Aerosol size and relative humidity: Water uptake by mixtures of salts

The water uptake by salt mixtures at large relative humidities is discussed. Numerous results of measurements of the lowering of water vapour pressure over multicomponent electrolyte solutions have been evaluated to obtain a simple mixture rule. Results from literature and own measurements were used. It turns out that for a given relative humidity the water uptake of a mixture of salts is almost equal to the sum of the water uptakes of the pure components of the mixture when separate from each other. This rule holds for the most cases at relative humidities larger than approximately 0.85 to 0.9 with an accuracy better than 10 to 15%. A second rule with a smaller range of application is discussed in the paper.