Crystallization Kinetics of HNO3/H2O Films Representative of Polar Stratospheric Clouds

Abstract

The crystallization of binary HNO3/H2O aerosols may be an important step in the formation of type Ia polar stratospheric clouds (PSCs). We have used Fourier transform infrared (FTIR) spectroscopy to probe the crystallization kinetics of supercooled 2:1 and 3:1 H2O:HNO3 films to nitric acid dihydrate (NAD) and trihydrate (NAT), respectively. Nitric acid/ice films were grown on a silicon substrate at temperatures near the glass point. The substrate was then warmed to a temperature above the glass point, and the crystallization of the film was measured using FTIR absorption spectroscopy. The temperature dependence of the crystallization rate was used to determine the activation energy for a nitrate ion to cross the liquid−solid phase boundary. The activation energies ranged from 22 to 41 kcal mol-1 for NAD crystallization at temperatures between 177 and 168 K and from 22 to 35 kcal mol-1 for α-NAT crystallization at temperatures between 171 and 161 K. In addition, activation energies at higher temperatures were derived from recent viscosity measurements obtained between 225 and 298 K. A parametrization was then developed to fit all of the activation energy data between 160 and 298 K. Finally, homogeneous nucleation calculations were performed using the temperature-dependent activation energies in conjunction with previous measurements of aerosol nucleation rates at 190-202 K to determine the freezing rates for HNO3/H2O particles over a wide temperature range. These calculations indicate that nucleation of NAD from supercooled 2:1 H2O:HNO3 aerosols is rapid at relevant stratospheric temperatures. Nucleation of α-NAT from supercooled 3:1 H2O:HNO3 aerosols could occur as rapidly as within 1 day, but the uncertainty in the interfacial energy used for α-NAT is large.