N. Larsen

Large nitric acid particles at the top of an Arctic stratospheric cloud

measurements approximately 200 km upwind of the in situ measurements indicate a similar vertical structure for the cloud. These in situ measurements represent, to our knowledge, the most comprehensive in situ observations of all phases of polar stratospheric cloud particles, while the large particles at cloud top have not been previously observed and may have implications for producing particles large enough to remove reactive nitrogen from the polar stratosphere. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0340 Atmospheric Composition and Structure: Middle atmosphere— composition and chemistry; KEYWORDS: polar stratospheric clouds, in situ stratospheric cloud measurements, Arctic stratospheric clouds, polar stratospheric cloud composition, balloon-borne aerosol measurements, large polar stratospheric cloud particles

Ozone depletion in the Arctic stratosphere in early 1993

During the first three months of 1993 measurements of the vertical ozone profile, with balloon-borne sensors, were performed in Greenland. A substantial ozone depletion took place from the end of January to the end of March in the altitude range between approximately 14 and 20 km. An ozone decrease of about 1% per day is ascribed to chemical destruction inside the polar vortex based on air parcel trajectory analysis, using measurements of lower stratospheric aerosols as dynamical tracers to correct for diabatic subsidence. The column-integrated total ozone loss was found to be about 48 Dobson units or 12%. These measurements are in good agreement with satellite observations, and further document the 1993 springtime stratospheric ozone depletion as the most severe and long lasting yet reported for the Arctic

In situ mountain-wave polar stratospheric cloud measurements: Implications for nitric acid trihydrate formation

0.2 cm 3 , median radii of 1 to 2 mm and volumes up to 1 mm 3 cm 3 . A comparison between optical PSC data and optical simulations based on the measured particle size distribution indicates that the NAT particles were aspherical with an aspect ratio of 0.5. The NAT particle properties have been compared to another PSC observation on 25 January 2000, where NAT particle number densities were about an order of magnitude higher. In both cases, microphysical modeling indicates that the NAT particles have formed on ice particles in the mountain-wave events. Differences in the NAT particle number density can be explained by the meteorological conditions. We suggest that the higher NAT number density on 25 January can be caused by stronger wave activity observed on that day, larger cooling rates and therefore higher NAT supersaturation. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0340 Atmospheric Composition and Structure: Middle atmosphere— composition and chemistry; KEYWORDS: polar stratospheric cloud (PSC), nitric acid trihydrate (NAT), ozone, gravity wave, PSC formation

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.

Non‐equilibrium compositions of liquid polar stratospheric clouds in gravity waves

On 25 January 1998 mountain induced gravity waves developed over Scandinavia leading to the formation of mesoscale polar stratospheric clouds (PSCs). Balloon-borne mass spectrometric measurements of particle composition and optical backscatter measurements were performed above Kiruna/Sweden. PSCs were encountered twice, showing a correlated increase in the condensed phase water, nitric acid and the backscatter ratio. Thermodynamic modeling allows the PSC particles to be unambiguously identified as supercooled ternary solution (STS) droplets, but cannot account for the measured scatter in the particulate HNO3∶H2O mole ratio. Simultaneous temperature measurements show that the particles were subject to rapid atmospheric temperature fluctuations of ±1 K and cooling/heating rates exceeding 1 K/min caused by the gravity waves. Micro-physical non-equilibrium modeling of STS droplet distributions reveals that the observed temperature perturbations cause particle compositions in close agreement with the measured HNO3∶H2O variations. Non-equilibrium compositions of liquid PSC particles are thus a principal stratospheric characteristic related to gravity waves affecting particle evolution.

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. 13 refs., 5 figs., 1 tab.

Three-dimensional studies of the 1991/1992 northern hemisphere winter using domain-filling trajectories with chemistry

We describe a new and computationally efficient technique for global three-dimensional modeling of stratospheric chemistry. This technique involves integrating a photochemical package along a large number of independent trajectories to produce a Lagrangian view of the atmosphere. Although Lagrangian chemical modeling with trajectories is an established procedure, this extension of integrating chemistry along a large number of domain-filling trajectories is a novel technique. This technique is complementary to three-dimensional Eulerian chemical transport modeling and avoids spurious mixing caused by low resolutions or diffusive transport schemes in these models. We illustrate the technique by studying the chlorine activation in the Arctic winter lower stratosphere. A photochemical model was integrated along large ensembles of calculated trajectories between 20 and 100 mbar for the 1991/1992 winter in order to produce a three-dimensional chemical picture. Large amounts of chlorine was activated at low altitudes (80 to 100 mbar) as well as altitudes near 50 mbar. This activated air was well contained at all levels, with little indication of mixing into lower latitudes. Model results for early January 1992 were compared to daily Microwave Limb Sounder (MLS) ClO observations at 465 K. The structure and evolution of the activated chlorine was well reproduced, giving faith in the technique, although absolute modeled ClO amounts were smaller than the MLS data. A larger number of domain-filling isentropic trajectories were also run at 475 K to produce a higher-resolution picture of vortex evolution in late January 1992. The model successfully reproduced the wave breaking events which characterized this period causing transport of activated air to lower latitudes.

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.

Trajectory model studies of ClOx activation during the 1991/92 northern hemispheric winter

The authors present the result of chemical box model calculations carried out along air mass trajectories between November 1991, and January 1992. The calculations show that reactive chlorine compounds, such as chlorine oxide, built up in these air masses over time. The calculated values are compared with measurements from satellite platforms.

Observations of correlated behavior of stratospheric ozone and aerosol at Thule during winter 1991-1992

Using a recently installed lidar, a series of measurements of aerosol concentrations have been carried out between 12-1991 and 3-1992. This work was in conjunction with the European Arctic Stratospheric Ozone Experiment. Scattering ratios were very high because of the presence of aerosols from the Mt Pinatubo volcanic eruption. Ozone observations were made in this period by sondes. This paper reports on correlations between these observations. The correlations observed were not always positive. Both observations saw distinct layered structures representing density variations with height.