A new method for calculating number concentrations of cloud condensation nuclei based on measurements of a three-wavelength humidified nephelometer system

Abstract. The number concentration of cloud condensation nuclei (CCN) plays a fundamental role in cloud physics. Instrumentations of direct measurements of CCN number concentration (N_CCN) based on chamber technology are complex and costly; thus a simple way for measuring N_CCN is needed. In this study, a new method for N_CCN calculation based on measurements of a three-wavelength humidified nephelometer system is proposed. A three-wavelength humidified nephelometer system can measure the aerosol light-scattering coefficient (σ_sp) at three wavelengths and the light-scattering enhancement factor (fRH). The Ångström exponent (Å) inferred from σ_sp at three wavelengths provides information on mean predominate aerosol size, and hygroscopicity parameter (κ) can be calculated from the combination of fRH and Å. Given this, a lookup table that includes σ_sp, κ and Å is established to predict N_CCN. Due to the precondition for the application, this new method is not suitable for externally mixed particles, large particles (e.g., dust and sea salt) or fresh aerosol particles. This method is validated with direct measurements of N_CCN using a CCN counter on the North China Plain. Results show that relative deviations between calculated N_CCN and measured N_CCN are within 30 % and confirm the robustness of this method. This method enables simplerN_CCN measurements because the humidified nephelometer system is easily operated and stable. Compared with the method using a CCN counter, another advantage of this newly proposed method is that it can obtain N_CCN at lower supersaturations in the ambient atmosphere.