Vertical Observation And Analysis Of Aerosol Optical Properties And Direct Radiative Forcing Over Hangzhou

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 among aerosol optical properties,aerosol radiative forcing?ARF?and aerosol vertical structure during 2013–2015 over the YRD were investigated by employing ground-based Microwave Pulse Lidar?MPL?and CE-318 sun-photometer observations.Combining MODIS and CALIPSO satellite products reanalysis wind fields,enhanced haze pollution events affected by different types of aerosol over the YRD region were analyzed through vertical structures,spatial distributions,backward trajectories,the Potential Source Contribution Function?PSCF?and Concentration Weighted Trajectory?CWT?models.The results show that aerosols in the YRD are dominated by fine-mode particles,except in March.The aerosol optical depth?AOD?in June?1.20±0.55?and September?1.07±0.54?is higher due to high single scattering albedo?SSA?from hygroscopic growth,and the aerosol radiative forcing at the bottom?ARF-BOA?and the top of atmosphere?ARF-TOA?are-118±43 W/m²,-80±24 W/m² and-37±28 W/m²,-50±25 W/m² respectively.But AOD is lower in July?0.66±0.52?and August?0.59±0.34?due to wet deposition from precipitation,whose ARF-BOA and ARF-BOA are-88±35 W/m²,-95±33 W/m² and-23±30 W/m²,-16±21W/m².The 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 ARF 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 ARF at the surface and the enhanced cooling effect on near-surface layer in turn depresses the PBL.The PBL height?PBLH?is greater?means ranging from 1.23 to 1.84 km?and more variable in the warmer months of March to August,due to the stronger diurnal cycle and exchange of heat.Northern fine-mode pollutants are brought to the YRD at a height of 1.5km.The SSA increases blocking the radiation to the surface,and cooling the surface,thereby weakening turbulence,lowering the PBL,and in turn accelerating the accumulation of pollutants,creating a feedback to the cooling effect.Originated from the deserts in Xinjiang and Inner Mongolia,long-range transported dust masses are seen at heights of about 2 km over the YRD region with an SSA_440nm below 0.84,which heat air and upraise PBL,accelerating the diffusion of dust particles.Regional transport from biomass burning spots to the south of the YRD region bring mixed aerosol particles at a height below 1.5 km,resulting in an SSA_440nm below 0.89.