Abstract
Abstract. The number concentration of cloud condensation nuclei (CCN) plays a fundamental role in cloud physics. Instrumentations of direct measurements of CCN number concentration (NCCN) based on chamber technology are complex and costly; thus a simple way for measuring NCCN is needed. In this study, a new method for NCCN 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 (σsp) at three wavelengths and the light-scattering enhancement factor (fRH). The Ångström exponent (Å) inferred from σsp 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 σsp, κ and Å is established to predict NCCN. 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 NCCN using a CCN counter on the North China Plain. Results show that relative deviations between calculated NCCN and measured NCCN are within 30 % and confirm the robustness of this method. This method enables simplerNCCN 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 NCCN at lower supersaturations in the ambient atmosphere.
Highlights
Cloud condensation nuclei (CCN) are the aerosol particles that form cloud droplets by hygroscopic growth
In the new method of this paper, using Å and κc to indicate the influence of aerosol size and aerosol hygroscopicity on aerosol CCN activity will increase the deviation of number concentration (NCCN) calculation, which is much larger than the deviation due to the assumption of aerosol mixing state
NCCN is a key parameter of cloud microphysics and aerosol indirect radiative effect
Summary
Cloud condensation nuclei (CCN) are the aerosol particles that form cloud droplets by hygroscopic growth. The direct measurement of NCCN is achieved in a chamber under supersaturated conditions (Hudson, 1989; Nenes et al, 2001; Rose et al, 2008). Due to the requirement of high accuracies of working conditions like temperatures, vapors and flow rates in chambers, the direct measurement of NCCN is complex and costly (Rose et al, 2008; Lathem and Nenes, 2011). There are methods for calculating NCCN based on measurements of aerosol optical properties. NCCN as well as the hygroscopicity parameter (κ) can be calculated based on measurements of a humidified nephelometer system in combination with aerosol particle number size distribution (PNSD; Ervens et al, 2007; Chen et al, 2014).
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