Probing the mechanism of prolonged ozone pollution in arid and semi-arid urban areas

Tropospheric ozone has significant impacts on human health, atmospheric oxidizing capacity and global climate change. Despite stringent control measures, summer ozone pollution in the arid and semi-arid regions of China has increased. The average ozone (90th percentile of the daily maximum 8-h sliding average) concentration in Lanzhou increased by 53 μg m⁻³ from 2013 to 2021. The primary factors driving changes in ozone are not clear, hampering the implementation of effective control strategies. This study aims to address the above issue by using the WRF-Chem model, as well as observational data in three arid and semi-arid urban stations (Lanzhou, Yinchuan, and Xining) from 2013 to 2021. The results indicated that changes in anthropogenic emissions such as NO_X (nitrogen oxides), VOC_S (volatile organic compounds), and particulate matter, led to a 0.98 ppb increase while meteorological variations contributed to a 1.68 ppb increase in summer daytime ozone concentration in Lanzhou from 2013 to 2021. Anthropogenic emissions primarily affect ozone levels by influencing the chemical and vertical mixing processes in the atmosphere. Changes in ozone concentration in Lanzhou from 2013 to 2021 were primarily influenced by meteorological variations rather than changes in anthropogenic emissions. Observational data in Lanzhou, Yinchuan, and Xining showed that summer ozone concentration exhibited stronger correlation with relative humidity than with temperature, which is a unique phenomenon compared to those cities in humid regions in eastern China. This study enhances our understanding of the mechanisms underlying ozone pollution in industrial cities located in arid and semi-arid regions.