Properties of polar stratospheric clouds obtained by combined ACE-FTS and ACE-Imager extinction measurements

International audienceWe report the compositions and size distributions of aerosol particles in typical polar stratospheric clouds (PSCs) observed between 24 January and 28 February 2005 in the Arctic stratosphere. The results are obtained by combining the extinction measurements made by the Atmospheric Chemistry Experiment (ACE) Fourier-Transform Spectrometer and the visible/near IR imagers on the SCISAT satellite. The extended wavenumber range provided by this combination (750 to 20 000 cm?1) enables the retrieval of aerosol particle sizes between 0.05 and 10 ?m as well as providing extensive information about the compositions. Our results indicate that liquid ternary solutions with a high (>30 wt%) content of HNO3 were the most probable component of the clouds at the (60?70° N) latitudes accessible by ACE. The mean size of these ternary aerosol particles is in the range of 0.3 to 0.8 ?m. Less abundant, although still frequent, were clouds composed of NAT particles having radii in the range of 1 ?m and clouds of ice particles having mean radii in the 4?5 ?m range. In some cases, these last two types were found in the same observation

Thermodynamics of heterogeneous crystal nucleation in contact and immersion modes

One of most intriguing problems of heterogeneous crystal nucleation in droplets is its strong enhancement in the contact mode (when the foreign particle is presumably in some kind of contact with the droplet surface) compared to the immersion mode (particle immersed in the droplet). Many heterogeneous centers have different nucleation thresholds when they act in contact or immersion modes, indicating that the mechanisms may be actually different for the different modes. Underlying physical reasons for this enhancement have remained largely unclear. In this paper we present a model for the thermodynamic enhancement of heterogeneous crystal nucleation in the contact mode compared to the immersion one. To determine if and how the surface of a liquid droplet can thermodynamically stimulate its heterogeneous crystallization, we examine crystal nucleation in the immersion and contact modes by deriving and comparing with each other the reversible works of formation of crystal nuclei in these cases. As a numerical illustration, the proposed model is applied to the heterogeneous nucleation of Ih crystals on generic macroscopic foreign particles in water droplets at T=253 K. Our results show that the droplet surface does thermodynamically favor the contact mode over the immersion one. Surprisingly, our numerical evaluations suggest that the line tension contribution to this enhancement from the contact of three water phases (vapor-liquid-crystal) may be of the same order of magnitude as or even larger than the surface tension contribution

Quasi-linear least squares and computer code for numerical evaluation of relaxation time distribution from broadband dielectric spectra

We present a new fitting procedure and computer code for numerical evaluation of dielec. relaxation time distribution functions. The technique is based on linear least squares minimization and aims primarily at the anal. of compd. exptl. spectra of complex dielec. permittivity. It is fast, robust, and easy to use. No prior knowledge about the no. of relaxation modes, their characteristic times, relaxation strengths, or the functional form of the underlying relaxation time distribution function is required, the procedure dets. these parameters instead. The method is tested by both synthetic spectra with well-defined parameters of dielec. relaxation and exptl. wide-band dielec. spectra of different types. We believe that this new fitting instrument, which allows an unbiased approach to the formal description of dielec. spectra, may be of interest in many areas of dielec. spectroscopy

Thermodynamics of homogeneous nucleation of ice particles in the polar summer mesosphere

We present the hypothesis of homogeneous nucleation of ice nano-particles in the polar summer mesosphere. The nucleation of condensed phase is traced back to the first step on the formation pathway, which is assumed to be the transition of water vapor to amorphous cluster. Amorphous clusters then freeze into water ice, likely metastable cubic ice, when they reach the critical size. The estimates based on the equilibrium thermodynamics give the critical size (radius) of amorphous water clusters as about 1.0 nm. The same estimates for the final transition step, that is the transformation of cubic to hexagonal ice, give the critical size of about 15 nm at typical upper mesospheric conditions during the polar summer (temperature <i>T</i>=150 K, water vapor density <i>ρ</i><sub>vapor</sub>=10<sup>9</sup> cm<sup>−3</sup>)