THE LIFETIME OF LEEWAVE-INDUCED ICE PARTICLES IN THE ARCTIC STRATOSPHERE .2. STABILIZATION DUE TO NAT-COATING

Large ice particles with radii of 1-5 mum and particle number densities up to 1 cm-3 were observed by balloonborne particle counters at altitudes 20-24 km about 250 km SE of Kiruna (northern Sweden). The particles were probably formed during strong adiabatic cooling in leewaves over the Norwegian mountains. At the time of measurement, temperatures were 5-10 K above the frost-point. Pure ice particles of this size would evaporate within a few minutes; however, these particles somehow survived the passage of 2-3 hours from the Norwegian mountains to the Gulf of Bothnia. The slow evaporation could be explained if at some point the particles had acquired a coating of nitric acid trihydrate (NAT). In air cooling slowly the condensation of NAT will precede the condensation of ice. However, if the air cools very rapidly then nitric acid and ice could condense simultaneously, with the nitric acid embedded in the ice crystal in the form of NAT clusters. In particular large HNO3/H2O ratios in the solid phase would be reached if the formation of pure NAT during initial cooling is suppressed, e.g. when solid NAT nuclei are absent. During warming the preferential evaporation of water would expose the clusters. leaving eventually an almost complete surface layer of NAT. Model calculations suggest that for particles undergoing vertical movement in leewaves this could happen several times in succession, greatly prolonging their lifetime. Thus leewave cloud particles may persist much further downstream than has been previously thought.