Interactions between HCl, NO_<i>x</i> and H₂O ice in the Antarctic stratosphere: Implications for ozone

<jats:p>Thermodynamic properties of solid phases containing H<jats:sub>2</jats:sub>O and HCl are examined for conditions expected over Antarctica in winter. It is shown that solid solutions with 0.02–0.035 mole fraction HCl in H<jats:sub>2</jats:sub>O ice will form in the stratosphere at temperatures between 193° and 190°K. Crystalline HNO<jats:sub>3</jats:sub>·H<jats:sub>2</jats:sub>O and/or HNO<jats:sub>3</jats:sub>·3H<jats:sub>2</jats:sub>O form in the same temperature range. Thus condensation of small quantities of H<jats:sub>2</jats:sub>O (<1 ppm) leads to nearly complete removal of both HCl and HNO<jats:sub>3</jats:sub> from the gas phase. Formation of ice‐HCl solid solutions provides a favorable setting for the heterogeneous reaction of HCl with ClNO<jats:sub>3</jats:sub>, initiating a rapid sequence of reactions that converts solid‐phase HCl into gaseous ClNO<jats:sub>3</jats:sub> or chlorine oxide radicals. Odd nitrogen species are simultaneously converted into solid‐phase nitrates. The cycle producing reactive chlorine gases is likely to be completed before particles grow large enough to fall from the stratosphere. Hence precipitation of particles from the polar stratospheric clouds is expected to remove odd nitrogen from the stratosphere efficiently, leaving chlorine gases behind. High concentrations of reactive chlorine oxide radicals are rapidly produced by heterogeneous reactions if the initial concentration of HCl exceeds a critical value, 0.5 times the concentration of NO<jats:sub><jats:italic>x</jats:italic></jats:sub> before onset of condensation. Production of unreactive ClNO<jats:sub>3</jats:sub> and HOCl is favored if HCl levels are lower than this value. The onset of Antarctic ozone depletion in the late 1970s may in part reflect growth of HCl levels beyond this threshold.</jats:p>