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Abstract. In recent years, ozone pollution has become one of the most severeenvironmental problems in China. Evidence from observations have showedincreased frequency of high O3 levels in suburban areas of the YangtzeRiver Delta (YRD) region. To better understand the formation mechanism oflocal O3 pollution and investigate the potential role of isoprenechemistry in the budgets of ROx (OH+HO2+RO2) radicals,synchronous observations of volatile organic compounds (VOCs), formaldehyde(HCHO), and meteorological parameters were conducted at a suburban site ofthe YRD region in 2018. Five episodes with elevated O3 concentrationsunder stagnant meteorological conditions were identified; anobservation-based model (OBM) with the Master Chemical Mechanism was appliedto analyze the photochemical processes during these high O3 episodes.The high levels of O3, nitrogen oxides (NOx), and VOCs facilitatedstrong production and recycling of ROx radicals with the photolysis ofoxygenated VOCs (OVOCs) being the primary source. Our results suggest thatlocal biogenic isoprene is important in suburban photochemical processes.Removing isoprene could drastically slow down the efficiency of ROx recyclingand reduce the concentrations of ROx. In addition, the absence of isoprenechemistry could further lead to a decrease in the daily average concentrationsof O3 and HCHO by 34 % and 36 %, respectively. Therefore, thisstudy emphasizes the importance of isoprene chemistry in the suburbanatmosphere, particularly with the participation of anthropogenic NOx.Moreover, our results provide insights into the radical chemistry thatessentially drives the formation of secondary pollutants (e.g., O3 andHCHO) in suburban areas of the YRD region.