Elevated aerosol layers within the daytime mixed layer over ecologically sensitive areas of northwest China: diurnal variation, formation mechanisms and regional effects

Abstract

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 the elevated aerosol layer (EAL) embedded within the daytime mixed layer over the Hexi Corridor, northwest China, defined as a high-concentration layer above the surface but within the 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 that EALs frequently occur at altitudes of 0.6–2 km during daytime. Excluding dust events, the occurrence frequencies were 82 % and 44 % at Dunhuang and Minqin, respectively; the layers were more frequently dust-influenced at Dunhuang and fine-mode/anthropogenic-aerosol-influenced at Minqin. 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 Extinction-derived PM10-equivalent mass concentration within EAL (203.3 ± 106.6 µg m−3) at ∼1.2 km is five times higher than that at surface (40.8 ± 30.4 µg m−3). Furthermore, the EALs enhance the atmospheric shortwave heating rate (up to 0.7 K d−1), indicating a potential impact on regional snowmelt/glacier retreat. These findings suggest that the 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.