Ozone pollution mechanism and VOCs emission characteristics in an oil industry city of northwestern China: analysis of a photochemical event using an observation-based model

Ozone (O₃) pollution is a critical environmental issue, especially in rapidly developing industrial regions such as Dushanzi, an oil and petrochemical hub in northwestern China. This paper investigated the first 4-day ozone pollution event in Dushanzi in June 2023. Meteorological parameters, O₃ and its precursor concentrations were utilized, combined with the Observation-based model (OBM), Positive Matrix Factorization (PMF), and backward trajectory analysis, to investigate the causes of ozone pollution in the region. The OBM-MCM model assesses the sensitivity of ozone and its generation-loss mechanisms, revealing that O₃ is mainly generated through the HO₂+NO pathway and lost via the NO₂+RO₂ pathway. On polluted days, the region falls into the VOCs-NOx collaborative control zone, with an optimal VOCs to NOx reduction ratio of 1:2. Under this reduction ratio, focusing on emissions reduction from the petrochemical industry, particularly controlling OVOCs, would be beneficial for reducing ozone pollution. Based on OFP and PMF analyses, petrochemical emissions were identified as the primary source of O₃ pollution, with OVOCs contributing the most to O₃ formation. Meteorological data revealed that high temperatures, low humidity and low wind speeds are external triggers of ozone pollution in the region. Meanwhile, the impact of long-range transport in the eastern Shihezi-Changji region cannot be ignored. This study provides important scientific basis for local governments to formulate O₃ pollution control strategies.