Measurement report: Vertical and temporal variability of near-surface ozone production rate and sensitivity in an urban area in Pearl River Delta (PRD) region, China.

Understanding the near-ground vertical and temporal photochemical O₃ formation mechanism is important to mitigate the O₃ pollution. Here, we measured the vertical profiles of O₃ and its precursors at six different heights from 5-335 m using a newly built vertical observation system in Pearl River Delta (PRD) region, China. The net photochemical ozone production rate (P (O₃)_net) and O₃ formation sensitivities at various heights were diagnosed using an observation-based model coupled with the Master Chemical Mechanism (MCM v3.3.1). Moreover, for the assessment of model performance and the causative factors behind O₃ pollution episodes, the net photochemical ozone production rate (P (O₃)_net) was measured at 5 m ground level utilizing a custom-built detection system. In total three O₃ pollution episodes and two non-episodes were captured. The identified O₃ pollution episodes were found to be jointly influenced by both photochemical production and physical transport, with local photochemical reactions play a dominate role. The high index of agreement (IOA) calculated from comparing the modelled and measured P (O₃)_net values indicated the rationality to investigate the vertical and temporal variability of O₃ formation mechanism using modelling results. However, the measured P (O₃)_net values were generally higher than the modelled P (O₃)_net values, particularly under high NOx conditions, which may indicate a potential underestimation of total RO₂ by the model. Throughout the measurement period, the contribution of different reaction pathways to O₃ production remained consistent across various heights, with HO₂+NO as the major O₃ production pathway, followed by RO₂+NO. We saw P (O₃)_net decreased with the increase of the measurement height, primarily attributed to the decreased O₃ precursors anthropogenic organic compounds (AVOC) and oxygenated volatile organic compounds (OVOC). O₃ formation regimes were similar at different heights during both episodes and non-episodes, which was located either in volatile organic compounds (VOCs) sensitive regime or in transition regime and more sensitive to VOCs. Diurnally, photochemical O₃ formation typically remained in the VOCs sensitive regime during the morning and noon time, but in the transitional regime and more sensitive to VOCs in the afternoon at around 16:00 LT. The vertical and temporal O₃ formation are most sensitive to AVOC and OVOC, which suggests that targeting VOCs, especially AVOC and OVOC, for control measures is more practical and feasible at the observation site. The vertical temporal analysis of O₃ formation mechanisms near the ground surface in this study provides critical foundational knowledge for formulating effective short-term emergency and long-term strategies to combat O₃ pollution in the PRD region of China. [ABSTRACT FROM AUTHOR]