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.