The transition from a nitrogen oxides-limited regime to a volatile organic compounds-limited regime in the petrochemical industrialized Lanzhou City, China

Heavy ozone (O-3) pollution is often observed in the petrochemical industrial city of Lanzhou, located in the semiarid and mountainous northwest China. The mountain-valley topography, special meteorological conditions, and high precursor emissions from the petrochemical industry across the city caused significantly increasing surface ozone concentrations in recent years. We investigated temporal distributions of O-3 and its precursors and explored volatile organic compounds (VOCs) 'composition and its contribution to O-3 formation. The Ozone Isopleth Plotting Research (OZIPR) model was used to illustrate the ozone isoconcentration curves and assess the responses and sensitivity of O-3 concentrations to its precursors. The results show that, in summer 2018, the VOCs/ nitrogen oxides (NOx) ratio defined in the ridgeline of the Empirical Kinetic Modeling Approach (EKMA) curve was about 13.8:1, indicating that ozone was formed in the NOx-limited regime, whereas in summer 2019, this ratio was 15:1, indicating that ozone was formed in the VOCs-limited regime, featuring a transition from a NOx-limited regime in summer 2018 to a VOCs-limited regime in summer 2019. We explored potential causes of this transition and attributed it to a large-scale instrument overhaul that resulted in significantly lower OH radical concentrations in summer 2019 and lower VOC emission after the overhaul. The modeling results using the Regional Atmospheric Chemical Mechanism version 2 (RACM2) photochemical box model show that the peak OH radical concentration in the summer of 2018 was much higher than that of 2019, reaching 12.38 x 10(6 )molecules.cm(-3). The result was confirmed by the Community Multiscale Air Quality (CMAQ) modeling system.