Homogeneous freezing of concentrated aqueous nitric acid solutions at polar stratospheric temperatures

The freezing behavior of aqueous nitric acid solutions was investigated in order to elucidate the formation mechanism of solid polar stratospheric clo uds (PSCs). Drops with composition ranging from 40 to 60 wt % HNO3 were pre pared and their phase transitions were monitored with an optical microscope . Homogeneous nucleation rates of nitric acid dihydrate (J(NAD)) and nitric acid trihydrate (J(NAT)) at temperatures between 175 and 195 K were estima ted from the data. Classical nucleation theory was used to parametrize the results into simple equations to calculate J(NAT) and J(NAD) for different temperatures and concentrations of the liquid. The nucleation rate of the n itric acid hydrates was found to depend predominantly on the saturation rat io of the liquid with respect to the solid: higher saturation ratios corres pond to higher nucleation rates. Both NAD and NAT can preferentially nuclea te in binary nitric acid solutions, depending on the temperature and the co mposition of the liquid; also, NAD appears to catalyze the nucleation of NA T below similar to 183 K. The results suggest that the largest drops in a P SC will freeze homogeneously if the stratospheric temperature remains near 190 K for more than 1 day, forming mixed liquid-solid clouds. In addition, the results indicate that nonequilibrium quasi-binary nitric acid solutions will not freeze in the stratosphere unless the temperature drops below 180 K.