Peroxy radical chemistry during ozone photochemical pollution season at a suburban site in the boundary of Jiangsu-Anhui-Shandong-Henan region, China.

Ambient peroxy radical (RO = HO + RO) concentrations were measured at a suburban site in a major prefecture-level city (Huaibei) in the boundary of Jiangsu-Anhui-Shandong-Henan region, which is the connecting belt of air pollution in the Beijing-Tianjin-Hebei region and the Yangtze River Delta. Measurements were carried out during the period of September to October 2021 to elucidate the formation mechanism of O pollution. The observed maximum concentration of peroxy radicals was 73.8 pptv. A zero-dimensional box model (Framework for 0-Dimensional Atmospheric Modeling, F0AM) based on Master Chemical Mechanism (MCM3.3.1) was used to predict radical concentrations for comparison with observations. The model reproduced the daily variation of peroxy radicals well, but discrepancies still appear in the morning hours. As in previous field campaigns, systematic discrepancies between modelled and measured RO concentrations are observed in the morning for NO mixing ratios higher than 1 ppbv. Between 6:00 and 9:00 am, the model significantly underpredicts RO by a mean factor of 7.2. This underprediction can be explained by a missing RO source of 1.2 ppbv h which originated from the photochemical conversion of an alkene-like chemical species. From the model results it shows that the main sources of RO (= OH + HO + RO) are the photolysis of oxygenated volatile organic compounds (OVOCs, 33 %), O and HONO (25 %), and HCHO (24 %). And the major sinks of RO transitioned from a predominant reaction of radicals with NO in the morning to a predominant peroxy self- and cross-reaction in the late afternoon. The introduction of an alkene-like species increased RO radical concentration and resulted in 14 % increase in net daily integrated ozone production, indicating the possible significance of the mechanism of alkene-like species oxidation to peroxy radicals. This study provides important information for subsequent ozone pollution control policies in Jiangsu-Anhui-Shandong-Henan region.