ALBERT

Description: Black carbon (BC) is a dominant absorber in visible spectrum and a potent factor in climatic effects. Vertical profiles of BC were measured using a micro-aethalometer attached to a tethered balloon during the Vertical Observations of trace Gases and Aerosols (VOGA) field campaign, in summer 2014 at a semirural site in the North China Plain (NCP). The diurnal cycle of BC vertical distributions following the evolution of the mixing layer was investigated for the first time in the NCP region. Statistical parameters including identified mixing height (Hm) and average mass concentrations within the mixing layer (Cm) and in free troposphere (Cf) were obtained for a selected dataset of 67 BC vertical profiles. Hm was usually lower than 0.2 km in the early morning and rapidly rose thereafter due to strengthened turbulence. The maximum height of the ML was reached in late afternoon. The top of a full developed ML exceeded 1 km on sunny days in summer, while stayed much lower on cloudy days. The sunset triggered the collapse of the ML and a stable nocturnal boundary layer (NBL) gradually formed. Accordingly, the highest level Cm was found in the early morning and the lowest in the afternoon. In the daytime, BC almost uniformly distributed within the ML and significantly decreased above the ML. During the field campaign, Cm averaged about 5.16±2.49 μg m−3, with a range of 1.12 to 14.49 μg m−3, comparable with observational results in many polluted urban areas. As evening approached, BC gradually built up near the surface and exponentially declined with height. In contrast to the large variability found both in Hm and Cm, Cf stayed relatively unaffected through the day. Cf was less than 10 % of the ground level under clean conditions, while amounted to half of the ground level in some polluted cases. In-situ measurements of BC vertical profiles would hopefully have an important implication for accurately estimating direct radiative forcing by BC and improving the retrieval of aerosol optical properties by remote sensing in this region.