Characterization of vertical distribution and radiative forcing of ambient aerosol over the Yangtze River Delta during 2013-2015.

As the central part of eastern China, the Yangtze River Delta (YRD) region, with its rapid economic growth and industrial expansion, has experienced severe air quality issues. In this study, the monthly variation and interaction between aerosol direct radiative forcing (ADRF) and aerosol vertical structure during 2013-2015 over the YRD were investigated using ground-based observations from a Micro Pulse Lidar (MPL) and a CE-318 sun-photometer. Combining satellite products from MODIS and CALIPSO, and reanalysis wind fields, an integrated discussion of a biomass burning episode in Hangzhou during August 2015 was conducted by applying analysis of optical properties, planetary boundary layer (PBL), spatial-temporal and vertical distributions, backward trajectories, Potential Source Contribution Function (PSCF), and Concentration Weighted Trajectory (CWT). The results reveal that a shallower PBL coincides with higher scattering extinction at low altitude, resulting in less heating to the atmosphere and radiative forcing to the surface, which in turn further depresses the PBL. In months with a deeper PBL, the extinction coefficient decreases rapidly with altitude, showing stronger atmospheric heating effects and ADRF to the surface, facilitating the turbulence and vertical diffusion of aerosol particles, which further reduces the extinction and raises the PBL. Because of the hygroscopic growth facilitated by high relative humidity, June stands out for its high scattering extinction coefficient and relatively low PBL, and the reduced ADRF at the surface and the enhanced cooling effect on near-surface layer in turn depresses the PBL. Absorptive aerosols transported from biomass burning events located in Zhejiang, Jiangxi, and Taiwan provinces at 1.5 km, result in high ADRF efficiency for atmospheric heating. And the enhanced heating effect on near-surface layer caused by absorptive particles facilitates PBL development in August over the YRD.