Type Ia Polar Stratospheric Clouds Research Articles

CALIPSO observations of wave‐induced PSCs with near‐unity optical depth over Antarctica in 2006–2007

Ground‐based and satellite observations have hinted at the existence of polar stratospheric clouds (PSCs) with relatively high optical depths, even if optical depth values are hard to come by. This study documents a type II PSC observed from spaceborne lidar, with visible optical depths up to 0.8. Comparisons with multiple temperature fields, including reanalyses and results from mesoscale simulations, suggest that intense small‐scale temperature fluctuations due to gravity waves play an important role in its formation, while nearby observations show the presence of a potentially related type Ia PSC farther downstream inside the polar vortex. Following this first case, the geographic distribution and microphysical properties of PSCs with optical depths above 0.3 are explored over Antarctica during the 2006 and 2007 austral winters. These clouds are rare (less than 1% of profiles) and concentrated over areas where strong winds hit steep ground slopes in the Western Hemisphere, especially over the peninsula. Such PSCs are colder than the general PSC population, and their detection is correlated with daily temperature minima across Antarctica. Lidar and depolarization ratios within these clouds suggest they are most likely ice‐based (type II). Similarities between the case study and other PSCs suggest they might share the same formation mechanisms.

Heterogeneous Nucleation of Nitric Acid Trihydrate on Clay Minerals: Relevance to Type Ia Polar Stratospheric Clouds

Although critical to atmospheric modeling of stratospheric ozone depletion, selective heterogeneous nuclei that promote the formation of Type Ia polar stratospheric clouds (PSCs) are largely unknown. While mineral particles are known to be good ice nuclei, it is currently not clear whether they are also good nuclei for PSCs. In the present study, a high-vacuum chamber equipped with transmission Fourier transform infrared spectroscopy and a quadrupole mass spectrometer was used to study heterogeneous nucleation of nitric acid trihydrate (NAT) on two clay minerals-Na-montmorillonite and kaolinite-as analogs of atmospheric terrestrial and extraterrestrial minerals. The minerals are first coated with a 3:1 supercooled H2O/HNO3 solution prior to the observed nucleation of crystalline NAT. At 220 K, NAT formation was observed at low SNAT values of 12 and 7 on kaolinite and montmorillonite clays, respectively. These are the lowest SNAT values reported in the literature on any substrate. However, NAT nucleation exhibited significant temperature dependence. At lower temperatures, representative of typical polar stratospheric conditions, much higher supersaturations were required before nucleation was observed. Our results suggest that NAT nucleation on mineral particles, not previously treated with sulfuric acid, may not be an important nucleation platform for Type Ia PSCs under normal polar stratospheric conditions.

Discriminating Types Ia and Ib polar stratospheric clouds in POAM satellite data

A method for discriminating Types Ia and Ib Polar Stratospheric Clouds (PSCs) from Polar Ozone and Aerosol Measurement (POAM) satellite occultation measurements of aerosol extinction coefficient is described. The method has been validated by applying several statistical tests to the results and by using Differential Absorption Lidar (DIAL) and Ozone Lidar Experiment (OLEX) lidar observations made during Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment/Third European Stratospheric Experiment on Ozone II (SOLVE/THESEO 2000). Type Ia PSCs are believed to be composed of large nitric‐acid‐containing particles that will sediment out of the stratosphere, causing denitrification and facilitating ozone depletion. Type Ib PSCs are believed to be much smaller and will not sediment out of the stratosphere. Discriminating between these two types of PSCs is significant because it will permit a better understanding of ozone depletion today and predict the fate and effect of PSCs using the more continuous temporal coverage and larger areal coverage that can be obtained from satellites. The method is made possible by the character of POAM observations when plotted as normalized extinction versus wavelength dependence. As the extinction increases, observations of Types Ia and Ib PSCs bifurcate. This behavior is also observed in idealized simulations of the formation of Supercooled Ternary Solutions (STS) and nitric acid trihydrate (NAT) particles, which are believed to make up Types Ib and Ia PSCs, respectively. Analysis of POAM observations from the 1999/2000 Arctic winter using the PSC discrimination algorithm revealed that the number of PSC observations peaked in January. In November, December, and January, the ratio of Type Ia to Ib PSCs was about 3. In February and March, this ratio was about 0.3. The average altitude of Type Ia PSCs descended more than the Type Ib, especially in the spring where the Type Ia observations were 2–3 km below the Type Ib observations. This is consistent with observations of denitrification during the 1999/2000 winter. The PSC discrimination algorithm is applicable to previous winters in both hemispheres and will work with SAGE III observations as well. This will permit a more extensive study of the statistical significance of some features of the PSCs observed during the 1999/2000 Arctic winter. It is our belief that the present method of analyzing satellite data to discriminate Type I PSCs will be of great utility in the study of PSCs and ozone depletion.

Stratospheric ozone loss in the 1996/1997 Arctic winter: Evaluation based on multiple trajectory analysis for double‐sounded air parcels by ILAS

Quantitative chemical ozone loss rates and amounts in the Arctic polar vortex for the spring of 1997 are analyzed based on ozone profile data obtained by the Improved Limb Atmospheric Spectrometer (ILAS) using an extension of the Match technique. In this study, we calculated additional multiple trajectories and set very strict criteria to overcome the weakness of the satellite sensor data (lower vertical resolution and larger sampling air mass volume) and to identify more accurately a double‐sounded air mass. On the average inside the inner edge of the vortex boundary (north of about 70°N equivalent latitude), the local ozone loss rate was 50–80 ppbv/day at the maximum during late February between the levels of 450 and 500 K potential temperatures. The integrated ozone loss during February to March reached 2.0 ± 0.1 ppmv at 475–529 K levels, and the column ozone loss between 400 and 600 K during the 2 months was 96 ± 0.3 DU. Using a relative potential vorticity (rPV) scale, the vortex was divided into some rPV belts, and it was shown that the magnitude of the ozone loss increased gradually toward the vortex center from the edge. The maximum ozone loss rate of 6.0 ± 0.6 ppbv/sunlit hour near the vortex center was higher than near the vortex edge by a factor of 2–3. When we expanded the area of interest to include all the data obtained inside the vortex edge (north of about 65°N equivalent latitude), the local ozone loss rate was about 50 ppbv/day at the maximum. This value is slightly larger than that estimated by the Match analysis using ozonesondes for the same winter by ∼10 ppbv/day. Temperature histories of double‐sounded air parcels indicated that the extreme ozone loss in the innermost part of the vortex was observed when the air parcel experienced temperatures below TNAT during the two soundings and had experienced temperatures near Tice in the 10 days prior to the first sounding. These facts suggest that the high ozone loss rate deep inside the vortex in the 1997 Arctic early spring correlates with the presence of type Ia polar stratospheric clouds (PSCs).

Lidar observations of polar stratospheric clouds over northern Greenland in the period 1990–1997

Observations of the stratosphere were carried out at Thule (76.5°N, 68.8°W), Greenland, in the period 1990–1997 with an elastic backscatter/depolarization lidar. Polar stratospheric clouds (PSCs) were never detected during the winters 1991, 1992, 1993, and 1994. Small nondepolarizing PSCs could have been masked, however, by the large volcanic aerosol cloud that was in the stratosphere during 1992, 1993, and 1994. During 1995, 1996, and 1997, PSCs were observed on a total of 32 days. The evolution of the PSCs during each winter was mainly modulated by the behavior of the polar vortex. PSC signatures were often detected in regions where, assuming a 5 ppmv concentration of water vapor and a 10 ppbv concentration of nitric acid, condensation was not possible. Thus larger concentrations of these gases, or significant differences between the true and the radiosonde/analysis temperatures, were experienced. All observed PSCs could be classified as type I PSCs. Type Ia PSCs, sometimes found as stable homogeneous layers, were common. These layers formed when the air mass temperature reached the frost point during the days preceding the passage over Thule. Type Ib PSCs were generally associated with an articulated cloud structure, consisting of several layers of type Ib and type Ia particles. Circumstantial indications suggest that the development of these articulated clouds is associated with oscillations in the temperature profile, and with rapid temperature changes along the isentropic trajectories. The behavior of the depolarization ratio appears to depend on the minimum temperature experienced by the air mass during the past few days, and possibly on the condensation of ice prior to the PSC observation. The properties of a large type Ib PSC have been studied by means of a ternary solution model, and theoretical calculations of the backscattering. The measured cloud properties are not satisfactorily described by the ternary solution model, and a droplet composition with a water fraction larger than expected may be implied. The type Ib PSC properties are, however, consistent with a cloud composed of a large number of small particles, with a narrow size distribution. In a single case a cloud exhibiting relatively large backscatter ratio and depolarization, classified as type Ic, was detected; the isentropic trajectory of the air mass shows that the particles probably reached the frost point a few days before arriving at Thule. Theoretical calculations of the backscatter ratio from this cloud indicate that it was probably constituted of a relatively large number of submicron to micron‐sized crystals.

Analysis of polar stratospheric clouds using temperature and aerosols measured by the Alomar R/M/R lidar

In this work we analyze and interpret stratospheric temperature and polar stratospheric cloud measurements made with the ALOMAR Rayleigh Mie Raman (R/M/R) lidar located at Andøya (69°N, 16°E) in northern Norway. The ALOMAR R/M/R lidar is in operation since July 1994. Vertical profiles of polar stratospheric clouds (PSCs) are measured at three different wavelengths and used to classify PSC types. Vertical temperature profiles of the stratosphere are also obtained with the lidar by using the rotational Raman technique. The backscatter ratios at three different wavelengths and the Angstrom coefficients derived from backscatter ratio are used to study the dependence of the PSC aerosol properties on the measured air temperature. We observed PSCs during the cold winters 1994/1995, 1995/1996, and 1996/1997. The clouds are classified in three types, Ia, Ib, and II. The temperature at which type Ib PSCs appear fits well with Supercooled Ternary Solution (STS) theory, while the type Ia PSCs are often measured at temperatures above TNAT. Trajectory analysis indicates that the type Ia and type Ib PSCs are characterized by different thermal histories. The last part of the paper is devoted to the interpretation of the results that are not in full agreement with the current theories of the formation of PSCs. In fact, it is found that a subtype of PSC, characterized by different values of optical parameters is measured inside clouds composite of different PSCs types. Moreover, it is observed that for a large fraction of the type Ia PSC observations the prevailing air temperature is above the thermodynamic equilibrium temperature for NAT PSCs

Nonequilibrium coexistence of solid and liquid particles in Arctic stratospheric clouds

Observations of polar stratospheric clouds (PSCs) from Ny Ålesund, Spitsbergen, have been examined to quantify the occurrence of solid particles. The polarized backscatter ratio was found to be a more sensitive indicator of the presence of non‐spherical (solid) particles in a PSC than the aerosol depolarization, which approaches zero for clouds containing a large volume of liquid droplets. The analysis corroborates our previous finding that type Ia PSCs cannot be composed of nitric acid trihydrate particles in equilibrium with the gas phase, which would lead to a much too high backscatter ratio. Conversely, type Ib PSCs, previously thought to be composed solely of liquid droplets, often contain a very small fraction of solid particles. These clouds develop a high parallel backscatter, arising from Mie scattering by droplets, and a nonvanishing perpendicular signal due to the few solid particles. Other type Ib PSCs appear to contain only liquid particles. The mixed liquid/solid clouds are observed on the smallest spatial and temporal scale resolvable by the lidar instrument (30 m, l min), implying that their properties are not a result of spatial averaging of different cloud types. Comparison of the observations with optical calculations shows that such nonequilibrium particle distributions are to be expected, as temperature changes are sufficiently rapid to prevent the particles from assuming equilibrium sizes. The observed optical characteristics of type Ia and type Ib clouds can be reproduced in a model by assuming that a very small fraction of the particles are composed of nitric acid hydrate, with the majority being binary H2SO4/H2O or ternary HNO3/H2SO4/H2O droplets.

Constraints on PSC particle microphysics derived from lidar observations

Based on extensive T-matrix computations of light scattering by polydispersions of randomly oriented, rotationally symmetric nonspherical particles, we analyze existing lidar observations of polar stratospheric clouds (PSCs) and derive several constraints on PSC particle microphysical properties. We show that sharp-edged nonspherical particles (finite circular cylinders) exhibit less variability of lidar backscattering characteristics with particle size and aspect ratio than particles with smooth surfaces (spheroids). For PSC particles significantly smaller than the wavelength, the backscatter color index α and the depolarization color index β are essentially shape independent. Observations for type Ia PSCs can be reproduced by spheroids with aspect ratios larger than 1.2, oblate cylinders with diameter-to-length ratios greater than 1.6, and prolate cylinders with length-to-diameter ratios greater than 1.4. The effective equal-volume-sphere radius for type Ia PSCs is about 0.8 μm or larger. Type Ib PSCs are likely to be composed of spheres or nearly spherical particles with effective radii smaller than 0.8 μm . Observations for type II PSCs are consistent with large ice crystals (effective radius greater than 1 μm ) modeled as cylinders or prolate spheroids.

Studies of polar stratospheric cloud formation.

Stratospheric ozone depletion was first reported in 1985. Early on, researchers identified polar stratospheric clouds (PSCs) as being important in chemistry related to ozone depletion. PSCs exist as crystalline water-ice particles (type II), and as crystalline (type Ia) or liquid (type Ib) particles stable above the water-ice frost point. Uncertainty remains concerning the composition and formation mechanism of the most common PSC, type Ia. Here, we consider likely formation mechanisms for type Ia PSCs.

Role of lee waves in the formation of solid polar stratospheric clouds: Case studies from February 1997

Recent theories of solid polar stratospheric clouds (PSCs) formation have shown that particles could remain liquid down to 3 K or 4 K below the ice frost point. Such temperatures are rarely reached in the Arctic stratosphere at synoptic scale, but nevertheless, solid PSCs are frequently observed. Mesoscale processes such as mountain‐induced gravity waves could be responsible for their formation. In this paper, a microphysical‐chemical Lagrangian model (MiPLaSMO) and a mountain wave model (NRL/MWFM) are used to interpret balloon‐borne measurements made by an optical particle counter (OPC) and by the Absorption par Minoritaires Ozone et NOx (AMON) instrument above Kiruna on February 25 and 26, 1997, respectively. The model results show good agreement with the particle size distributions obtained by the OPC in a layer of large particles, and allow us to interpret this layer as an evaporating mesoscale type Ia PSC (nitric acid trihydrate) mixed with liquid particles. The detection of a layer of solid particles by AMON is also qualitatively reproduced by the model and is interpreted to be frozen sulfate acid aerosols (SAT). In this situation, the impact of mountain waves on chlorine activation is studied. It appears that mesoscale perturbations amplify significantly the amount of computed ClO, as compared to synoptic runs. Moreover, MiPLaSMO chemical results concerning HNO3 and HCl agree with measurements made by the Limb Profile Monitor of the Atmosphere (LPMA) instrument on February 26 at a very close location to AMON, and explain part of the differences between LPMA measurement and Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) model outputs.

Aircraft lidar observations of an enhanced type Ia polar stratospheric clouds during APE‐POLECAT

Polar stratospheric clouds (PSCs) which do not fit into the standard type Ia/Ib scheme were measured by the airborne lidar OLEX (Ozone Lidar Experiment) on board the Deutsches Zentrum für Luft‐ und Raumfhart (DLR) Falcon during the Airborne Polar Experiment and Polar stratospheric clouds, Leewaves, Chemistry Aerosol and Transport (APE‐POLECAT) campaign. In contrast, the standard classification is satisfied by almost all observations for four winters at Ny Ålesund, Spitsbergen, which is one of the most comprehensive data sets of ground station lidar measurements presently available. The cloud observed by the Falcon south of Spitsbergen on December 31, 1996, was a 400‐km long type I cloud with backscatter ratio S = 2.5 and aerosol depolarization δA = 15%, which is clearly distinct from the Ny Ålesund 4 year record. Using a combination of microphysical and optical modeling, we investigate the possible evolution of this cloud assuming either in situ freezing of ternary HNO3/H2SO4/H2O droplets as nitric acid trihydrate, or the formation of the clouds in mountain waves over the east coast of Greenland, as suggested by a mountain wave model. Best agreement with the observations was obtained by assuming mountain‐wave‐induced cloud formation, which yields nitric acid trihydrate particles with much higher total mass than achieved by assuming synoptic‐scale freezing. Our analysis suggests that this rare type of PSC, which we term type Ia‐enh, is characterized by nitric acid hydrate particles rather close to thermodynamic equilibrium, while the more common type Ia PSCs appear to contain much less mass than representative of equilibrium.

Optical classification, existence temperatures, and coexistence of different polar stratospheric cloud types

Multispectral lidar measurements of polar stratospheric clouds (PSCs) from two winter campaigns in 1994/1995 and 1996'1997 at Sodankylä, Finland, have been evaluated together with temperature data from local radiosondes to find optical parameters for a PSC classification of different particle types and their existence temperatures. Precise depolarization measurements show that both solid and liquid particles exist below the NAT (nitric acid trihydrate) temperature. A comparison of temperatures at the PSC base and at the cloud top shows a good agreement with the NAT‐existence temperature for solid type Ia clouds and a 3–4 K lower temperature for liquid type Ib clouds. The two particle families are therefore consistent with solid NAT particle formation and condensational growth of HNO3, H2O and H2SO4 liquid ternary solutions. The coexistence of solid and liquid particles has been observed by means of the temporal development of parallel and perpendicular polarized lidar signals. These time series of subsequent lidar measurements show stronger and faster fluctuations in the liquid particle mode compared to the solid particles and thus indicate a higher sensitivity toward temperature fluctuations for the liquid PSCs. While the optical properties of most observations are consistent with the definition of PSC type Ia (solid) and type Ib (liquid) clouds, a third type has been observed which does not fit into the current type Ia/Ib optical classification. This cloud type consists of solid particles but has a higher backscatter than type Ia PSC.

Polar stratospheric clouds climatology over Dumont d'Urville between 1989 and 1993 and the influence of volcanic aerosols on their formation

The first polar stratospheric clouds (PSCs) climatology ever established from lidar data and relative to a specific site is presented here. It is based on lidar backscatter and depolarization measurements of PSCs carried out between 1989 and 1993 at Dumont d'Urville (66°S, 140°E), which is a primary station of the Network for Detection of Stratospheric Changes (NDSC). The climatology was subdivided based on the stratospheric sulphuric acid aerosol content (background aerosols in 1989–1991 and volcanic aerosols in 1992–1993 following the Mount Pinatubo eruption). PSCs were mainly observed in July and August. Very few water ice clouds (type II) were detected. Most of the PSCs tended to form around the peak in sulphuric acid aerosol, between 17 and 23 km in 1989–1991 and between 11 and 20 km in 1992–1993. This tendency suggests that sulphuric acid aerosols are very likely to act as condensation nuclei for PSCs. As shown by previous lidar studies [Browell et al, 1990], two type I subclasses were identified: depolarizing (nonspherical) particles (type Ia) and nondepolarizing (spherical) particles (type Ib). No type Ia PSCs were detected above the nitric acid trihydrate (NAT) saturation temperature, TNAT, lending support to the theory that NAT is the main component of type Ia PSCs. There was also no evidence of the existence of sulphuric acid tetrahydrate (SAT) in the data. Some type Ib PSCs were observed close to the frost point, showing that supersaturation with respect to NAT is a necessary, but not sufficient, condition for the existence of solid PSCs. No type Ib PSCs were clearly detected above TNAT in 1989–1991 whereas 18% of the PSCs seem to be found at temperatures above TNAT in 1992–1993. This difference might be linked to the HNO3 uptake by volcanic sulphuric acid particles starting at higher temperatures. The fraction of type Ia out of the total PSCs observations was lower in 1992–1993 than in 1989–1991. This difference was not found to be highly statistically significant.

Dissolution of sulfuric acid tetrahydrate at low temperatures and subsequent growth of nitric acid trihydrate

Crystalline sulfuric acid tetrahydrate (SAT) has been observed to change phase at temperatures below its melting point, in agreement with recent theoretical predictions of deliquescence. Dissolution of SAT was observed in 63% of experiments expected to show a phase change, leading to formation of a ternary HNO3/H2SO4/H2O solution. This solution, which still contained a portion of the original solid SAT, crystallized to form nitric acid trihydrate (NAT). NAT then continued to grow by condensation of additional nitric acid and water at temperatures several degrees above the ice frost point. This process of SAT dissolution followed by NAT crystallization and growth may offer a mechanism for the formation of type Ia polar stratospheric clouds on frozen sulfate aerosols when SNAT>15.

Polar stratospheric cloud threshold temperatures in the 1995–1996 arctic vortex

Balloon‐borne backscattersondes have been used to study the relationship between particle scattering and ambient temperature near the vertical edge of arctic polar stratospheric clouds (PSCs) as well as to delineate the cloud type occurrence probability as a function of temperature. The observed typical threshold temperatures as a function of altitude are about1°K warmer than the temperature TSTS expected for rapid growth of supercooled ternary solution aerosols. A more descriptive analysis shows that the threshold temperatures occur over a definable range of temperatures and tend to cluster near, but somewhat warmer than, TSTS. Considering the experimental and theoretical uncertainties, this difference may not be significant. The probability of type Ib PSC occurrence shows a dramatic increase at TSTS±1°K, while for type Ia PSCs the probability is roughly constant at 10% for temperatures below the formation point of nitric acid trihydrate (TNAT).

Crystallization Kinetics of HNO3/H2O Films Representative of Polar Stratospheric Clouds

The crystallization of binary HNO3/H2O aerosols may be an important step in the formation of type Ia polar stratospheric clouds (PSCs). We have used Fourier transform infrared (FTIR) spectroscopy to probe the crystallization kinetics of supercooled 2:1 and 3:1 H2O:HNO3 films to nitric acid dihydrate (NAD) and trihydrate (NAT), respectively. Nitric acid/ice films were grown on a silicon substrate at temperatures near the glass point. The substrate was then warmed to a temperature above the glass point, and the crystallization of the film was measured using FTIR absorption spectroscopy. The temperature dependence of the crystallization rate was used to determine the activation energy for a nitrate ion to cross the liquid−solid phase boundary. The activation energies ranged from 22 to 41 kcal mol-1 for NAD crystallization at temperatures between 177 and 168 K and from 22 to 35 kcal mol-1 for α-NAT crystallization at temperatures between 171 and 161 K. In addition, activation energies at higher temperatures were derived from recent viscosity measurements obtained between 225 and 298 K. A parametrization was then developed to fit all of the activation energy data between 160 and 298 K. Finally, homogeneous nucleation calculations were performed using the temperature-dependent activation energies in conjunction with previous measurements of aerosol nucleation rates at 190-202 K to determine the freezing rates for HNO3/H2O particles over a wide temperature range. These calculations indicate that nucleation of NAD from supercooled 2:1 H2O:HNO3 aerosols is rapid at relevant stratospheric temperatures. Nucleation of α-NAT from supercooled 3:1 H2O:HNO3 aerosols could occur as rapidly as within 1 day, but the uncertainty in the interfacial energy used for α-NAT is large.

Observational constraints on the formation of type ia polar stratospheric clouds

On January 11, 1989 during AASE I, Type Ia polar stratospheric clouds (PSCs) were observed over a vast area in the Arctic. Synoptic scale temperature histories for this flight are obtained using the Goddard trajectory model. Mesoscale temperature fluctuations (MTF) are divided into two major categories of background and lee waves. The statistics of background MTFs are derived from measurements by the microwave temperature profiler aboard the ER‐2 aircraft. A forecast model is used to determine the occurrence of lee waves. The MTFs are then superimposed upon the Goddard synoptic scale temperatures. Statistical correlations between temperature histories and Type Ia PSC observations indicate that MTFs can not be solely responsible for the formation of Type Ia PSCs in the stratosphere. Instead, on a synoptic scale, the exposure time of air masses to temperatures below the nitric acid trihydrate (NAT) frost point for ∼1 day or more should be the main criteria for predicting the occurrence of Type Ia clouds in the stratosphere.

Crystallization Kinetics of Nitric Acid Dihydrate Aerosols

Type Ia polar stratospheric clouds (PSCs) are thought to consist of HNO3/H2O mixtures, usually assumed to be crystalline nitric acid trihydrate (NAT). However, it has recently been suggested that metastable nitric acid dihydrate (NAD) may form preferentially in the atmosphere due to a lower nucleation barrier. We have used Fourier transform infrared spectroscopy to investigate the crystallization kinetics of NAD aerosols. The crystallization rates were measured under two experimental regimes. In the first, we formed glassy 2:1 H2O:HNO3 aerosols in a cryostat held at 77 K and measured the rates of crystalline NAD formation when the aerosols were warmed to stratospheric temperatures. The crystal growth rates were then used to estimate the activation energy for diffusional transfer of HNO3 across the solid/liquid phase boundary, Δgd. We found Δgd = 13.3 kcal mol-1 for the temperature range 190−202 K. We have also measured the crystallization rate of NAD aerosols nucleated at stratospheric temperatures. We used homogeneous nucleation theory and our estimate of Δgd to determine the interfacial surface energy, σ, between NAD and a supercooled 2:1 H2O:HNO3 solution. We found the interfacial surface energy for NAD to be σ = 22 erg cm-2, much lower than that estimated previously for NAT of σ = 44 erg cm-2. If the interfacial surface energy for NAD is indeed this much lower than that of NAT, nucleation of NAD from liquid HNO3/H2O aerosols may be an important step in the formation mechanism for crystalline type Ia PSCs.