Strong dependence of cubic ice formation on droplet ammonium to sulfate ratio

We show that the phase of ice that crystallizes in solution droplets, under conditions relevant for the upper troposphere (UT) and tropopause region (typically > 188 K), is strongly dependent on the ammonium to sulfate ratio (ASR) of the solute. Droplets of aqueous (NH4)(3)H(SO4)(2) (ASR = 1.5) freeze dominantly to cubic ice over a range of temperatures relevant for the UT and tropopause; whereas aqueous NH4HSO4 (ASR = 1.0) droplets do not freeze dominantly to ice I-c at temperatures relevant for the UT and tropopause region. We also show that the amount of cubic ice formed in the aqueous solution droplets at temperatures below 200 K is independent of droplet size, whereas a size dependence was observed at higher temperatures. The implications of these results for the phase of ice that forms in upper tropospheric ice clouds and the potential impact on climate, water vapor, and ozone are briefly discussed.