[Changes in O₃-VOCs-NO 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). In this study, we conducted online observations of O₃, VOCs, and NO 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 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 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 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 and VOCs indicated that O₃ production was controlled by both VOCs and NO. 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 control measures, the O₃-VOCs-NO 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.