Temporally and spatially resolved investigations of the efflorescence of equimolar mixtures of ammonium sulfate (AS) and ammonium nitrate (AN) aerosol droplets were performed by confocal Raman spectroscopy combined with high-speed photography. Information was obtained on the dynamic process of morphological changes and the heterogeneous distribution of efflorescence products. The supersaturated AS/AN mixed droplet was found to first precipitate as the double salt 3AN·AS when the relative humidity was 50%. The excess AS was irregularly distributed on the surface of the double salt as an inclusion structure. No formation of 2AN·AS was observed throughout the efflorescence process. Temporally and spatially resolved investigations of the efflorescence of equimolar mixtures of ammonium sulfate (AS) and ammonium nitrate (AN) aerosol droplets were performed by confocal Raman spectroscopy combined with high-speed photography. Information was obtained on the dynamic process of morphological changes and the The supersaturated AS / AN mixed droplet was found to first precipitate as the double salt 3AN · AS when the relative humidity was 50%. The excess AS was irregularly distributed on the surface of the double salt as an inclusion structure No formation of 2AN · AS was observed throughout the efflorescence process.