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

摘要

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 (sigma(sp)) at three wavelengths and the light-scattering enhancement factor (fRH). The angstrom ngstrom exponent (angstrom) inferred from sigma(sp) at three wavelengths provides information on mean predominate aerosol size, and hygroscopicity parameter (kappa) can be calculated from the combination of fRH and angstrom. Given this, a lookup table that includes sigma(sp), kappa and angstrom 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 simpler N-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.