Th. Peter

Stratospheric aerosol growth and HNO3 gas phase depletion from coupled HNO3 and water uptake by liquid particles

Aqueous sulphuric acid droplets, which constitute the background stratospheric aerosol, strongly absorb HNO3 and HCl under cold conditions. A thermodynamic model is used to predict partitioning of HNO3, HCl and H2O between gas and aerosol phases, and show that a 50-fold increase in aerosol volume, observed in the Arctic stratosphere as temperature approached the frost point (188.9 K), can be explained in terms of uptake of HNO3 and H2O by liquid aerosols. Calculated degrees of saturation of the droplets with respect to solid hydrates, taking into account the reduction in vapour phase HNO3, suggest that the droplets remain liquid to the frost point. Near this temperature, they can yield larger aerosol volumes than would have been the case for solid NAT (HNO3•3H2O) particles. The depletion of gas phase HNO3 into enhanced volumes of liquid aerosols resulting from volcanic eruptions may hamper NAT formation.

Do stratospheric aerosol droplets freeze above the ice frost point

Laboratory experiments are presented which show that liquid stratospheric aerosol droplets under polar winter conditions do not freeze for temperatures higher than the water ice saturation temperature (frost point). Calorimetric measurements of the freezing of supercooled H2SO4/HNO3/H2O bulk solutions with concentrations typical of the polar stratospheric aerosol exhibit very small freezing rates, which excludes the possibility of homogeneous freezing of the droplets for temperatures above the frost point. Even heterogeneous formation of H2SO4 and HNO3 hydrates at these temperatures is a very inefficient process unless the stratosphere offers nuclei better suited for nucleation than those present in the laboratory experiments, which appears to be unlikely. Only ice was found to be a potential nucleus suited for the formation of the hydrates, which could cause the hydrates to freeze at temperatures below the frost point.

Size-dependent stratospheric droplet composition in lee wave temperature fluctuations and their potential role in PSC freezing

Rapid temperature fluctuations are shown to cause liquid H2SO4/HNO3/H2O stratospheric aerosols to depart considerably from thermodynamic equilibrium. While HNO3 uptake by larger droplets is diffusively hindered, small droplets can approach the composition of a pure binary HNO3/H2O solution with up to 52 wt% HNO3, 48 wt% H2O and very small amounts of H2SO4. The stoichiometry of these droplets is close to that of nitric acid trihydrate (NAT) and freezing experiments suggest that this could be a suitable pathway for the formation of frozen polar stratospheric clouds (PSCs) of type-Ia.

On the potential importance of the gas phase reaction CH3O2 + ClO → ClOO + CH3O and the heterogeneous reaction HOCl + HCl → H2O + Cl2 in “ozone hole” chemistry

We call attention to the great importance of the gas phase reaction ClO + CH3O2 → ClOO + CH3O and the heterogeneous reaction HCl + HOCl → Cl2 + H2O on polar stratospheric cloud (PSC) particles. These reactions may accomplish the almost complete conversion of HCl into ClOx radicals, thus leading to rapid destruction of ozone.

Chlorine chemistry and the potential for ozone depletion in the Arctic stratosphere in the winter of 1991/92

We present an analysis of chlorine chemistry in the Arctic stratosphere during the winter of 1991/92 and assess its potential implications for ozone depletion. In accordance with observations of total organic chlorine, ClONO2 and HCl, box model results indicate the following: (1) An almost complete activation of chlorine during the cold winter period. (2) A possible contribution from the heterogeneous reaction HOCl + HCl and the gas-phase reaction CH3O2 + ClO to the complete conversion of HCl to active chlorine. (3) A strong buildup of ClONO2 following PSC disappearance which remains the main chlorine reservoir for about a month, after which HCl becomes dominant. (4) Appreciable chemical ozone loss in the lower stratosphere inside the polar vortex is conceivable for the winter of 1991/92.

Increase in the PSC‐formation probability caused by high‐flying aircraft

The saturation temperature Tsat for the formation of polar stratospheric clouds (PSC) strongly depends on the local partial pressures of nitric acid and water vapour, and thus is sensitive to NOx and H2O-injection due to exhaust from aircraft in the stratosphere. The present paper investigates this effect, using daily stratospheric temperature data from the northern hemisphere compiled over the last 25 years by the Free University of Berlin. For Type-I PSC the data were examined for temperatures T < Tsat - 3 K, which would allow for a supercooling of about the 3 K measured in a previous arctic winter balloon mission, in effect corresponding to a large HNO3-supersaturation. We show that this is required to overcome the heterogeneous nucleation energy barrier. We compare the analyses of both a background atmosphere and one perturbed by a fleet of 600 stratospheric aircraft flying at ∼22 km altitude. The result is that between December and March in the polar cap region there might be more than a doubling in the occurence of Type-I PSCs and an even stronger increase of Type-II PSCs, and accordingly a substantial enhancement in the potential ozone destruction by chlorine radicals.

Contrail formation: Homogeneous nucleation of H2SO4/H2O droplets

Homogeneous nucleation of sub-nanometer H2SO4/H2O germs, their growth and freezing probability in the cooling wake of a subsonic jet aircraft at tropopause altitude are investigated. Heteromolecular condensation, water uptake, and coagulation cause a small subset of the germs to grow into nm-sized solution droplets which overcome the Kelvin barrier. These droplets efficiently take up water vapor from the gas phase, dilute rapidly, and may eventually freeze as water ice. However, results discussed for the case of a B 747 airliner suggest that under threshold conditions for the onset of contrail formation, a visible contrail is not likely to be produced by this mechanism. The sensitivity of the calculations to uncertainties in the nucleation theory underlines the need for detailed measurements of the particle microphysics in young aircraft plumes.

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 fur 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 Alesund, 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 Alesund 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.

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.

Model-guided Lagrangian observation and simulation of mountain polar stratospheric clouds

Gravity-wave-induced polar stratospheric clouds (PSCs) were observed over the Scandinavian mountains by airborne lidar on January 9, 1997. Guided by the forecasts of a mesoscale dynamical model, a flight path was chosen to lead through the coldest predicted region parallel to the wind at the expected PSC level (23–26 km). Because of the nearly stationary nature of the wave-induced PSC the individual filaments visible in the backscatter data of the clouds can be interpreted as air parcel trajectories. Assuming dry adiabatic behavior and fixing the absolute temperature to the ice frost point in the ice part of the cloud enables detailed microphysical simulations of the whole life cycle of the cloud particles. Optical calculations are used to adjust open parameters in the microphysical model by optimizing the agreement with the multichannel lidar data. This case is compared with former work from the Arctic winter 1994/1995. The influence of the stratospheric H2SO4 content and the cooling rate on the type of cloud particles (liquid ternary solution droplets or solid nitric acid hydrates) released from the ice part of the cloud is evaluated.