Why observed and modelled ozone production rates and sensitives differ, a case study at rural site in China

Abstract. Ground-level ozone (O3) pollution has recently become of increasing concern in China. Studies have shown that conventional models often fail to predict accurately the net O3 production rate (P(O3)net) due to the absence of certain mechanisms, particularly the kinetics from missing reactive volatile organic compounds (VOCs) species, and hence affects the reliability of evaluation for O3 formation sensitivity (OFS). Therefore, we conducted a field observation of P(O3)net and OFS using a P(O3)net (NPOPR) detection system based on a dual-channel reaction chamber technique at the Guangdong Atmospheric Supersite of China in Heshan, Pearl River Delta (PRD) in autumn of 2023. The in-situ monitoring data were then compared with results from a zero-dimensional model incorporating the Master Chemical Mechanism (MCM v3.3.1). We tested the model performance by incorporating parameterization for 4 processes including HO2 uptake by ambient aerosols, dry deposition, N2O5 uptake, and ClNO2 photolysis, and found that the discrepancies between the modelled P(O3)net (P(O3)net_Mod) and measured data (P(O3)net_Mea) did not change evidently, the maximum daily P(O3)net differed by ∼ 44.8 %. Meanwhile, we found that the agreement of OFS assessment results between the direct measurements and the modelling study was lower in the P(O3)net rising phase (08:00–09:00 LT, 63.6 %) than in the P(O3)net stable phase (10:00–12:00 LT, 72.7 %) and P(O3)net declining phase (13:00–17:00 LT, 72.7 %). The results in this study reflected that unmeasured oxygenated VOCs (OVOCs) were the most effective compensating factor for the discrepancies between observed and computed P(O3)net and OFS, hinting clearly at the importance of quantitative understanding the total reactivity of VOCs in O3 chemistry.