Single freezing and triple melting of micrometre-scaled (NH4)2SO4/H2O droplets.

Atmospheric aerosol droplets containing NH4+and SO42−ions are precursors of cirrus ice clouds. However, the low-temperature phase transformation of such droplets is not understood yet. Here we show for the first time that micrometre-scaled (NH4)2SO4/H2O droplets produce one freezing event but three melting events which are the melting of (i) pure ice, (ii) eutectic ice/(NH4)2SO4, and (iii) eutectic ice/(NH4)3H(SO4)2. We also find that the melting of ice/(NH4)3H(SO4)2consists of two eutectic melting events, presumably ice/letovicite-II and ice/letovicite-III. [ABSTRACT FROM AUTHOR]