Aqueous aerosol may build up an elevated upper tropospheric ice supersaturation and form mixed-phase particles after freezing.

Observations often reveal large clear-sky upper tropospheric ice supersaturation, S(i), which sometimes reaches 100%. However, a water activity criterion (Nature 2000, 406, 611) does not allow the buildup of large S(i) by cooled aqueous aerosol. According to the criterion, S(i) produced by aqueous aerosol increases from approximately 52% at 220 K to only approximately 67% at 185 K. The nature of the formation of large upper tropospheric S(i) remains unclear. Here we present the results of the study of micrometer-scaled three-, four-, and five-component droplets containing different weight fractions of H(2)O, H(2)SO(4), HNO(3), (NH(4))(2)SO(4), (NH(4))HSO(4), NH(4)NO(3), and (NH(4))(3)H(SO(4))(2). The study was performed between 133 and 278 K at cooling rates of 3, 0.1, and 0.05 K/min using differential scanning calorimetery. We find that complex phase transformations, which include one, two, and three freezing and melting events, glass transition on cooling, and devitrification and crystallization-freezing on warming, can occur during the cooling and warming of droplets. Using the measured freezing temperature of ice, T(i), and the thermodynamic E-AIM model, we calculate the largest clear-sky S(i) which would be formed immediately prior to the formation of ice cirrus by homogeneous freezing of multicomponent aerosol. The calculations show that multicomponent aerosol of some compositions may produce S(i) >80% at temperatures higher than 185 K. We also find that similar to that of H(2)SO(4)/H(2)O and H(2)SO(4)/HNO(3)/H(2)O aerosol the freezing of multicomponent aerosol can also produce mixed-phase cirrus particles: an ice core + a residual solution coating.