Particulate matter (PM) and ozone air quality in Urumqi of Northwest China: Seasonality, trends, and sources

Abstract

The city of Urumqi experiences severe air pollution, for which the variations and causes remain unclear. Here, we comprehensively investigate the seasonality, trends, and drivers of fine and coarse particulate matter (PM_2.5 and PM_2.5-10) and ozone (O₃) in Urumqi in the Chinese national context by statistical analysis of surface and satellite observations during 2015–2023. Wintertime PM_2.5 in Urumqi is twice as high as that averaged over Eastern China due to intensive emissions and unfavourable topography and meteorology, monthly PM_2.5-10 in Urumqi of above 40 μg m⁻³ year-round is driven by spring and autumn natural dust episodes and ubiquitous anthropogenic fugitive dust. During 2015–2023, PM_2.5 and PM_2.5-10 decreased by 34–38% credited to emissions control. However, the maximum daily 8h average (MDA8) O₃ in Urumqi increases rapidly at a rate of 2.5 ppbv yr⁻¹ due to increased volatile organic compounds (VOCs), decreased nitrogen oxides (NO_x) in earlier years of 2015–2016, and decreased PM_2.5. Impacts from VOCs and NO_x emission changes are supported by summertime O₃ formation regime transits from VOCs-limited to NO_x and VOCs co-limited during 2015–2023 as depicted by surface O₃-nitrogen dioxide (NO₂) correlations and satellite formaldehyde (HCHO)/NO₂ ratios. Responses of PM_2.5, PM_2.5-10, O₃, and related gases during the 2020 winter and summer epidemic lockdowns conform to our findings for their 2015–2023 trends. This study concludes that joint NO_x and VOCs emissions control would be particularly effective in reducing PM_2.5 and O₃ in Urumqi. This study also provides references for studying air quality in other places with limited observations.