Characteristics of Weak Anthropogenic Influences on High Background Ozone Concentrations in the Desert Cities of Western China

There are large gaps in the domestic and international research on the characteristics of ozone (O₃) concentration evolution in desert areas. Alxa Left Banner is a typical desert city in western China; it has high background O₃ concentrations because of the low intensity of anthropogenic activities. From 2015 to 2022, the annual growth of daily maximum 8-h average ozone (MDA8 O₃) was approximately 2 μg m⁻³. The average MDA8 O₃ values in spring, summer, autumn, and winter were 111.3 μg m⁻³, 132.3 μg m⁻³, 101.8 μg m⁻³, and 82.9 μg m⁻³, respectively. The existence of favorable conditions, namely, high temperature and strong radiation, promotes the atmospheric photochemical reaction of O₃ and enhances the secondary production of O₃; therefore, O₃ pollution incidents mainly occur in the summer. The O₃ concentrations in Alashan Left Banner were 93–106 μg m⁻³ on summer nights under the influence of the NO titration effect, indicating that this region featured a high background O₃ level. The greater the distance from the observation point to the ground, the higher the O₃ concentration. The observations showed that the near-surface anthropogenic emissions have a direct and significant influence on the vertical O₃ concentrations at three stations (Huanbaojuxinlou, Mengeryou, and Xihuayuan stations) with different heights over the ground surface. The O₃ concentration at Huanbaojuxinlou station (22 m above the ground) was set as a criterion. The O₃ concentration at Mengeryou station, 15 m above the ground, was lower by 5–25 μg m⁻³ during the night and higher by 5 μg m⁻³ at noon. The O₃ concentration at Xihuayuan station, 12 m above the ground, was lower by 10–25 μg m⁻³ during the night and higher by 5 μg m⁻³ at noon. A strong NO titration effect was observed in the near-ground atmosphere in the urban areas at night. However, high O₃ concentrations were observed in the near-ground atmosphere because of convective diffusion and rapid photochemical reactions. Thus, the weak effect of human activities on O₃ concentration was illustrated, and the vertical distribution characteristics of O₃ in desert cities were captured.