MECHANISM OF INCORPORATION OF TRACE GASES IN ICE GROWN FROM THE GAS-PHASE

The equilibrium solubilities and diffusion coefficients of HCl and HNO 3 in ice have been measured at -15 degrees C as a function of the trac e 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-H NO3, is greater, about 10(-10) cm(2)s(-1). The solubility values are c ompared with field measurements performed in central Greenland where H Cl in snow was found to be out of equilibrium with the atmosphere, whi le HNO3 in snow was in equilibrium A mechanism of incorporation of tra ce gases in ice growing from the gas phase is proposed from these obse rvations: ice grows by the deposition of layers of a given thickness, whose composition is determined by condensation kinetics. The layers t hen tends to equilibrate by solid state diffusion until they are cover ed by the following layer. Implications of these results for ice cloud chemistry and ice core analyses are discussed.