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We present a method to describe the relationship between particle drydiameter and cloud condensation nuclei (CCN) activity using a singlehygroscopicity parameter κ. Values of the hygroscopicity parameter are between0.5 and 1.4 for highly-CCN-active salts such as sodium chloride, between0.01 and 0.5 for slightly to very hygroscopic organic species, and 0 fornonhygroscopic components. Observations indicate that atmosphericparticulate matter is typically characterized by 0.1<κ<0.9. Ifcompositional data are available and if the hygroscopicity parameter of eachcomponent is known, a multicomponent hygroscopicity parameter can becomputed by weighting component hygroscopicity parameters by their volumefractions in the mixture. In the absence of information on chemicalcomposition, experimental data for complex, multicomponent particles can befitted to obtain the hygroscopicity parameter. The hygroscopicity parametercan thus also be used to conveniently model the CCN activity of atmosphericparticles, including those containing insoluble components. We confirm theapplicability of the hygroscopicity parameter and its mixingrule by applying it to published hygroscopic diameter growth factor andCCN-activation data for single- and multi-component particles containingvarying amounts of inorganic, organic and surface active compounds. Wesuggest that κ may be fit to CCN data assuming σs/a=0.072 J m−2 andpresent a table of κ derived for this value and T=298.15 K. The predictedhygroscopicities for mixtures that contain the surfactant fulvic acid agreewithin uncertainties with the measured values. It thus appears that thisapproach is adequate for predicting CCN activity of mixed particlescontaining surface active materials, but the generality of this assumptionrequires further verification

A single parameter representation of hygroscopic growth and cloud condensation nucleus activity