Analysis of Ozone Pollution Characteristics and Transport Paths in Xi’an City

Xi’an is a key city for air-pollution prevention and control in China, and its near-ground ozone (O3) pollution has become a key issue whose resolution is urgent. The spatial and temporal variations in ozone pollution and their relationship with meteorological factors, transport pathways, and potential source distribution in Xi’an City were investigated in this study using the backward trajectory clustering analysis, potential source contribution function (PSCF), and concentration weight trajectory (CWT) methods coupled with the hourly ozone-mass concentration observations and meteorological data from 2014 to 2020. The results indicate that Xi’an City has suffered from increasingly severe ozone pollution in recent years. Overall, the annual average mass concentration of O3_8h_max presented an upward trend, exhibiting a 34.94% increase from 2014 to 2020. A seasonal variation peak occurred in summer, and the monthly variations featured an inverted “V” shape. Furthermore, the diurnal variation was significantly affected by the near-surface atmospheric photochemical process, showing a discernible single-peak and single-valley distribution with a peak between 15:00 and 17:00. The O3 concentration demonstrated a significant linear positive correlation with surface temperature and sunshine duration. When the relative humidity was 30~50%, the temperature was above 30 °C, and the wind speed was ≤4 m/s, high-concentration O3 pollution was liable to occur. The transmission mainly relied on short-distance airflow during periods of heavy O3-polluted weather. In this context, in addition to local O3 generation, high O3 concentrations were also affected by the large NOx and VOC emissions from heavy industries in neighboring cities in Shaanxi Province, southwest Shanxi Province, and northern Henan Province. Therefore, joint prevention and control measures on the O3 pollution in Xi’an City must be adopted throughout the Fenwei Plain area with the aim of strictly controlling the emissions from regional pollution sources.