Wind shear structures and their association with the vertical distribution of PM_2.5 in Taiyuan Basin

Vertical wind shear (VWS) is a key feature of the dynamical structure of the atmospheric boundary layer, yet its association with the vertical distribution of pollution remains insufficiently understood in complex terrain. This study examined VWS structures and their relationships with PM_2.5 in Taiyuan Basin using surface observations, wind profiler radar, microwave radiometer, aerosol lidar, radiosonde, unmanned aerial vehicle measurements, and Weather Research and Forecasting model simulations. Unsupervised clustering identified three distinct VWS regimes. Class_1 featured strong near-surface shear dominated by directional changes and was associated with pronounced near-surface PM_2.5 accumulation, strong stability, and the shallowest boundary layer. Class_2 exhibited enhanced wind speed shear above 500 m and an elevated PM_2.5 maximum near this height. Class_3 showed strong wind speed shear through most of the layer below 2000 m, together with stronger winds and lower PM_2.5 concentrations. UAV observations within 0-500 m confirmed distinct PM_2.5 profiles among the three regimes. A pollution episode on 17 January 2024 further showed that strong VWS did not necessarily enhance vertical exchange when the shear layer remained above a shallow, stable boundary layer. Its influence became more evident as daytime heating deepened the boundary layer toward the shear layer, whereas evening boundary-layer contraction renewed near-surface accumulation. These results demonstrate that the relationship between VWS and PM_2.5 depends on shear height and composition and on the evolving thermal structure of the boundary layer.