Ice Frost Research Articles

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.

Particle microphysics and chemistry in remotely observed mountain polar stratospheric clouds

Polar stratospheric clouds (PSCs) at 22–26 km were observed over the Norwegian mountains by airborne lidar on January 15, 1995. Simulations using a mesoscale model reveal that they were caused by mountain‐induced gravity waves. The clouds had a highly detailed filamentary structure with bands as thin as 100 m in the vertical, and moved insignificantly over 4 hours, suggesting them to be quasi‐stationary. The aircraft flight path was parallel or close to parallel with the wind at cloud level. Such a quasi‐Lagrangian observation, together with the presence of distinct aerosol layers, allows an air parcel trajectory through the cloud to be constructed and enables the lidar images to be simulated using a microphysical box model and light scattering calculations. The results yield detailed information about particle evolution in PSCs and suggest that water ice nucleated directly from liquid HNO3/H2SO4/H2O droplets as much as 4 K below the ice frost point. The observation of solid nitric acid hydrate particles downwind of the mountains shows that such mesoscale events can generate solid PSC particles that can persist on the synoptic scale. We also draw attention to the possible role of mesoscale PSCs in chlorine activation and subsequent ozone destruction.

Deliquescence of sulfuric acid tetrahydrate following volcanic eruptions or denitrification

A thermodynamic model is employed to investigate the effects of cooling sulfuric acid tetrahydrate (SAT) when the polar stratosphere is perturbed by denitrification or volcanic eruptions. Also, survey experiments are carried out to study the plausibility of the predictions of this model. In contrast to an atmosphere rich in HNO3 for which Koop and Carslaw [1996] calculated that SAT deliquesces upon cooling to form a ternary solution, in a denitrified atmosphere the liquid begins to form ∼3 K colder, and SAT is not predicted to deliquesce completely above the ice frost point. Likewise, in an atmosphere with a high sulfate loading, solid/liquid aerosols are predicted to coexist over a large temperature range. For these two cases, SAT particles should thus deliquesce only partially upon cooling, and hence, should return to solid form upon warming. Furthermore, the results of laboratory experiments suggest that HNO3‐induced deliquescence of SAT might be too slow to matter even under normal stratospheric conditions.

Freezing of polar stratospheric clouds in orographically induced strong warming events

Results from laboratory experiments and microphysical modeling are presented that suggest a potential freezing nucleation mechanism for polar stratospheric cloud (PSC) particles above the water ice frost point (Tice). The mechanism requires very high HNO3 concentrations of about 58 wt% in the droplets, and leads to the freezing of nitric acid dihydrate (NAD) in a highly selective manner in the smallest droplets of an ensemble. In the stratosphere such liquid compositions are predicted to occur in aerosol droplets which are warmed adiabatically with rates of about +150 K/h from below 190 K to 194 K. Such rapid temperature changes have been observed in mountain leewaves that occur frequently in the stratosphere, clearly demonstrating the need for a stratospheric gravity wave climatology.

Formation and implications of ice particle nucleation in the stratosphere

The classical nucleation theory of homogeneous freezing along with a thermodynamic model of aqueous HNO3 is used to calculate the critical temperatures and compositions at which ice crystals will nucleate from an aqueous HNO3 droplet. Assuming that stratospheric aerosols are composed of aqueous HNO3 near the ice frost point, we show that ice particle formation in the stratosphere can only occur if the air mass is cooled to about 2 to 3 K below the equilibrium condensation point of ice. A simple function is given for predicting the nucleation temperature of ice particle formation in the stratosphere from the ambient water vapor concentration. Microphysical calculations show that only fast cooling rates (> 500 K day−1) encountered in cold lee waves are rapid enough for nucleating the majority of the background aerosols into ice particles. The likelihood that ice crystals formed in lee waves may result in the subsequent crystallization of HNO3 in the droplets is discussed. The barrier to ice formation by heterogeneous gas phase nucleation is compared to that of homogeneous freezing nucleation.

Ammonium Bisulfate/Water Equilibrium and Metastability Phase Diagrams

is monitored continuously using a capacitance manometer. The change in the particle’s composition as the temperature or relative humidity (RH) is varied is determined by measuring the mass-to-charge ratio (with an accuracy of1%). A vertically polarized HeNe laser is focused on the particle. The intensity of the scattered light is continuously monitored at a9 0 °angle from the incident laser, and the phase (solid or liquid) is determined from the Mie scattering. To assure uniform temperature, the electrodynamic trap and the interaction chamber are constructed of copper. The chamber is insulated with a vacuum shroud and is in contact with a liquid nitrogen reservoir via heat transfer straps. An automated feedback heating loop is used to control the temperature to better than 0.2 K. The temperature is monitored at two points, above and below the particle, with the difference kept to less than 0.3 K. The reported temperature is the average between the two points. The system can be used to perform experiments at constant temperature or to scan temperature at a prescribed rate. Both modes were utilized to obtain the data presented here. When the temperature was scanned, the water vapor pressure was maintained at equilibrium with ice, by coating the cell walls with ice. The entire pressure and temperature monitoring system was calibrated against the ice frost line. The results reported here represent observations of over20 particles.

Freezing behavior of single sulfuric acid aerosols suspended in a quadrupole trap

The freezing properties of sulfuric acid droplets were studied by suspending single 20‐ to 30‐μm‐diameter particles in a quadrupole trap and cooling them to stratospheric temperatures (≥191.5 K). Each particle's dc balance voltage was measured to determine the particle composition as a function of temperature and map out the particle's trajectory relative to the sulfuric acid phase diagram. Angularly resolved optical scattering patterns were monitored to detect freezing events. Particles cooled through the sulfuric acid tetrahydrate region (35–70 wt % H2SO4) did not freeze and remained spherical liquid droplets for several hours. Only particles cooled through the ice‐liquid equilibrium region (<35 wt% H2SO4) showed evidence of freezing. This supports previous experimental and field observations that stratospheric sulfuric acid aerosols are likely to remain liquid to within a few degrees of the ice frost point.

Freezing of HNO3/H2SO4/H2O Solutions at Stratospheric Temperatures: Nucleation Statistics and Experiments

Calorimetric freezing experiments with aqueous sulfuric and nitric acid solutions are presented and applied to the formation of polar stratospheric clouds (PSCs). We show that the nucleation of hydrates from these solutions is a stochastic process and that nucleation rates and their uncertainties can be determined using Poisson statistics. Under thermodynamic equilibrium conditions above the ice frost point, the homogeneous nucleation rates of stratospheric aerosols are exceedingly low, ruling out homogeneous freezing as a pathway for PSC formation. Several stratospherically important substrates were tested concerning their ability to induce heterogeneous nucleation. None of the experiments indicated a relevant enhancement of the freezing probability of liquid aerosols. Moreover, the experiments reveal that the freezing process of the solutions under stratospheric conditions is limited by the nucleation rates of the hydrates, rather than their crystal growth rates, thus ruling out the possibility of a gla...

Physical State and Composition of Polar Stratospheric Clouds Inferred from Airborne Lidar Measurements During SESAME

The airborne lidar LEANDRE was deployed from Kiruna (Sweden) in January1995, during Phase III of the Second European Stratospheric Arctic andMid-latitude Experiment (SESAME). Polar Stratospheric Clouds (PSC) weredetected on several flights. Four lidar retrievals, on two different days, arepresented together with the thermal history of air masses, derived from 5-daybackward trajectories calculated from the TOPCAT model, using ECMWF analyses.The first PSC case was observed at the synoptic scale and at a temperatureclose to the ice frost point, with no significant depolarization, suggestingthat particles were in the liquid phase, probably liquid ternaryH2SO4/HNO3/H2Osolution forming a Type 1b cloud. Locally, on the same day, a Type II PSC (icecloud) was detected above the Scandinavian Mountains and was related tomesoscale processes. On another day, the third lidar retrieval only showedbackground aerosols signature, whereas the last one was performed at the NATequilibrium temperature and its measured depolarization ratio was consistentwith few solid state Type Ia particles, probably composed of NAT. From thethermal histories, the different behaviour of the two Type I clouds isexplained in terms of cooling rates and compositional hysteresis, driven bytemperature.

Melting of H 2 SO 4 ⋅4H 2 O Particles upon Cooling: Implications for Polar Stratospheric Clouds

Polar stratospheric clouds (PSCs) are important for the chemical activation of chlorine compounds and subsequent ozone depletion. Solid PSCs can form on sulfuric acid tetrahydrate (SAT) (H2SO4·4H2O) nuclei, but recent laboratory experiments have shown that PSC nucleation on SAT is strongly hindered. A PSC formation mechanism is proposed in which SAT particles melt upon cooling in the presence of HNO3 to form liquid HNO3-H2SO4-H2O droplets 2 to 3 kelvin above the ice frost point. This mechanism offers a PSC formation temperature that is defined by the ambient conditions and sets a temperature limit below which PSCs should form.

Balloonborne backscatter observations of type 1 PSC formation: Inference about physical state from trajectory analysis

Ballonborne backscatter measurements of polar stratospheric clouds (PSC) from four Arctic winters have been used together with isentropic airparcel trajectories to assess recent theories of PSC formation. Assuming unperturbed HNO3 concentrations, observations performed in the month of January, reveal that PSC formation initiates 3–4 K below the nitric acid trihydrate (NAT) condensation temperature (TNAT). According to a suggested scenario for PSC formation [Tabazcdeh et al., 1995] liquid supercooled ternary solution particles may freeze upon an increase in temperature after previously being close to the ice frost point. Liquid particles, identified by the temperature histories according to this scenario, show a relatively compact relationship between measured temperature and backscatter ratio, indicating a substantial growth at 3–4 K below the NAT‐temperature as expected by ternary solution particles. Newly formed solid particles are of small size, in contrast to aged solid particles which appear to retain large sizes up to the NAT‐temperatures during evaporation and grow by condensation at temperatures slightly below TNAT, indicating the composition to be NAT.

Laboratory studies of the formation of polar stratospheric clouds: Nitric acid condensation on thin sulfuric acid films

Thin sulfuric acid films were exposed to 5 × 10−8 − 8 × 10−7 torr HNO3 and 2 − 3 × 10−4 torr H2O and cooled to temperatures near the ice frost point. Fourier transform infrared (FTIR) spectroscopy was used to probe the condensed‐phase species during isothermal experiments, and gas pressures were monitored with mass spectrometry. Supercooled liquid sulfuric acid films exposed to HNO3 (6 ≤ SNAT ≤ 114) showed indications of HNO3 uptake to form ternary solutions of approximately 4 wt % HNO3, 38 wt % H2SO4, and 59 wt % H2O, followed by crystallization of nitric acid trihydrate (NAT). NAT crystallization did not initiate significant crystallization of the supercooled H2SO4, but the H2SO4 often crystallized to sulfuric acid tetrahydrate (SAT) upon warming. In contrast, when crystalline SAT films were exposed to HNO3 and water, NAT did not condense within several hours, even at HNO3 saturation ratios of 30 or higher. Calculations of the contact parameter from experimental data indicate that m <0.76 for NAT on SAT. Our film studies suggest that crystalline polar stratospheric cloud (PSC) growth is most easily accomplished when stratospheric sulfate aerosols (SSAs) remain liquid, absorb HNO3, and produce crystalline nitric acid trihydrate via heterogeneous nucleation. If SSAs crystallize to SAT at some point during the winter, nitric acid condensation is hindered, and PSC formation could become more difficult.

On the theories of type 1 polar stratospheric cloud formation

Several mechanisms for the production of polar stratospheric cloud (PSC) particles are investigated using the classical theories of nucleation and freezing and the multicomponent condensation theory. These mechanisms invoke particle compositions ranging from binary (H2SO4/H2O) solution, solid sulfuric acid tetrahydrate (SAT) and ternary (HNO3/H2O/H2SO4) solution to binary (HNO3/H2O) solution and solid nitric acid trihydrate (NAT). Empirical relations, derived from classical nucleation studies, are used to calculate the surface energies required in calculations of nucleation and freezing. Using these data, we calculate that the nucleation of nitric acid solutions or solid phases onto SAT particles is not efficient. Homogeneous freezing of SAT or NAT from ternary solutions does not occur under stratospheric conditions. Homogeneous freezing of water ice can occur at temperatures near the frost point of pure water. Heterogeneous freezing is a strong function of the contact parameter between the emergent crystal and the initiating seed particle. Heterogeneous freezing of the stratospheric aerosol to SAT and NAT at temperatures above the frost point is not ruled out by our calculations. If formed, NAT can deplete the gas phase nitric acid concentration, by condensational growth, more efficiently than ternary droplets. We conclude that the most likely route to type 1 PSC particles is via condensational growth of ternary solution droplets followed by rapid freezing to NAT, SAT, and water ice at temperatures near the ice frost point. The particles formed are then stable and can reduce nitric acid vapor pressures to the saturation vapor pressure over NAT at all temperatures below the NAT point. Such a mechanism is consistent with observations.

Deliquescence and freezing of stratospheric aerosol observed by balloonborne backscattersondes

Stratospheric sulfate aerosols, originating from the Mt. Pinatubo volcanic eruption, have been observed during three winters in the Arctic by balloonborne backscattersondes. A measured color index, defined as the quotient between the aerosol backscatter ratios at wavelengths 940 and 480 nm, provides information of the size of the observed particles. The effects of liquid particle growth, by water vapor uptake, clearly show up as changes in the color index, whereas measurements on other days indicate the particles to be frozen. Air parcel trajectories have been calculated, providing the temperature history of the observed particles. Evidences appear of a temperature hysteresis in the freezing and melting cycle of the aerosol, indicating melting temperatures around 215–220 K in good agreement with laboratory measurements, and freezing of the particles within less than 5 K above the ice frost point. The changes in color index of the liquid particles are in good agreement with predictions from theoretical model calculations of growth by water vapor uptake.

Composition and freezing of aqueous H2SO4/HNO3 solutions under polar stratospheric conditions

The results of laboratory investigations of the freezing behavior of aqueous acid solutions indicate that in the stratosphere H2SO4/H2O aerosol droplets would not freeze at temperatures above the ice frost point in the absence of HNO3; however, in the presence of typical levels of HNO3 liquid sulfuric acid aerosols take up significant amounts of HNO3 and H2O vapors and freeze much more readily. This is a consequence of the very rapid change in composition of the liquid droplets as the temperature drops to within two to three degrees of the equilibrium temperature at which HNO3 and H2O vapors would co‐condense to form a liquid solution. In the high latitude stratosphere this HNO3/H2O ‘dew point’ is typically around 192–194 K at 100 mbar.

Fourier transform‐infrared studies of thin H2SO4/H2O films: Formation, water uptake, and solid‐liquid phase changes

Fourier transform‐infrared (FTIR) spectroscopy was used to examine films representative of stratospheric sulfuric acid aerosols. Thin films of sulfuric acid were formed in situ by the condensed phase reaction of SO3 with H2O. FTIR spectra show that the sulfuric acid films absorb water while cooling in the presence of water vapor. Using stratospheric water pressures, the most dilute solutions observed were >40 wt % before simultaneous ice formation and sulfuric acid freezing occurred. FTIR spectra also revealed that the sulfuric acid films crystallized mainly as sulfuric acid tetrahydrate (SAT). Crystallization occurred either when the composition was about 60 wt % H2SO4 or after ice formed on the films at temperatures 1–4 K below the ice frost point. Finally, we determined that the melting point for SAT depended on the background water pressure and was 216–219 K in the presence of 4 × 10−4 Torr H2O. Our results suggest that once frozen, sulfuric acid aerosols in the stratosphere are likely to melt at these temperatures, 30 K colder than previously thought.

Physical Chemistry of the H 2 SO 4 /HNO 3 /H 2 O System: Implications for Polar Stratospheric Clouds

Polar stratospheric clouds (PSCs) play a key role in stratospheric ozone depletion. Surface-catalyzed reactions on PSC particles generate chlorine compounds that photolyze readily to yield chlorine radicals, which in turn destroy ozone very efficiently. The most prevalent PSCs form at temperatures several degrees above the ice frost point and are believed to consist of HNO(3) hydrates; however, their formation mechanism is unclear. Results of laboratory experiments are presented which indicate that the background stratospheric H(2)SO(4)/H(2)O aerosols provide an essential link in this mechanism: These liquid aerosols absorb significant amounts of HNO(3) vapor, leading most likely to the crystallization of nitric acid trihydrate (NAT). The frozen particles then grow to form PSCs by condensation of additional amounts of HNO(3) and H(2)O vapor. Furthermore, reaction probability measurements reveal that the chlorine radical precursors are formed readily at polar stratospheric temperatures not just on NAT and ice crystals, but also on liquid H(2)SO(4) solutions and on solid H(2)SO(4) hydrates. These results imply that the chlorine activation efficiency of the aerosol particles increases rapidly as the temperature approaches the ice frost point regardless of the phase or composition of the particles.

Formation of model polar stratospheric cloud films

Fourier transform infrared spectroscopy was used to examine the competitive growth of films representative of polar stratospheric clouds. These experiments show that either crystalline nitric acid trihydrate (β‐NAT) or amorphous films with H2O:HNO3 ratios close to 3:1 formed at temperatures 3–7 K warmer than the ice frost point under stratospheric pressure conditions. In addition, with higher HNO3 pressures we observed nitric acid dihydrate (NAD) formation at temperatures warmer than ice formation. However, our experiments also show that NAD surfaces converted to β‐NAT upon exposure to stratospheric water pressures. Finally, we determined that the net uptake coefficient for HNO3 on β‐NAT is close to unity, whereas the net uptake coefficient for H2O is much less.

Fourier transform infrared studies of model polar stratospheric cloud surfaces: Growth and evaporation of ice and nitric acid/ice

Fourier Transform Infrared (FTIR) surface studies have been used to probe the microphysical properties of nitric acid trihydrate (NAT) and ice films representative of type I and II polar stratospheric clouds (PSCs), respectively. The infrared spectra of NAT and nitric acid monohydrate (NAM) are obtained by dosing calibrated 3:1 and 1:1 mixtures of water and nitric acid onto a support at −90°C. The spectra, along with previously reported spectra for NAT and NAM, are used to monitor growth, transformation, and evaporation of ice and NAT films. Under our experimental pressure and temperature conditions, NAT is observed to grow at temperatures around 5°C higher than the ice frost point. This is in agreement with the conclusions drawn by Hanson and Mauersberger (1988a,b) from vapor pressure studies over ice and NAT solids. At temperatures 2°C warmer than NAT appearance, we observe growth of a film recently assigned by Ritzhaupt and Devlin (1990) as nitric acid dihydrate (NAD). Because it is formed at the highest temperatures, it is possible that NAD plays a role in PSC formation. However, the differences between the pressures used in the present study and atmospheric pressures, as well as uncertainties in measurements of absolute temperature, could explain NAD formation in our experiment that may not occur in the atmosphere. FTIR surface studies have also been used to probe the condensed products that result from the interaction of ice with gaseous nitric acid. The experiments indicate that upon initial exposure of ice to 1.8×10−6 torr of HNO3, a layer of ice is quantitatively converted to NAT. However, conversion of ice to NAT does not proceed indefinitely, but rather the system reaches saturation. For longer exposures or higher HNO3 pressures, NAM becomes the dominant nitric acid containing species on the surface. Evaporation studies were performed to test the feasibility of a recent denitrification mechanism proposed by Wofsy et al. (1990). The results indicate that ice coated with 0.02 μm of NAT evaporates at a temperature about 4°C higher than uncoated ice. This is consistent with the mechanism for stratospheric denitrification proposed by Wofsy et al. (1990).

Field observations of soil temperature and water tension feedback effects on needle ice nights

Numerous laboratory studies have indicated that thermal gradients may produce appreciable soil water movement in the absence of a strong suction field. In addition the soil water flow and thermal properties are physically interrelated. This paper presents field data taken during needle ice events at Vancouver, Canada, and yields an indication of the magnitude of interaction between the thermal and water flow properties. This feedback determines the state of soil frost hazard in a location where damage to plant material is produced by ice frost hazard in a location where damage to plant material is produced by ice needle growth. Further apparently anomalous interruption in the normal parabolic temperature-time pattern during radiation events is interpreted as the product of water flow down a thermal gradient.