Cloud droplet activation of mixed model HULIS and NaCl particles: Experimental results and κ-Köhler theory

•The kappa for NAFA for supersaturated conditions is lower than expected.•Mixed NAFA–NaCl particles have lower CCN activity than expected from kappa addition.•Surface tension (σ) depression of NAFA in bulk solution is enhanced with NaCl present.•CCN activity modeled from bulk measurements of water activity and σ is overestimated.•Modeled CCN activities assuming a σ of pure water agree well with measurements.

Significant amounts of humic-like substances (HULIS) are present in marine submicrometer particles. The cloud condensation nuclei (CCN) activation was investigated for marine model particles comprised of Nordic Aquatic Fulvic Acid Reference (NAFA) and sodium chloride (NaCl) in mass ratios of 100:0, 80:20, 50:50, 20:80 and 0:100 respectively. The CCN activity of NAFA was found to be represented by a κ value of 0.028. The CCN activities of the mixed particles were overestimated by volume weighted addition of the κ values of the pure compounds, which indicates that synergistic effects of the mixtures tend to lower the CCN activity. Parameterizations of water activity (aw) and surface tension (σ) versus solute concentration were obtained from measurements on aqueous solutions. The CCN activity was modeled on the basis of the parameterizations of aw and σ using Köhler theory. For the particles containing 50% or more NAFA the model overpredicted the CCN activity compared to observations. Reasonable model results were obtained by assuming a surface tension of pure water.