The paper presents a method to describe the relationship between particle dry diameter and cloud condensation nuclei (CCN) activity using a single hygroscopicity parameter, κ. The parameter κ ranges from 0.5 to 1.4 for highly CCN-active salts like sodium chloride, 0.01 to 0.5 for slightly to very hygroscopic organic species, and 0 for non-hygroscopic components. Observations show that atmospheric particulate matter typically has a κ value between 0.1 and 0.9. If compositional data are available, the hygroscopicity parameter of each component can be weighted by its volume fraction to compute a multicomponent hygroscopicity parameter. The method is validated by applying it to published hygroscopic diameter growth factor and CCN-activation data for single- and multi-component particles containing varying amounts of inorganic, organic, and surface-active compounds. The authors confirm the applicability of the hygroscopicity parameter and its mixing rule, suggesting that κ can be fit to CCN data assuming σs/μ = 0.072 J m−2. They also present a table of κ values derived for this assumption and T = 298.15 K, showing good agreement with measured values for mixtures containing the surfactant fulvic acid. The approach is deemed adequate for predicting CCN activity of mixed particles containing surface active materials, but further verification is needed.The paper presents a method to describe the relationship between particle dry diameter and cloud condensation nuclei (CCN) activity using a single hygroscopicity parameter, κ. The parameter κ ranges from 0.5 to 1.4 for highly CCN-active salts like sodium chloride, 0.01 to 0.5 for slightly to very hygroscopic organic species, and 0 for non-hygroscopic components. Observations show that atmospheric particulate matter typically has a κ value between 0.1 and 0.9. If compositional data are available, the hygroscopicity parameter of each component can be weighted by its volume fraction to compute a multicomponent hygroscopicity parameter. The method is validated by applying it to published hygroscopic diameter growth factor and CCN-activation data for single- and multi-component particles containing varying amounts of inorganic, organic, and surface-active compounds. The authors confirm the applicability of the hygroscopicity parameter and its mixing rule, suggesting that κ can be fit to CCN data assuming σs/μ = 0.072 J m−2. They also present a table of κ values derived for this assumption and T = 298.15 K, showing good agreement with measured values for mixtures containing the surfactant fulvic acid. The approach is deemed adequate for predicting CCN activity of mixed particles containing surface active materials, but further verification is needed.