Influence of the external mixing state of atmospheric aerosol on derived CCN number concentrations

Abstract

We derived the range of particle hygroscopicities (κ) that occurs in the atmosphere, based on literature data of measured hygroscopic growth or based on chemical composition. The derived κ‐values show that the atmospheric aerosol often is an external mixture with respect to hygroscopicity. Mean κ were derived for urban, rural, and marine aerosols for the different hygroscopic modes. Using these κ and exemplary particle number size distributions for the different aerosols, the number concentration of cloud condensation nuclei (NCCN) was derived for two cases, (1) accounting for the less hygroscopic fraction of particles and (2) assuming all particles to have κ of the more hygroscopic mode. NCCN derived from measured particle hygroscopicity is overestimated for case (2). Overestimation of NCCN is largest for fresh continental aerosol and less pronounced for marine aerosol. With κ derived from bulk aerosol composition data, only the hygroscopicity of more soluble aerosol particles is captured. Bulk or even size‐resolved composition data will be insufficient to predict NCCN under many conditions unless independent information about particle mixing state is available.