CRYSTALLIZATION KINETICS OF NITRIC-ACID DIHYDRATE AEROSOLS

Type Ia polar stratospheric clouds (PSCs) are thought to consist of HN O3/H2O mixtures, usually assumed to be crystalline nitric acid trihydr ate (NAT). However, it has recently been suggested that metastable nit ric acid dihydrate (NAD) may form preferentially in the atmosphere due to a lower nucleation barrier. We have used Fourier transform infrare d spectroscopy to investigate the crystallization kinetics of NAD aero sols. The crystallization rates were measured under two experimental r egimes. In the first, we formed glassy 2:1 H(2)0:HNO3 aerosols in a cr yostat held at 77 K and measured the rates of crystalline NAD formatio n when the aerosols were warmed to stratospheric temperatures. The cry stal growth rates were then used to estimate the activation energy for diffusional transfer of HNO3 across the solid/liquid phase boundary, Delta g(d). We found Delta g(d) = 13.3 kcal mol(-1) for the temperatur e range 190-202 K. We have also measured the crystallization rate of N AD aerosols nucleated at stratospheric temperatures. We used homogeneo us nucleation theory and our estimate of Delta g(d) to determine the i nterfacial surface energy, sigma, between NAD and a supercooled 2:1 H2 O:HNO3 solution. We found the interfacial surface energy for NAD to be sigma = 22 erg cm(-2), much lower than that estimated previously for NAT of sigma = 44 erg cm(-2). If the interfacial surface energy for NA D is indeed this much lower than that of NAT, nucleation of NAD from l iquid HNO3/H2O aerosols may be an important step in the formation mech anism for crystalline type Ia PSCs.