Characteristics of surface air quality over provincial capital cities in Northwestern China during 2013–2020

In this study, we have reported spatial and temporal variation in particulate matter (PM), sulfur dioxide (SO₂), nitrogen dioxide (NO₂), carbon monoxide (CO) and ozone (O₃) over five provincial capital cities in northwestern China during 2013–2020. Regarding the seasonal variation, all pollutants (except ozone) exhibited the lowest concentration during summer and the highest concentration during winter, which could be attributed to increased anthropogenic activities (like coal burning) and conducive meteorological features. The highest monthly mean concentrations were primarily observed during December-February, whereas ozone exhibited the highest concentration during April-August, with different cities experiencing the highest concentration during different seasons. Regarding the diurnal variation exhibited by the pollutants, the lowest concentration of pollutants (except O₃) was observed during the late afternoon (17:00–18:00) period. Ozone posed the urban site diurnal variation characteristic (peak during afternoon hour) over all sites. Urumqi had the highest PM_2.5/PM₁₀ ratio during November-March and the lowest ratio during April-October. Compared to the WHO revised guideline, the annual mean PM_2.5 concentration was about 8–12 times higher, whereas the annual PM₁₀ concentration was exceeded by a factor of up to 7. Most pollutants exhibited reduced concentration during the spring festival period. Analysis using HYSPLIT back trajectories indicated that the air masses affecting the five sites primarily originated from the northwestern area of China, although the impact of long-range pollution transport from remote regions should not be overlooked.