[Changes in O3-VOCs-NOx Sensitivity and VOCs Sources at an Urban Site of Nanjing Between 2020 and 2021]

The synergistic control of PM_2.5 and ozone (O₃) are the focus of air quality improvement during the 14^th Five-Year Plan in China. The production of O₃ shows a highly nonlinear relationship with its precursors volatile organic compounds (VOCs) and nitrogen oxides (NO_x). In this study, we conducted online observations of O₃, VOCs, and NO_x at an urban site in downtown Nanjing from April to September of 2020 and 2021. The average concentrations of O₃ and its precursors between these two years were compared, and then the O₃-VOCs-NO_x sensitivity and the VOCs sources were analyzed using the observation-based box model (OBM) and positive matrix factorization (PMF), respectively. The results showed that the mean daily maximum O₃ concentrations, VOCs, and NO_x concentrations decreased by 7% (P=0.031), 17.6% (P<0.001), and 14.0% (P=0.004) from April to September of 2021 compared with those from the same period in 2020, respectively. The average relative incremental reactivity (RIR) values of NO_x and anthropogenic VOCs during the O₃ non-attainment days in 2020 and 2021 were 0.17 and 0.14 and 0.21 and 0.14, respectively. The positive RIR values of NO_x and VOCs indicated that O₃ production was controlled by both VOCs and NO_x. The O₃ production potential contours (EKMA curves) based on the 50×50 scenario simulations also supported this conclusion. The PMF results showed that industrial and traffic-related emissions were the main sources of VOCs. The five PMF-resolved factors were identified as industrial emissions, including industrial liquefied petroleum gas (LPG) use, the benzene-related industry, petrochemistry, toluene-related industry, and solvent and paint use, which contributed 55%-57% of the average mass concentration of total VOCs. The summed relative contributions of vehicular exhaust and gasoline evaporation were 43%-45%. Petrochemistry and solvent and paint use showed the two highest RIR values, suggesting that VOCs from these two sources should be reduced with priority to control O₃. With the implementation of VOCs and NO_x control measures, the O₃-VOCs-NO_x sensitivity and VOCs sources have changed, and therefore we still need to follow their variations in the future to timely adjust O₃ control strategies during the 14^th Five-Year Plan.

[Ozone Formation and Key VOCs of a Continuous Summertime O3 Pollution Event in Ji'nan]

In the summer of 2019, field measurements of ozone (O₃) and its precursors[volatile organic compounds (VOCs) and nitrogen oxides (NO_x)] were carried out at an urban site in Ji'nan. We found that the daily maximum 8-hour averages φ(O₃) were (103.0±14.5)×10^-9. The average φ(NO_x) and φ(VOCs), which are ozone precursors, were (16.7±11.3)×10^-9and (22.4±9.4)×10^-9, respectively. The ·OH reactivity of VOCs was determined (9.6±3.8) s^-1. Ji'nan suffered from serious O₃ pollution. An observation-constrained chemical box model was deployed to evaluate in situ photochemical O₃ production, which indicated that chemical reactions made positive contributions to O₃ production rates between 07:00 and 19:00 LT, with the average hourly O₃ production rate of 35.6×10^-9 h^-1. To evaluate the effectiveness of various ozone precursor control strategies in reducing ozone pollution, we combined the observation-based model (OBM) with the relative incremental reactivity (RIR) method. The key indicators that affect the local ozone production rate were identified. Ji'nan was under VOC-limited conditions and the key VOC precursors were alkenes. The O₃ formation mechanism changed from the VOC-limited regime in the morning to the transitional regime in the afternoon. Correspondingly, the simulated local O₃ production rate was increased from 18.3×10^-9 h^-1 to 29.6×10^-9 h^-1. To further explore the role of anthropogenic emissions in ozone pollution, we used the positive matrix factorization (PMF) model to identify the major sources contributing to VOCs. The major sources in Ji'nan were vehicular exhaust and gasoline evaporation, accounting for more than 50% of the observed VOCs. Therefore, constraints on vehicular emissions is the most effective strategy to control O₃ pollution in Ji'nan.