Complex refractive indices of supercooled liquid water at 240, 253, 263, and 273 K, and ice at 200, 210, and 235 K in the mid infrared from 460 to 4000 cm(-1) are reported. The results were obtained from the extinction spectra of small (micron-size) aerosol particles, recorded using the cryogenic flow tube technique. An improved iterative procedure for retrieving complex refractive indices from extinction measurements is described. The refractive indices of ice determined in the present study are in good agreement with data reported earlier. The temperature region and range of states covered in the present work are relevant to the study of upper tropospheric and stratospheric aerosols and clouds.

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We have measured the freezing curve of liquid H2SO4/H2O aerosol droplets having average radii of approximately 0.2 μm. We form the aerosol by the reaction of SO3 with H2O and flow it through a temperature-controlled flow tube equipped with reentrant windows, through which we make observations by FTIR extinction spectroscopy. At the freezing point, a microcrystallite of pure ice (H2O(s)) nucleates in the aerosol droplet, and this causes a small change in the spectrum near 3250 cm-1. By recording the temperatures at which the crystallites appear for different acid concentrations, we are able to map out the freezing curve. In the following account, we describe the experimental technique and report the freezing curve for the concentration range up to 35 wt % H2SO4, which corresponds to the first eutectic point on the phase diagram of the bulk material. We find that the aerosol supercools by about 35 K below the temperature at which the corresponding bulk material freezes. Our data show that the overall freezing mechanism is similar to that of the bulk solution:  after nucleation, the crystallite grows with decreasing temperature, causing the remaining acid to become more concentrated due to the removal of H2O until eventually a eutectic mixture forms.

Abstract. Temperature-dependent volume nucleation rate coefficients for supercooled water droplets, J V (T ), are derived from infrared extinction measurements in a cryogenic laminar aerosol flow tube using a microphysical model. The model inverts water and ice aerosol size distributions retrieved from experimental extinction spectra by considering the evolution of a measured initial droplet distribution via homogeneous nucleation and the exchange of vapour-phase water along a well-defined temperature profile. Experiment and model results are reported for supercooled water droplets with mean radii of 1.0, 1.7, and 2.9 µm. Values of mass accommodation coefficients for evaporation of water droplets and vapour deposition on ice particles are also determined from the model simulations. The coefficient for ice deposition was found to be 0.031 ± 0.001, while that for water evaporation was 0.054 ± 0.012. Results are considered in terms of the applicability of classical nucleation theory to the freezing of micrometre-sized droplets in cirrus clouds, with implications for the parameterization of homogeneous ice nucleation in numerical models.

Abstract. The freezing of a submicron-sized aerosol composed of H20 and HNO 3 in a precise 2:1 concentration ratio has been measured using Fourier transform infrared extinction spectroscopy. The measurements were carried out in a flow tube operating at temperatures and pressures appropriate to the polar stratosphere. On the timescale of this measurement, about 15 s, detectable nucleation occurred at 179 _+ 1.6 K. Ten percent of the sample was frozen after 15 s at a temperature of 178.8 K; 50% was frozen at 177.5 K, and 90% was frozen at 175.8 K. Using the known (constant) aerosol flow velocity, the nucleation rate constant was obtained from the freezing point measurements. Values of this rate constant are reported over the temperature range between 176 K and 179 K. In this range the freezing temperature is in excellent agreement with that measured by Barton et al. [1993], and the temperature dependence of the nucleation rate constant agrees well with that calculated using the method of MacKenzie et al. [1997]. It does not agree with that reported by Tisdale et al. [1997]. Extrapolation of the rates indicate that nitric acid dihydrate nucleation from liquid aerosol droplets having a 2:1 H20:HNO 3 composition would occur on the stratospherically relevant timescales of 1 hour and 1 day at temperatures of 183 and 185 K, respectively.