The Impact of Multi-Scale Turbulence Structures within the Urban Canopy of a Basin City on Haze Pollution Processes

Abstract. The urban canopy layer (UCL) is strongly affected by human activities, with rough elements and anthropogenic heat sources enhancing turbulence complexity. In basin cities, the terrain further complicates the influence of turbulence structures on the haze pollution processes, which still remains unclear. Based on field observations in Lanzhou, China, this study identified a typical evolution pattern of haze pollution in the UCL: PM_2.5 concentrations rose from 08:00–13:00, declined from 14:00–18:00, rose again from 19:00–21:00, and declined from 22:00–07:00. The occurrence of unexpected thermal plumes promotes the increase in PM_2.5 concentrations during 10:00–13:00. The rapid removal by small-scale eddies with timescales less than 15 min or even 2 min contributes to the reduction of PM_2.5 concentrations during 14:00–18:00. At the same time, thermal driven turbulent eddies form organized coherent structures, ejections and sweeps, which affected the exchange of pollutants interior and exterior of the UCL. Turbulence intensity was generally weak, coupled with evening peak emissions leading to a rise in PM_2.5 concentrations during 19:00–21:00. Finally, sub-mesoscale motions can enhance pollutant dispersion by driving intermittent bursts of turbulence, or reduce pollutant concentrations by forming organized coherent structures during 22:00–07:00. These complex multi-scale atmospheric motions interact and collectively influence the evolution of haze pollution within the basin UCL. These findings are of great significance for understanding the dynamics and thermodynamics of UCL, improving the accuracy of urban air pollution forecasting.