Accurate Elucidation of Oxidation Under Heavy Ozone Pollution: A Full Suite of Radical Measurement In the Chemical-complex Atmosphere

Abstract. The Yangze River Delta (YRD) in China encountered with prolonged ozone pollution in September 2020, which had significant impacts on the respiratory, dermatological, and visual health of local residents. To accurately elucidate the limitations of oxidation processes in the chemical-complex atmosphere, a full suite of radical measurements (OH, HO₂, RO₂, and k_OH) was established in YRD region for the first time. The diurnal peaks of radicals exhibited considerable variation due to environmental factors, showing ranges of 3.6 to 27.1×10⁶ cm^-3 for OH, 2.1 to 33.2×10⁸ cm^-3 for HO₂, and 4.9 to 30.5×10⁸ cm^-3 for RO₂. At a heavy ozone pollution episode, the oxidation capacity reached an intensive level compared with other sites, and the simulated OH, HO₂, and RO₂ radicals provided by the RACM2-LIM1 mechanism failed to adequately match the observed data both in concentration and coordinate ratios. Sensitivity tests based on the full suite of radical measurement revealed that the X mechanism accelerated OH regeneration, and the introduction of larger RO₂ isomerization steps alleviated the RO₂-related imbalance by 2 to 4 times. The hypothesis of sensitivity analysis can be chemically validated by the special HCHO contribution to oxidation. Constraining HCHO increased the ChL from 1.94 to 4.45, leading to a 51.54 % increase in ozone production during the heavy pollution. The incorporation of complex processes enabled better coordination of HO₂/OH, RO₂/OH, and HO₂/RO₂ ratios comparable to the observed values, and adequately addressed the deficiency in the ozone generation mechanism within a certain range. The full-chain radical detection untangled a gap-bridge between the photochemistry and the intensive oxidation level in the chemical-complex atmosphere, enabling a deeper understanding of the tropospheric radical chemistry at play.