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.

Articles you may be interested inDynamics of the C ( D 1 ) + D 2 reaction: A comparison of crossed molecular-beam experiments with quasiclassical trajectory and accurate statistical calculationsThe O ( 1 D)+ H 2 reaction at 56 meV collision energy: A comparison between quantum mechanical, quasiclassical trajectory, and crossed beam results Probing the effect of the H 2 rotational state in O ( 1 D)+ H 2 →OH+H : Theoretical dynamics including nonadiabatic effects and a crossed molecular beam studyThe dynamics of the reactions O( 1 D)ϩH 2 →OHϩH and O( 1 D)ϩD 2 →ODϩD have been investigated in crossed molecular beam experiments with mass spectrometric detection at the collision energies of 1.9 and 3.0 kcal/mol, and 5.3 kcal/mol, respectively. From OH͑OD͒ product laboratory angular and velocity distribution measurements, center-of-mass product translational energy and angular distributions were derived. The angular distributions are nearly backwardforward symmetric with a favored backward peaking which increases with collision energy. About 30% of the total available energy is found to be channeled into product translational energy. The results are compared with quasiclassical trajectory calculations on a DIM ͑diatomic-in-molecules͒ potential energy surface. Related experimental and theoretical works are noted. Insertion via the 1 1 AЈ ground state potential energy surface is the predominant mechanism, but the role of a second competitive abstraction micromechanism which should evolve on one of ͑or both͒ the first two excited surfaces 1 AЉ and 2 1 AЈ is called into play at all the investigated energies to account for the discrepancy between theoretical predictions and experimental results.

Abstract. The nucleation rates for submicron-diameter nitric acid trihydrate (NAT) aerosol particles were measured under stratospheric conditions using a temperatureprogramed flow tube and Fourier transform infrared extinction spectroscopy to detect the phase change from liquid to solid. The temperature range from 175 to 155 K was 3 1 examined. The measured nucleation rate constants (in units of cm-s-) are between 3.8 +_ 1.8 x 10 •ø and 9.7 +_ 6.3 x 10 TM for the temperature range between 167.2 and 163.5 K. The experiments show directly that critical nuclei form rapidly in NAT droplets at these temperatures, but subsequent crystallization is very slow, so the nucleated droplets can persist as liquids for long times unless their temperature is raised to increase the crystal growth rate. In these experiments, complete crystallization of the nucleated droplets was achieved in the observation time (about 10 s) by raising their temperature to near 180 K. Although demonstrable in the laboratory, the freezing of NAT by homogeneous nucleation at low temperature and subsequent warming are unlikely to be important for type 1 polar stratospheric cloud formation because the temperatures required for nucleation are lower than those commonly found in the lower stratosphere. IntroductionSince the suggestion that nitric acid plays a role in the formation of polar stratospheric clouds ( , 1998]. With these measurements we hope to provide a systematic and selfconsistent data set for use in the parameterization of models and in the interpretation of PSC field measurements. ExperimentThe cryogenic flow tube and associated apparatus used for

Abstract. The relative roles of volume and surface nucleation were investigated for the homogeneous freezing of pure water droplets. Experiments were carried out in a cryogenic laminar aerosol flow tube using supercooled water aerosols with maximum volume densities at radii between 1 and 3 µm. Temperature-and size-dependent values of volumeand surface-based homogeneous nucleation rates between 234.8 and 236.2 K were derived using a microphysical model and aerosol phase compositions and size distributions determined from infrared extinction measurements in the flow tube. The results show that the contribution from nucleation at the droplet surface increases with decreasing droplet radius and dominates over nucleation in the bulk droplet volume for droplets with radii smaller than approximately 5 µm. This is interpreted in terms of a lowered free energy of ice germ formation in the surface-based process. The implications of surface nucleation for the parameterization of homogeneous ice nucleation in numerical models are considered.