Assessment of ozone formation sensitivity and possible bias analysis in an urban ambient environment using a direct measurement technique

Effectively identifying ozone (O3) formation sensitivity to its precursors, volatile organic compounds (VOCs) and nitrogen oxides (NOx=NO+NO2), is key to developing effective O3 pollution control strategies. However, assessments of photochemical O3 formation can be significantly biased by neglecting certain photochemical reactions in existing atmospheric chemical mechanisms, which in turn markedly influences the determination of O3 formation sensitivity (OFS). This study improved a dual-channel net photochemical ozone production rate (P(O3)net, NPOPR) detection system to measure OFS continuously in Dongguan, a typical industrial city in China. Three OFS regimes were identified by comparing P(O3)net levels in a base ambient air scenario to those with added NOx/VOCs—VOC-limited (37.5%), NOx-limited (12.5%), and transition (50.0%)—with notable differences observed in radical chain lengths and CO/HCHO ratios. Daily patterns showed a transition regime in early morning, shifting to NOx-limited regime in the afternoon. Measured and modelled OFS showed approximately 50% agreement, with modelled OFS often overestimating VOC-limited regimes. Further analysis indicated that unmeasured oxygenated volatile organic compounds (OVOCs) with high photolysis frequencies and anthropogenic VOCs (such as branched alkenes) act as important sources of missing VOCs reactivities (VOCR), which are potential contributors to the estimation bias of OFS. This study emphasizes the importance of comprehensive VOCs measurement for accurate OFS estimation through modelling methods, thereby offering valuable insights for developing effective O3 pollution control policies.