Accurate elucidation of oxidation under heavy ozone pollution: a full suite of radical measurements in the chemically complex atmosphere

Abstract

The Yangze River Delta (YRD) in China encountered prolonged ozone pollution in September 2020. To accurately elucidate the limitations of oxidation processes in the chemically complex atmosphere, a full suite of radical measurements (OH, HO2, RO2, and kOH) was established in the 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. The simulated results provided by RACM2-LIM1 failed to adequately match the observed data in both radical concentration and experimental budget for a heavy ozone pollution episode. Sensitivity tests utilizing a comprehensive set of radical measurements revealed that the Higher Aldehyde Mechanism (HAM) effectively complements the regeneration of OH radicals, yielding enhancements of 4.4 %-6.0 % compared to the base scenario, while the concentrations of HO2 and RO2 radicals have shown increments of about 7.4 % and 12.5 %, respectively. It is noteworthy that under the constraints of kOH measurement, the inclusion of oxygenated volatile organic compounds (OVOCs) and larger alkoxy radicals derived from monoterpenes improved the model-measurement consistency for ozone formation, reducing the discrepancy under high NO conditions from 4.17 to 2.39. This outcome corroborates the hypothesis of sensitivity analysis as it pertains to ozone formation. Moving forward, by implementing a comprehensive radical detection approach, further investigations should concentrate on a broader range of OVOCs to rectify the imbalance associated with RO2 radicals, thereby providing a more precise understanding of oxidation processes during severe ozone pollution episodes.