The deliquescence behaviour of ternary aerosols composed of ammonium sulfate (AS) and water, internally mixed with malonic acid (MOA), maleic acid (MEA) and glutaric acid (GAA), has been studied using a new surface aerosol microscope setup (SAM) as well as an electrodynamic balance (EDB). In each of the systems studied the addition of the organic acids to ammonium sulfate leads to a decrease of the deliquescence relative humidities (DRH). However, the observed behaviour of the DRH values over a large range of acid concentrations is complex and indicates a eutectic behaviour. Moreover, the ternary AS/MOA/water aerosols show a two step deliquescence process whose magnitude and concentration dependence have been quantitatively investigated for the first time. The results suggest that previous DRH interpretations underestimate the strength and the atmospheric implications of the MOA influence. In addition to the deliquescence behaviour, effloresced ternary aerosols were studied with respect to their morphology and crystal behaviour using environmental scanning electron microscopy (ESEM) and Raman microscopy (RM), respectively. It is found that in each case crystalline mixtures consisting of the pure AS and pure organic acid are formed. However, the crystalline appearances of the solids formed are different from those of the effloresced pure acids. Moreover, a maximum size of the single crystallites formed during the efflorescence of these complex ternary aerosols has been assigned.
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