Thomas Peter

Water activity as the determinant for homogeneous ice nucleation in aqueous solutions

The unique properties of water in the supercooled (metastable) state are not fully understood. In particular, the effects of solutes and mechanical pressure on the kinetics of the liquid-to-solid phase transition of supercooled water and aqueous solutions to ice have remained unresolved. Here we show from experimental data that the homogeneous nucleation of ice from supercooled aqueous solutions is independent of the nature of the solute, but depends only on the water activity of the solution—that is, the ratio between the water vapour pressures of the solution and of pure water under the same conditions. In addition, we show that the presence of solutes and the application of pressure have a very similar effect on ice nucleation. We present a thermodynamic theory for homogeneous ice nucleation, which expresses the nucleation rate coefficient as a function of water activity and pressure. Recent observations from clouds containing ice are in good agreement with our theory and our results should help to overcome one of the main weaknesses of numerical models of the atmosphere, the formulation of cloud processes.

An analytic expression for the composition of aqueous HNO3‐H2SO4 stratospheric aerosols including gas phase removal of HNO3

An analytic expression is given for the composition and volume of stratospheric HNO3-H2SO4-H2O aerosols in terms of the temperature and total amounts of H2O, HNO3 and H2SO4, taking into account conservation of HNO3. In contrast to previous parameterisations of equilibrium vapour pressures, the present scheme for calculating the aerosol composition does not require an iteration procedure, thus computation times are significantly reduced, making it ideal for inclusion in large-scale atmospheric models. Calculated compositions are compared with the results of a comprehensive thermodynamic model for all stratospheric conditions and maximum deviations are presented.

Increased stratospheric ozone depletion due to mountain-induced atmospheric waves

Chemical reactions on polar stratospheric cloud (PSC) particles are responsible for the production of reactive chlorine species (chlorine ‘activation’) which cause ozone destruction. Gas-phase deactivation of these chlorine species can take several weeks in the Arctic winter stratosphere, so that ozone destruction can be sustained even in air parcels that encounter PSCs only intermittently,. Chlorine activation during a PSC encounter proceeds much faster at low temperatures when cloud particle surface area and heterogeneous reaction rates are higher. Although mountain-induced atmospheric gravity waves are known to cause local reductions in stratospheric temperature of as much as 10–15 K (refs 5-9), and are often associated with mesoscale PSCs, their effect on chlorine activation and ozone depletion has not been considered. Here we describe aircraft observations of mountain-wave-induced mesoscale PSCs in which temperatures were 12 K lower than expected synoptically. Model calculations show that despite their localized nature, these PSCs can cause almost complete conversion of inactive chlorine species to ozone-destroying forms in air flowing through the clouds. Using a global mountain-wave model, we identify regions where mountain waves can develop, and show that they can cause frequent chlorine activation of air in the Arctic stratosphere. Such mesoscale processes offer a possible explanation for the underprediction of reactive chlorine concentrations and ozone depletion rates calculated by three-dimensional models of the Arctic stratosphere.

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.

Modeling the composition of liquid stratospheric aerosols

There is extensive evidence to suggest that stratospheric aerosols can remain liquid to very low stratospheric temperatures, despite being highly supercooled. Even polar stratospheric clouds, which are a key factor in the depletion of ozone in polar regions, can often consist of liquid rather than solid particles. It has been known since the 1960s that stratospheric aerosols are mostly concentrated sulfuric acid-water droplets, but the combination of recent laboratory measurements, field observations, and thermodynamic model calculations has led to a recognition that many species other than water vapor can partition into the aerosols, particularly at low temperatures. This has been shown to increase the aerosol size, to control their freezing properties, and to affect the rates of important liquid phase reactions. This in turn influences the formation of polar stratospheric clouds and the subsequent extent and duration of seasonal ozone depletion in the polar regions. We review thermodynamic models of the liquid phase that enable the partitioning of gases such as HCl, HBr, HOCl, and HNO3 into sulfuric acid aerosols to be calculated over the full range of stratospheric conditions. Such models have been used to show that the uptake of nitric acid vapor can lead to a rapid transition from mainly sulfuric-acid- to mainly nitric-acid-based liquid aerosols at low temperatures, a process that has changed our view of how polar stratospheric clouds form. Liquid aerosol composition at these low temperatures is still known largely from predictions made by thermodynamic models, rather than from observations, and even laboratory data under these conditions are limited. This and other uncertainties in calculated aerosol composition are estimated, and their effect on the interpretation of particle observations and predictions made by chemical stratospheric models is described.

The 1997 Arctic Ozone depletion quantified from three-dimensional model simulations

Three-dimensional simulations of total ozone are reported for the 1996-97 Arctic winter. The record low ozone values observed by satellite in late March are well reproduced by the chemistry-transport model. The comparison between the chemically integrated ozone and a passive tracer with identical initialization allows us to discrimate chemical changes from variations due to dynamical processes. In addition to a substantial total ozone chemical loss (60 to 120 Dobson Units), the simulation reveals an dynamically-induced reduction of ∼70 DU also responsible for the ozone minimum observed in the Arctic in late March 1997.

vapour pressures of H2SO4/HNO3/HCl/HBr/H2O solutions to low stratospheric temperatures

Vapor pressures of H2O, HNO3, HCl and HBr over supercooled aqueous mixtures with sulfuric acid have been calculated using an activity coefficient model, for 185 K less than T less than 235 K, 0 less than wt% (H2SO4) + wt% (HNO3) less than 70, and assuming HCl and HBr to be minor constituents. Predicted vapor pressures agree well with most laboratory data, and give confidence in the validity of the model. The results are parameterized as simple formulae, which reproduce the model results to within 40% and cover the entire stratospherically relevant range of composition and temperature.

Widespread Solid Particle Formation by Mountain Waves in the Arctic Stratosphere

Observations of polar stratospheric clouds (PSCs) by lidar show that the clouds often contain solid particles, which are most likely composed of nitric acid hydrates. However, laboratory experiments indicate that such hydrate particles are not easily formed under Arctic synoptic scale conditions, suggesting that solid PSC particles should be rather rare. Here we show results from a model study indicating that mountain-induced mesoscale temperature perturbations may be an important source of nitric acid hydrate particles in the Arctic. Multiple Arctic vortex trajectories were combined with a global mountain wave forecast model to calculate the potential for solid particle formation during December and January 1994/1995. The mountain wave model was used to calculate adiabatic cooling over several thousand ridge elements. Nitric acid hydrate particles were assumed to form in the mountain waves according to several microphysical mechanisms, and were then advected using polar vortex-filling synoptic trajectories to generate maps of solid particle occurrence. The calculations show that mountain waves may be a significant source of PSCs containing solid particles that are observed on the synoptic scale. In particular, the east coast of Greenland, the Norwegian mountains, and the Urals are found to be solid particle sources, with the PSCs often predicted to survive several thousand kilometers downstream.

The lifetime of leewave-induced ice particles in the Arctic stratosphere: II. Stabilization due to NAT-coating

This paper offers a possible explanation for the observation of ice particles in the size range 1 to 5 microns at stratospheric altitudes of 20 to 24 km in areas with temperatures 5 to 10 K above the frost point. The observations were made from balloon borne instruments launched from northern Sweden. The authors argue such ice particles would have to originate from cooled air masses over relatively near Norwegian mountain ranges. How would they live long enough to transport to this area They argue that if formation processes had allowed for a concurrent condensation of ice and nitric acid trihydrate (NAT), then the ice particles would have a suspension of NAT particles scattered through them. As the ice evaporates, it exposes more NAT clusters, which can eventually coat the surface, slowing evaporation from periods of minutes into the range of hours.

Densities and refractive indices of H2SO4/HNO3/H2O solutions to stratospheric temperatures

The Lorentz-Lorenz relation is used to estimate the refractive index of aqueous H2SO4/HNO3 solutions (5–70 wt%) for wavelengths from the near-ultraviolet to the near-infrared (0.35–2 µm) and for temperatures from 370 K to stratospheric temperatures (185 K). The molecular refractivities of the involved species are based on data of the binary solutions at room temperature. For the extrapolation to low temperatures, a careful estimate of the density of the ternary solutions has been performed. The accuracy of the model predictions is estimated to be better than 2%.