Elevated Aerosol Layers within the Daytime Mixed Layer Over Ecologically Sensitive Areas of Northwest China: Diurnal Variation, Formation Mechanisms and Regional Effects

Abstract. The cyclic processes of aerosol evolution have a profound impact on regional climate, water resources and ecosystems. However, studies on diurnal variations of aerosol vertical distribution in typical ecologically sensitive areas remain scarce due to limited availability of high-resolution profiles. This study first identifies elevated aerosol layer (EAL) within daytime mixed layer over the Hexi Corridor, northwest China, defined as a high-concentration layer above surface but within mixed layer. Based on intensive observation campaigns conducted in 2010 and 2012, we analyze the diurnal variation, formation mechanisms, air quality and radiative effects of EALs. The results show EALs frequently occur at altitudes of 0.6–2km during daytime. Excluding dust storms, occurrence frequencies reaches 81% (37%) in Dunhuang (Minqin) region , dominated by dust aerosol (anthropogenic pollutants). Driven by a thermodynamic coupling effect characterized by positive anomalies in potential temperature and negative anomalies in relative humidity, aerosols accumulate at the bottom of the stable stratification. Thus, a simplified conceptual model for prediction is proposed. The peak PM10 concentration within EAL (203.3±106.6μg/m³) at ~1.2km is five times higher than that at surface (40.8±30.4μg/m³). Furthermore, EALs enhance the atmospheric shortwave heating rate (up to 0.7K/day), and form a daily-scale heating pump that may accelerate snowmelt/glacier retreat in the region. These findings suggest that aerosol vertical evolution should receive greater consideration in air pollution prevention, ecosystem protection, water resource management, and wind/solar energy utilization in ecologically sensitive areas-particularly over complex terrains-rather than focusing solely on surface air pollution.