Abstract
Rapid temperature fluctuations in the vicinity of the nitric acid trihydrate (NAT) equilibrium temperature can lead to substantial deviations of the acid concentrations from the thermodynamic equilibrium values. Stoichiometry of NAT can be reached in the smallest droplets, which possibly leads to nucleation of NAT. This size-selective NAT nucleation mechanism is in contrast to previous studies, which postulated that the largest droplets would freeze first. This new mechanism would lead to the formation of two distinct radial modes, as the few frozen particles take up HNO3 at the expense of the numerous larger droplets, which would finally revert to almost binary H2SO4/H20. Clearly, further experiments are necessary to determine whether cooling of stratospheric air in lee waves is an important pathway for the formation of frozen polar stratospheric cloud (PSC) above the frost point. Whether they freeze or not, it is clear that mesoscale temperature fluctuations can strongly influence liquid aerosol composition and size distributions. Even synoptic temperature changes influence the composition of the droplets, whereas the total volume is hardly affected.
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