But volatile components, and above all water, are playing an important role for atmospheric aerosols. Increasing relative humidities in the atmosphere strongly alters the physical and chemical properties of the aerosol because of the growth of aerosol particles caused by the uptake of water. This hygroscopic growth influences the light scattering, the cloud formation and precipitation, the atmospheric lifetime, and the chemical reactivity of the individual particles.

This fact is well-known, and therefore the deliquescence behavior of different aerosol components becomes the focus of interest for a very long time. The deliquescence relative humidities (DRH) of the atmospheric most common salts (NaCl-, (NH₄)₂SO₄-, Na₂SO₄- and NH₄NO₃) are well- known. But under ambient conditions, mostly no pure salt particles are observed but rather mixtures. The deliquescence behavior of mixtures is very difficult to understand. There are not only mixtures of soluble salts, but also mixtures of salts and non soluble material, e.g. soot. Organic components can also alter the hygroscopic behavior of salt particles.

Furthermore, the deliquescence properties of the aerosol particles show hysteresis, but some particles will remain metastable in the liquid phase well below their deliquescence points. An adequate calculation of the water content of the individual water-soluble particles (or the exact deliquescence point) under ambient conditions is, thus, almost impossible. Therefore, a real in-situ-observation (optical and chemical) of the deliquescence process for particles in the µm and sub-µm range would be desirable.