Phase transitions in emulsified HNO3/H2O and HNO3/H2SO4/H2O solutions

The liquid-solid phase transitions of emulsified binary HNO3/H2O and ternar y HNO3/H2SO4/H2O acid solutions have been investigated using differential s canning calorimetry in order to elucidate the mechanism of ice formation fr om stratospheric aerosols. The results indicate that binary solution drople ts with less than 33 wt % HNO3 freeze to form ice at temperatures 37-65 K b elow the equilibrium ice melting temperatures. Droplets with higher concent rations (up to 65 wt %) do not freeze; instead, they form glasses at temper atures below 160 K. The results also show that the presence of H2SO4 in amo unts corresponding to less than 5 wt % depresses the freezing points of the 0-33 wt % HNO3 droplets by 5 to >30 K, in agreement with earlier observati ons. The implications of the new data for our understanding of the formatio n of polar stratospheric clouds have been investigated using a thermodynami c model. The results indicate that an ice saturation ratio of greater than 1.65 is required for ice particles to form from ternary aerosols at stratos pheric temperatures, corresponding to a cooling of about 3 K below the ice frost point.