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 densitie s up to 1 cm-3 were observed by balloonborne particle counters at alti tudes 20-24 km about 250 km SE of Kiruna (northern Sweden). The partic les were probably formed during strong adiabatic cooling in leewaves o ver the Norwegian mountains. At the time of measurement, temperatures were 5-10 K above the frost-point. Pure ice particles of this size wou ld evaporate within a few minutes; however, these particles somehow su rvived the passage of 2-3 hours from the Norwegian mountains to the Gu lf of Bothnia. The slow evaporation could be explained if at some poin t the particles had acquired a coating of nitric acid trihydrate (NAT) . In air cooling slowly the condensation of NAT will precede the conde nsation of ice. However, if the air cools very rapidly then nitric aci d and ice could condense simultaneously, with the nitric acid embedded in the ice crystal in the form of NAT clusters. In particular large H NO3/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 nu clei are absent. During warming the preferential evaporation of water would expose the clusters. leaving eventually an almost complete surfa ce layer of NAT. Model calculations suggest that for particles undergo ing vertical movement in leewaves this could happen several times in s uccession, greatly prolonging their lifetime. Thus leewave cloud parti cles may persist much further downstream than has been previously thou ght.