Crystallization Kinetics of Nitric Acid Dihydrate Aerosols

Abstract

Type Ia polar stratospheric clouds (PSCs) are thought to consist of HNO3/H2O mixtures, usually assumed to be crystalline nitric acid trihydrate (NAT). However, it has recently been suggested that metastable nitric acid dihydrate (NAD) may form preferentially in the atmosphere due to a lower nucleation barrier. We have used Fourier transform infrared spectroscopy to investigate the crystallization kinetics of NAD aerosols. The crystallization rates were measured under two experimental regimes. In the first, we formed glassy 2:1 H2O:HNO3 aerosols in a cryostat held at 77 K and measured the rates of crystalline NAD formation when the aerosols were warmed to stratospheric temperatures. The crystal growth rates were then used to estimate the activation energy for diffusional transfer of HNO3 across the solid/liquid phase boundary, Δgd. We found Δgd = 13.3 kcal mol-1 for the temperature range 190−202 K. We have also measured the crystallization rate of NAD aerosols nucleated at stratospheric temperatures. We used homogeneous nucleation theory and our estimate of Δgd to determine the interfacial surface energy, σ, between NAD and a supercooled 2:1 H2O:HNO3 solution. We found the interfacial surface energy for NAD to be σ = 22 erg cm-2, much lower than that estimated previously for NAT of σ = 44 erg cm-2. If the interfacial surface energy for NAD is indeed this much lower than that of NAT, nucleation of NAD from liquid HNO3/H2O aerosols may be an important step in the formation mechanism for crystalline type Ia PSCs.