Significant Vertical Difference in Aerosol Chemistry Within Urban Boundary Layer Triggered by Cold‐Air Pool: Insights From Simultaneous Mountain‐Valley Measurements

Abstract Valley‐basin terrains represent one of Earth's most prominent landforms and host numerous urban settlements. However, these topographically constrained regions frequently experience severe winter aerosol pollution. One critical challenge in elucidating the formation and evolution mechanisms of valley aerosol pollution lies in the precise quantification of its complex vertical difference in aerosol chemistry. To address this, we conducted simultaneous high‐resolution real‐time field measurements at two distinct elevations (urban surface and mountaintop sites with approximately 640 m vertical separation) in Lanzhou, a typical urban valley in northwest China, in January 2021. Significant vertical differences were observed in submicron aerosol (PM 1 ) chemical composition, sources, and temporal variations within this confined terrain. Primary emissions from residential cooking, traffic, and heating activities were major contributors to ground‐level PM 1 (averaging 42%), whereas secondary aerosols dominated (76%) at the mountaintop. Most notably, vertical differences in primary aerosol contributions reached ∼40% during persistent cold‐air pool (CAP) episodes characterized by strong temperature inversions and suppressed development of boundary layers. Our study quantitatively reveals the vertical variations in aerosol chemistry, demonstrating that synoptic systems and boundary layer dynamics critically govern air quality in valley cities by regulating vertical mixing. Furthermore, these findings highlight that combining precise CAP weather forecasts with targeted primary emission controls could be a highly effective strategy for mitigating winter aerosol pollution in similar topographically confined regions globally.