Vertical characteristics of aerosol hygroscopicity and impacts on optical properties over the North China Plain during winter

Abstract. The water-uptake on aerosol influences its optical depth and capacity of cloud formation, depending on the vertical profile of aerosol hygroscopicity because of different solar radiation received and supersaturation conditions at different atmospheric levels. Such information is lack over the polluted eastern Asian region. This study presents aircraft in-situ measured aerosol size distribution and chemical compositions by series of flights over Beijing area in wintertime. Under high relative humidity conditions (surface RH > 60 %, hRH), a significant enhancement of aerosol hygroscopicity parameter (κ) in the planetary boundary layer (PBL) was observed to increase by 50 % up to 0.32 from the surface to the top of PBL (vertical gradient of ~0.13 km−1), along with the dry particle effective diameter (Deff) increased by 71 % and activation ratio up to 0.23 (0.64) at SS = 0.05 % (0.1 %), in contrast with a lower vertical gradient of κ (0.05 km−1) and smaller Deff under low RH conditions (surface RH