Aqueous Aerosol May Build Up an Elevated Upper Tropospheric Ice Supersaturation andForm Mixed-Phase Particles after Freezing

摘要

Observations often reveal large clear-sky upper tropospheric ice supersaturation, S, which sometimes reaches100%. However, a water activity criterion (Nature 2000, 406, 611) does not allow the buildup of large S, bycooled aqueous aerosol. According to the criterion, S, produced by aqueous aerosol increases from -52% at220 K to only ~67% at 185 K. The nature of the formation of large upper tropospheric S, remains unclear.Here we present the results of the study of micrometer-scaled three-, four-, and five-component dropletscontaining different weight fractions of H20, H2SO4, HNO3, (NH4)2SO4, (NH4)HSO4, NH4NO3, and(NH_4)_3H(SO_2. The study was performed between 133 and 278 K at cooling rates of 3, 0.1, and 0.05 K/minusing 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 andcrystallization-freezing on warming, can occur during the cooling and warming of droplets. Using the measuredfreezing temperature of ice, T1, and the thermodynamic E-AIM model, we calculate the largest clear-sky S,which would be formed immediately prior to the formation of ice cirrus by homogeneous freezing ofmulticomponent aerosol. The calculations show that multicomponent aerosol of some compositions may produce > 80% at temperatures higher than 185 K. We also find that similar to that of H2SO4/H20 and H2SO4/HNO_3/H_2Oaerosol the freezing of multicomponent aerosol can also produce mixed-phase cirrus particles: anice core + a residual solution coating.