Observations and modeling of OH and HO₂ radicals in Chengdu, China in summer 2019

This study reports on the first continuous measurements of ambient OH and HO₂ radicals at a suburban site in Chengdu, Southwest China, which were collected during 2019 as part of a comprehensive field campaign 'CompreHensive field experiment to explOre the photochemical Ozone formation mechaniSm in summEr - 2019 (CHOOSE-2019)'. The mean concentrations (11:00-15:00) of the observed OH and HO₂ radicals were 9.5 × 10⁶ and 9.0 × 10⁸ cm^-3, respectively. To investigate the state-of-the-art chemical mechanism of radical, closure experiments were conducted with a box model, in which the RACM2 mechanism updated with the latest isoprene chemistry (RACM2-LIM1) was used. In the base run, OH radicals were underestimated by the model for the low-NO regime, which was likely due to the missing OH recycling. However, good agreement between the observed and modeled OH concentrations was achieved when an additional species X (equivalent to 0.25 ppb of NO mixing ratio) from one new OH regeneration cycle (RO₂ + X → HO₂, HO₂ + X → OH) was added into the model. Additionally, in the base run, the model could reproduce the observed HO₂ concentrations. Discrepancies in the observed and modeled HO₂ concentrations were found in the sensitivity runs with HO₂ heterogeneous uptake, indicating that the impact of the uptake may be less significant in Chengdu because of the relatively low aerosol concentrations. The ROx (= OH + HO₂ + RO₂) primary source was dominated by photolysis reactions, in which HONO, O₃, and HCHO photolysis accounted for 34%, 19%, and 23% during the daytime, respectively. The efficiency of radical cycling was quantified by the radical chain length, which was determined by the NO to NO₂ ratio successfully. The parameterization of the radical chain length may be very useful for the further determinations of radical recycling.