A novel method for deriving the aerosol hygroscopicity parameter based only on measurements from a humidified nephelometer system

Abstract. Aerosol hygroscopicity is crucial for understanding roles of aerosol particles in atmospheric chemistry and aerosol climate effects. Light-scattering enhancement factor f (RH,  λ ) is one of the parameters describing aerosol hygroscopicity, which is defined as f (RH,  λ ) =  σ sp (RH,  λ )∕ σ sp (dry,  λ ), where σ sp (RH,  λ ) or σ sp (dry,  λ ) represents σ sp at wavelength λ under certain relative humidity (RH) or dry conditions. Traditionally, an overall hygroscopicity parameter κ can be retrieved from measured f (RH,  λ ), hereinafter referred to as κ f (RH) , by combining concurrently measured particle number size distribution (PNSD) and mass concentration of black carbon. In this paper, a new method is proposed to directly derive κ f (RH) based only on measurements from a three-wavelength humidified nephelometer system. The advantage of this newly proposed approach is that κ f (RH) can be estimated without any additional information about PNSD and black carbon. This method is verified with measurements from two different field campaigns. Values of κ f (RH) estimated from this new method agree very well with those retrieved by using the traditional method: all points lie near the 1 : 1 line and the square of correlation coefficient between them is 0.99. The verification results demonstrate that this newly proposed method of deriving κ f (RH) is applicable at different sites and in seasons of the North China Plain and might also be applicable in other regions around the world.