Mechanism of incorporation of trace gases in ice grown from the gas phase

The equilibrium solubilities and diffusion coefficients of HCl and HNO3 in ice have been measured at -15 degrees C as a function of the trace gas partial pressures, P-HCl or P-HNO3. The diffusion coefficient of HCl is low. D-HCl is about 10(-12) cm(2)s(-1) while that of HNO3, D-HNO3, is greater, about 10(-10) cm(2)s(-1). The solubility values are compared with field measurements performed in central Greenland where HCl in snow was found to be out of equilibrium with the atmosphere, while HNO3 in snow was in equilibrium A mechanism of incorporation of trace gases in ice growing from the gas phase is proposed from these observations: ice grows by the deposition of layers of a given thickness, whose composition is determined by condensation kinetics. The layers then tends to equilibrate by solid state diffusion until they are covered by the following layer. Implications of these results for ice cloud chemistry and ice core analyses are discussed.