁶ molecules cm^-3, and peak HO₂ and RO₂ were in the range of 2-14 × 10⁸ molecules cm^-3 during ozone pollution episodes. During the ozone pollution episode, the average maximum of P(O₃) in suburban sites (about 30 ppbv h^-1.) was compared with urban sites, and the maximum of P(O₃) was 18 ppbv h^-1 in rural sites. The relative incremental reactivity (RIR) results demonstrate that it was a VOCs-limited regime in the central urban area of Chengdu, with NOx suppression effect in some regions. In the southern neighboring suburb of Chengdu, it was VOCs-limited as well. However, the northern suburban area was a transition region. In the remote rural areas of the southern CPUA, it was highly NOx-limited. Local ozone production driven by the photochemical process is crucial to the ozone pollution formation in 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.