Polar Stratospheric Clouds Composition Research Articles

Antarctic Polar Stratospheric Cloud Analysis of ACE‐FTS Data From 2005 to 2023

AbstractWe present an analysis of Antarctic polar winters from 2005 to 2023 as observed by the Atmospheric Chemistry Experiment (ACE). The unique broad band infrared spectral features in ACE “residual” spectra are used to classify the spectra of polar aerosols by composition into polar stratospheric clouds (PSCs) and sulfate aerosols. The spectra of PSCs are further classified into nitric acid trihydrate, supercooled ternary solutions, supercooled nitric acid, ice‐mix, and mixtures of PSCs. A breakdown of PSC composition is presented for each year. Antarctic winter seasons with unusual compositions are: 2011, in which volcanic ash mixed with PSCs was observed from July to August; 2019, which experienced a stratospheric warming event; 2020, the PSC season following the Australian Black Summer pyrocumulonimbus event; and 2023, which had unusually large sulfate aerosols following the Honga‐Tonga Honga Ha'apai eruption of 2022.

Antarctic polar stratospheric cloud composition as observed by ACE, CALIPSO and MIPAS

The composition of polar stratospheric clouds (PSCs) in the Antarctic is measured by infrared absorption spectroscopy by the Atmospheric Chemistry Experiment (ACE) satellite and compared to lidar (CALIPSO) and emission spectroscopy (MIPAS) results. ACE observations are compared first to CALIPSO for the period 2006–2020, then to MIPAS for the period 2005–2011, and then a set of triple coincidences is studied. In both CALIPSO-ACE and MIPAS-ACE comparisons, nitric acid trihydrate (NAT) detections are in excellent agreement (∼80 % of coincident compositions is found). Supercooled ternary solution (STS) detections are in good agreement for CALIPSO-ACE (∼60 %) but in poor agreement for MIPAS-ACE (39 %). This contrasts with ice detections, in poor agreement for CALIPSO-ACE (∼30 %) and in better agreement for MIPAS-ACE (45 %). Triple coincidences show that ACE STS detections are more reliable than MIPAS STS detections. The comparison between ACE observations and homogeneous CALIPSO PSCs shows that for 11 events, CALIPSO reports STS rather than supercooled nitric acid (SNA) clouds.

Particle shapes and infrared extinction spectra of nitric acid dihydrate (NAD) crystals: optical constants of the β-NAD modification

Abstract. Satellite- and aircraft-based mid-infrared measurements of polar stratospheric clouds (PSCs) have provided spectroscopic evidence for the presence of β-NAT (nitric acid trihydrate) particles. Metastable nitric acid hydrate phases such as α-NAT and α-NAD (nitric acid dihydrate) have been frequently observed in laboratory experiments but not yet detected as a constituent of PSCs in atmospheric measurements. As for the β-NAD modification, its formation was first observed in X-ray diffraction measurements when the low-temperature α-NAD phase was warmed to a temperature above 210 K. Its infrared spectrum has been reported, but so far no optical constants have been derived that could be used as input for infrared retrievals of PSC composition. In this work, we show that β-NAD particles were efficiently formed in isothermal heterogeneous crystallisation experiments at 190 K from supercooled HNO3/H2O solution droplets containing an embedded mineral dust or meteoric smoke particle analogue. An inversion algorithm based on a T-matrix optical model was used to derive for the first time the mid-infrared complex refractive indices of the β-NAD modification from the measured extinction spectrum of the particles. In contrast to the heterogeneous crystallisation experiments, the α-NAD phase was formed when the HNO3/H2O solution droplets did not contain a solid nucleus and crystallised homogeneously. Using a light-scattering detector that recorded two-dimensional scattering patterns of the crystallised NAD particles, we were able to determine predominant shapes of the α- and β-NAD crystals. We found that α-NAD grew into elongated, needle-shaped crystals, while β-NAD particles were compact in shape. This agrees with previously reported images of α- and β-NAD particles grown on the cryo-stage of an environmental scanning electron microscope. While direct evidence for the existence of metastable NAD in the polar stratosphere is still lacking, our experiments add to the wealth of previous laboratory studies that have identified various conditions for the rapid growth of metastable compositions. In the atmosphere, these could be intermediate states that transform into thermodynamically stable NAT on longer timescales in aged PSCs.

Infrared transmittance spectra of polar stratospheric clouds

Polar stratospheric clouds (PSCs) are responsible for polar ozone depletion. Infrared transmittance spectra of PSCs recorded by the Fourier transform spectrometer (FTS) on the Atmospheric Chemistry Experiment (ACE) satellite were used to determine the composition and properties of PSCs. These unique broad band infrared spectra identify PSCs as nitric acid trihydrate (NAT), supercooled ternary solutions (STS) of nitric and sulfuric acid, and ice, as expected. Quantitative modeling of these PSC spectra shows that supercooled nitric acid (SNA) is also common, i.e., “STS” with no observable sulfuric acid in their infrared spectra. ACE-FTS observations therefore classify PSCs into 4 basic spectral types, NAT, STS, SNA and ice, as well as their mixtures. As an example, this classification scheme was applied to the Antarctic for July to September 2019. The composition and particularly the phase of PSCs are critical to the understanding and detailed chemical modeling of polar stratospheric ozone depletion.

Quasi-coincident observations of polar stratospheric clouds by ground-based lidar and CALIOP at Concordia (Dome C, Antarctica) from 2014 to 2018

Abstract. Polar stratospheric clouds (PSCs) have been observed from 2014 to 2018 from the lidar observatory at the Antarctic Concordia station (Dome C), included as a primary station in the NDACC (Network for Detection of Atmospheric Climate Change). Many of these measurements have been performed in coincidence with overpasses of the satellite-borne CALIOP (Cloud Aerosol Lidar with Orthogonal Polarization) lidar, in order to perform a comparison in terms of PSC detection and composition classification. Good agreement has been obtained, despite intrinsic differences in observation geometry and data sampling. This study reports, to our knowledge, the most extensive comparison of PSC observations by ground-based and satellite-borne lidars. The PSCs observed by the ground-based lidar and CALIOP form a complementary and congruent dataset and allow us to study the seasonal and interannual variations in PSC occurrences at Dome C. Moreover, a strong correlation with the formation temperature of NAT (nitric acid trihydrate), TNAT, calculated from local temperature, pressure, and H2O and HNO3 concentrations is shown. PSCs appear at Dome C at the beginning of June up to 26 km and start to disappear in the second half of August, when the local temperatures start to rise above TNAT. Rare PSC observations in September coincide with colder air masses below 18 km.

Evaluation of polar stratospheric clouds in the global chemistry–climate model SOCOLv3.1 by comparison with CALIPSO spaceborne lidar measurements

Abstract. Polar stratospheric clouds (PSCs) contribute to catalytic ozone destruction by providing surfaces for the conversion of inert chlorine species into active forms and by denitrification. The latter describes the removal of HNO3 from the stratosphere by sedimenting PSC particles, which hinders chlorine deactivation by the formation of reservoir species. Therefore, an accurate representation of PSCs in chemistry–climate models (CCMs) is of great importance to correctly simulate polar ozone concentrations. Here, we evaluate PSCs as simulated by the CCM SOCOLv3.1 for the Antarctic winters 2006, 2007 and 2010 by comparison with backscatter measurements by CALIOP on board the CALIPSO satellite. The year 2007 represents a typical Antarctic winter, while 2006 and 2010 are characterized by above- and below-average PSC occurrence. The model considers supercooled ternary solution (STS) droplets, nitric acid trihydrate (NAT) particles, water ice particles and mixtures thereof. PSCs are parameterized in terms of temperature and partial pressures of HNO3 and H2O, assuming equilibrium between the gas and particulate phase. The PSC scheme involves a set of prescribed microphysical parameters, namely ice number density, NAT particle radius and maximum NAT number density. In this study, we test and optimize the parameter settings through several sensitivity simulations. The choice of the value for the ice number density affects simulated optical properties and dehydration, while modifying the NAT parameters impacts stratospheric composition via HNO3 uptake and denitrification. Depending on the NAT parameters, reasonable denitrification can be modeled. However, its impact on ozone loss is minor. The best agreement with the CALIOP optical properties and observed denitrification was for this case study found with the ice number density increased from the hitherto used value of 0.01 to 0.05 cm−3 and the maximum NAT number density from 5×10-4 to 1×10-3 cm−3. The NAT radius was kept at the original value of 5 µm. The new parameterization reflects the higher importance attributed to heterogeneous nucleation of ice and NAT particles following recent new data evaluations of the state-of-the-art CALIOP measurements. A cold temperature bias in the polar lower stratosphere results in an overestimated PSC areal coverage in SOCOLv3.1 by up to 40 %. Offsetting this cold bias by +3 K delays the onset of ozone depletion by about 2 weeks, which improves the agreement with observations. Furthermore, the occurrence of mountain-wave-induced ice, as observed mainly over the Antarctic Peninsula, is continuously underestimated in the model due to the coarse model resolution (T42L39) and the fixed ice number density. Nevertheless, we find overall good temporal and spatial agreement between modeled and observed PSC occurrence and composition. This work confirms previous studies indicating that simplified PSC schemes, which avoid nucleation and growth calculations in sophisticated but time-consuming microphysical process models, may also achieve good approximations of the fundamental properties of PSCs needed in CCMs.

Exploration of machine learning methods for the classification of infrared limb spectra of polar stratospheric clouds

Abstract. Polar stratospheric clouds (PSCs) play a key role in polar ozone depletion in the stratosphere. Improved observations and continuous monitoring of PSCs can help to validate and improve chemistry–climate models that are used to predict the evolution of the polar ozone hole. In this paper, we explore the potential of applying machine learning (ML) methods to classify PSC observations of infrared limb sounders. Two datasets were considered in this study. The first dataset is a collection of infrared spectra captured in Northern Hemisphere winter 2006/2007 and Southern Hemisphere winter 2009 by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument on board the European Space Agency's (ESA) Envisat satellite. The second dataset is the cloud scenario database (CSDB) of simulated MIPAS spectra. We first performed an initial analysis to assess the basic characteristics of the CSDB and to decide which features to extract from it. Here, we focused on an approach using brightness temperature differences (BTDs). From both the measured and the simulated infrared spectra, more than 10 000 BTD features were generated. Next, we assessed the use of ML methods for the reduction of the dimensionality of this large feature space using principal component analysis (PCA) and kernel principal component analysis (KPCA) followed by a classification with the support vector machine (SVM). The random forest (RF) technique, which embeds the feature selection step, has also been used as a classifier. All methods were found to be suitable to retrieve information on the composition of PSCs. Of these, RF seems to be the most promising method, being less prone to overfitting and producing results that agree well with established results based on conventional classification methods.

Polar stratospheric cloud climatology based on CALIPSO spaceborne lidar measurements from 2006 to 2017

Abstract. The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) satellite has been observing polar stratospheric clouds (PSCs) from mid-June 2006 until the present. The spaceborne lidar profiles PSCs with unprecedented spatial (5 km horizontal×180 m vertical) resolution and its dual-polarization capability enables classification of PSCs according to composition. Nearly coincident Aura Microwave Limb Sounder (MLS) measurements of the primary PSC condensables (HNO3 and H2O) provide additional constraints on particle composition. A new CALIOP version 2 (v2) PSC detection and composition classification algorithm has been implemented that corrects known deficiencies in previous algorithms and includes additional refinements to improve composition discrimination. Major v2 enhancements include dynamic adjustment of composition boundaries to account for effects of denitrification and dehydration, explicit use of measurement uncertainties, addition of composition confidence indices, and retrieval of particulate backscatter, which enables simplified estimates of particulate surface area density (SAD) and volume density (VD). The over 11 years of CALIOP PSC observations in each v2 composition class conform to their expected thermodynamic existence regimes, which is consistent with previous analyses of data from 2006 to 2011 and underscores the robustness of the v2 composition discrimination approach. The v2 algorithm has been applied to the CALIOP dataset to produce a PSC reference data record spanning the 2006–2017 time period, which is the foundation for a new comprehensive, high-resolution climatology of PSC occurrence and composition for both the Antarctic and Arctic. Time series of daily-averaged, vortex-wide PSC areal coverage versus altitude illustrate that Antarctic PSC seasons are similar from year to year, with about 25 % relative standard deviation in Antarctic PSC spatial volume at the peak of the season in July and August. Multi-year average, monthly zonal mean cross sections depict the climatological patterns of Antarctic PSC occurrence in latitude–altitude and also equivalent-latitude–potential-temperature coordinate systems, with the latter system better capturing the microphysical processes controlling PSC existence. Polar maps of the multi-year mean geographical patterns in PSC occurrence frequency show a climatological maximum between longitudes 90∘ W and 0∘, which is the preferential region for forcing by orography and upper tropospheric anticyclones. The climatological mean distributions of particulate SAD and VD also show maxima in this region due to the large enhancements from the frequent ice clouds. Stronger wave activity in the Northern Hemisphere leads to a more disturbed Arctic polar vortex, whose evolution and lifetime vary significantly from year to year. Accordingly, Arctic PSC areal coverage is distinct from year to year with no “typical” year, and the relative standard deviation in Arctic PSC spatial volume is >100 % throughout most of the season. When PSCs are present in the Arctic, they most likely occur between longitudes 60∘ W and 90∘ E, which is consistent with the preferential location of the Arctic vortex. Comparisons of CALIOP v2 and Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) Antarctic PSC observations show excellent correspondence in the overall spatial and temporal evolution, as well as for different PSC composition classes. Climatological patterns of CALIOP v2 PSC occurrence frequency in the vicinity of McMurdo Station, Antarctica, and Ny-Ålesund, Spitsbergen, are similar in nature to those derived from local ground-based lidar measurements. To investigate the possibility of longer-term trends, appropriately subsampled and averaged CALIOP v2 PSC observations from 2006 to 2017 were compared with PSC data during the 1978–1989 period obtained by the spaceborne solar occultation instrument SAM II (Stratospheric Aerosol Measurement II). There was good consistency between the two instruments in column Antarctic PSC occurrence frequency, suggesting that there has been no long-term trend. There was less overall consistency between the Arctic records, but it is very likely due to the high degree of interannual variability in PSCs rather than a long-term trend.

A multi-wavelength classification method for polar stratospheric cloud types using infrared limb spectra

Abstract. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument on board the ESA Envisat satellite operated from July 2002 until April 2012. The infrared limb emission measurements represent a unique dataset of daytime and night-time observations of polar stratospheric clouds (PSCs) up to both poles. Cloud detection sensitivity is comparable to space-borne lidars, and it is possible to classify different cloud types from the spectral measurements in different atmospheric windows regions. Here we present a new infrared PSC classification scheme based on the combination of a well-established two-colour ratio method and multiple 2-D brightness temperature difference probability density functions. The method is a simple probabilistic classifier based on Bayes' theorem with a strong independence assumption. The method has been tested in conjunction with a database of radiative transfer model calculations of realistic PSC particle size distributions, geometries, and composition. The Bayesian classifier distinguishes between solid particles of ice and nitric acid trihydrate (NAT), as well as liquid droplets of super-cooled ternary solution (STS). The classification results are compared to coincident measurements from the space-borne lidar Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument over the temporal overlap of both satellite missions (June 2006–March 2012). Both datasets show a good agreement for the specific PSC classes, although the viewing geometries and the vertical and horizontal resolution are quite different. Discrepancies are observed between the CALIOP and the MIPAS ice class. The Bayesian classifier for MIPAS identifies substantially more ice clouds in the Southern Hemisphere polar vortex than CALIOP. This disagreement is attributed in part to the difference in the sensitivity on mixed-type clouds. Ice seems to dominate the spectral behaviour in the limb infrared spectra and may cause an overestimation in ice occurrence compared to the real fraction of ice within the PSC area in the polar vortex. The entire MIPAS measurement period was processed with the new classification approach. Examples like the detection of the Antarctic NAT belt during early winter, and its possible link to mountain wave events over the Antarctic Peninsula, which are observed by the Atmospheric Infrared Sounder (AIRS) instrument, highlight the importance of a climatology of 9 Southern Hemisphere and 10 Northern Hemisphere winters in total. The new dataset is valuable both for detailed process studies, and for comparisons with and improvements of the PSC parameterizations used in chemistry transport and climate models.

Polar stratospheric cloud evolution and chlorine activation measured by CALIPSO and MLS, and modeled by ATLAS

Abstract. We examined observations of polar stratospheric clouds (PSCs) by CALIPSO, and of HCl and ClO by MLS along air mass trajectories, to investigate the dependence of the inferred PSC composition on the temperature history of the air parcels and the dependence of the level of chlorine activation on PSC composition. Several case studies based on individual trajectories from the Arctic winter 2009/2010 were conducted, with the trajectories chosen such that the first processing of the air mass by PSCs in this winter occurred on the trajectory. Transitions of PSC composition classes were observed to be highly dependent on the temperature history. In cases of a gradual temperature decrease, nitric acid trihydrate (NAT) and super-cooled ternary solution (STS) mixture clouds were observed. In cases of rapid temperature decrease, STS clouds were first observed, followed by NAT/STS mixture clouds. When temperatures dropped below the frost point, ice clouds formed and then transformed into NAT/STS mixture clouds when temperature increased above the frost point. The threshold temperature for rapid chlorine activation on PSCs is approximately 4 K below the NAT existence temperature, TNAT. Furthermore, simulations of the ATLAS chemistry and transport box model along the trajectories were used to corroborate the measurements and show good agreement with the observations. Rapid chlorine activation was observed when an air mass encountered PSCs. Usually, chlorine activation was limited by the amount of available ClONO2. Where ClONO2 was not the limiting factor, a large dependence on temperature was evident.

Simultaneous occurrence of polar stratospheric clouds and upper-tropospheric clouds caused by blocking anticyclones in the Southern Hemisphere

Abstract. Previous studies have reported that polar stratospheric clouds (PSCs) are frequently observed simultaneously with upper-tropospheric clouds (UCs) in the Southern Hemisphere. However, it has not yet been examined whether the UCs that simultaneously occur with PSCs are actually located below the height of the tropopause, which is modified by tropospheric disturbances. Furthermore, the mechanism of this simultaneous occurrence has not yet been clarified. This study statistically examines the simultaneous appearance of PSCs and UCs using the Cloud-Aerosol Lidar and Pathfinder Satellite Observation (CALIPSO) for the five austral winters of 2007–2011. From correlation analyses and statistical dependence tests, it is shown that the simultaneous occurrence frequencies of clouds with an altitude range of 15–25 km and 9–11 km are significant. The analyses based on tropopause-relative altitude suggest that the occurrence frequency of clouds at altitudes higher than 6 km above the local tropopause (i.e., PSCs) is significantly correlated with that of clouds around and slightly above the tropopause. These results indicate that the UCs observed simultaneously with PSCs reported in previous case studies are likely located around and slightly above the tropopause rather than in the troposphere. It is also shown that the simultaneous occurrence of PSCs and UCs is frequently associated with blocking highs that have large horizontal scales (several thousand kilometers) and tall structure (up to a height of ~15 km). The longitudinal variation of blocking high frequency accords well with that of the simultaneous occurrence frequency of PSCs and UCs. This fact suggests that the blocking highs provide a preferable condition for such simultaneous occurrences. Moreover, the composition of PSCs is investigated as a function of relative longitude of the anticyclones including blocking highs. It was discovered that relatively high proportions of STS (super-cooled ternary solutions), Ice, and Mix2 (mixture of nitric acid trihydrate and STS) types are distributed towards the windward, near, and leeward side of anticyclones in westerly background flows, respectively.

The effect of orographic gravity waves on Antarctic polar stratospheric cloud occurrence and composition

[1] A seasonal analysis of the relationship between mesoscale orographic gravity wave activity and polar stratospheric cloud (PSC) composition occurrence around the whole of Antarctica is presented. Gravity wave variances are derived from temperature measurements made with the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) Global Positioning System Radio Occultation (GPS-RO) satellites. Data from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite are used to determine the PSC composition class distribution and spatial volume. The results show intermittent large wave activity above the Antarctic Peninsula which is coincident with large volumes of H2O ice PSCs. These ice PSC volumes advect downstream, where increases in nitric acid trihydrate (NAT) PSC volumes occur, supporting the mountain wave seeding hypothesis. During winter 2007 in the latitude range 60°S–70°S, near the edge of the vortex and where temperatures are close to PSC formation thresholds, 30% of all PSCs are attributable to orographic gravity waves. In the separate composition classes, around 50% of both H2O ice PSCs and a high NAT number density liquid–NAT mixture class of PSCs are due to these waves. While we show that planetary waves are the major determinant of PSC presence at temperatures close to the NAT formation threshold, we also demonstrate the important role of mesoscale, intermittent orographic gravity wave activity in accounting for the composition and distribution of PSCs around Antarctica.

CALIPSO polar stratospheric cloud observations: second-generation detection algorithm and composition discrimination

Abstract. This paper focuses on polar stratospheric cloud (PSC) measurements by the CALIOP (Cloud-Aerosol LIdar with Orthogonal Polarization) lidar system onboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) spacecraft, which has been operating since June 2006. We describe a second-generation PSC detection algorithm that utilizes both the CALIOP 532-nm scattering ratio (ratio of total-to-molecular backscatter coefficients) and 532-nm perpendicular backscatter coefficient measurements for cloud detection. The inclusion of the perpendicular backscatter measurements enhances the detection of tenuous PSC mixtures containing low number densities of solid (likely nitric acid trihydrate, NAT) particles and leads to about a 15% increase in PSC areal coverage compared with our original algorithm. Although these low number density NAT mixtures would have a minimal impact on chlorine activation due to their relatively small particle surface area, these particles may play a significant role in denitrification and therefore are an important component of our PSC detection. In addition, the new algorithm allows discrimination of PSCs by composition in terms of their ensemble backscatter and depolarization in a manner analogous to that used in previous ground-based and airborne lidar PSC studies. Based on theoretical optical calculations, we define four CALIPSO-based composition classes which we call supercooled ternary solution (STS), ice, and Mix1 and Mix2, denoting mixtures of STS with NAT particles in lower or higher number densities/volumes, respectively. We examine the evolution of PSCs for three Antarctic and two Arctic seasons and illustrate the unique attributes of the CALIPSO PSC database. These analyses show substantial interannual variability in PSC areal coverage and also the well-known contrast between the Antarctic and Arctic. The CALIPSO data also reveal seasonal and altitudinal variations in Antarctic PSC composition, which are related to changes in HNO3 and H2O observed by the Microwave Limb Sounder on the Aura satellite.

Comparison between CALIPSO and MIPAS observations of polar stratospheric clouds

Polar stratospheric cloud (PSC) detection, top height, and composition as derived from measurements by the infrared emission limb sounder Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat and the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar have been compared. The comparisons are based on coincident observations from the 2006 and 2007 Antarctic and the 2006/2007 and 2007/2008 Arctic winters. During the middle of the Antarctic season, very good agreement in common PSC detection (around 90%) has been found. At the beginning and end of the Antarctic PSC season and in the Arctic, the frequency of common PSC detection is generally less (60–70%) which can be explained by cloud inhomogeneity and viewing geometry differences. MIPAS PSC top heights are about 0–2 km lower than CALIPSO top heights with larger offsets at higher altitudes. The negative bias of MIPAS PSC top heights can be modeled under the assumptions of limited horizontal cloud extent and a field‐of‐view–dependent sensitivity. The comparisons further show a high degree of consistency between PSC composition derived from the fundamentally different classification approaches of the two instruments. Remaining differences can be explained considering the physical limitations of each approach and the definition of composition boundaries within the classification scheme of each instrument.

Aerosols and clouds in the upper troposphere–lower stratosphere region detected by GOMOS and ACE: Intercomparison and analysis of the years 2004 and 2005

Satellite-based limb occultation measurements are well suited for the detection and mapping of polar stratospheric clouds (PSCs) and cirrus clouds. PSCs are of fundamental importance for the formation of the Antarctic ozone hole that occurs every year since the early 1980s in Southern Hemisphere spring. Despite progress in the observation, modeling and understanding of PSCs in recent years, there are still important questions which remain to be resolved, e.g. PSC microphysics, composition, formation mechanisms and long-term changes in occurrence. In addition, it has recently become clear that cirrus clouds significantly affect the global energy balance and climate, due to their influence on atmospheric thermal structure. Since 2002, two major space missions using the occultation method have been put into orbit: the European stellar occultation spectrometer GOMOS on board ENVISAT and the Canadian solar occultation instruments ACE-FTS/MAESTRO on board SCISAT-I. PSCs and cirrus clouds are detected both by ACE and GOMOS. The results of an intercomparison between retrieved aerosol extinction, PSCs and cirrus clouds are the subject of this paper. The cloud data are also used to examine the evolution of PSCs over the Antarctic vortex and the latitudinal variation of tropical cirrus for the years 2004 and 2005.

Spectroscopic evidence for NAT, STS, and ice in MIPAS infrared limb emission measurements of polar stratospheric clouds

Abstract. We have analyzed mid-infrared limb-emission measurements of polar stratospheric clouds (PSCs) by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) during the Antarctic winter 2003 with respect to PSC composition. Coincident Lidar observations from McMurdo were used for comparison with PSC types 1a, 1b and 2. Application of new refractive index data of β-NAT have allowed to accurately simulate the prominent spectral band at 820 cm-1 observed by MIPAS at the location where the Lidar instrument observed type 1a PSCs. Broadband spectral fits covering the range from 780 to 960 cm-1 and from 1220 to 1490 cm-1 showed best agreement with the MIPAS measurements when spectroscopic data of NAT were used to simulate the MIPAS spectra. MIPAS measurements collocated with Lidar observations of Type 1b and Type 2 PSCs could only be reproduced by assuming a composition of supercooled ternary H2SO4/HNO3/H2O solution (STS) and of ice, respectively. Particle radius and number density profiles derived from MIPAS were generally consistent with the Lidar observations. Only in the case of ice clouds, PSC volumes are partly underestimated by MIPAS due to large cloud optical thickness in the limb-direction. A comparison of MIPAS cloud composition and Lidar PSC-type determination based on all available MIPAS-Lidar coincident measurements revealed good agreement between PSC-types 1a, 1b and 2, and NAT, STS and ice, respectively. We could not find spectroscopic evidence for the presence of nitric acid dihydrate (NAD) from MIPAS observations of PSCs over Antarctica in 2003.

Simultaneous stratospheric gas and aerosol retrievals from broadband infrared occultation measurements

The inversion method for simultaneous gas (O3, NO2, HNO3, N2O, CH4, H2O, CFC-11, CFC-12, N2O5, and ClONO2) and aerosol retrievals from broadband continuous IR spectra of occultation measurements is described. Both gas and aerosol physical modeling with consideration of the multicomponent character of aerosol and polar stratospheric clouds (PSCs) are used to minimize the difference between measured and modeled transmittance spectra under smoothness constraints imposed on particle-size distributions for each PSC component and positive constraints on all gas and aerosol parameters. The method is tested by numerical simulations in which synthetic occultation measurements inherent to the improved limb atmospheric spectrometer are used. The study reveals that the method has significant advantages over other approaches based on offset or gas-window-channel aerosol correction for accurate gas retrievals and provides additional information on the particle-size composition, volume density, and chemical component character of PSCs.

Polar stratospheric cloud observations by MIPAS on ENVISAT: detection method, validation and analysis of the northern hemisphere winter 2002/2003

Abstract. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on ENVISAT has made extensive measurements of polar stratospheric clouds (PSCs) in the northern hemisphere winter 2002/2003. A PSC detection method based on a ratio of radiances (the cloud index) has been implemented for MIPAS and is validated in this study with respect to ground-based lidar and space borne occultation measurements. A very good correspondence in PSC sighting and cloud altitude between MIPAS detections and those of other instruments is found for cloud index values of less than four. Comparisons with data from the Stratospheric Aerosol and Gas Experiment (SAGE) III are used to further show that the sensitivity of the MIPAS detection method for this threshold value of cloud index is approximately equivalent to an extinction limit of 10-3km-1 at 1022nm, a wavelength used by solar occultation experiments. The MIPAS cloud index data are subsequently used to examine, for the first time with any technique, the evolution of PSCs throughout the Arctic polar vortex up to a latitude close to 90° north on a near-daily basis. We find that the winter of 2002/2003 is characterised by three phases of very different PSC activity. First, an unusual, extremely cold phase in the first three weeks of December resulted in high PSC occurrence rates. This was followed by a second phase of only moderate PSC activity from 5-13 January, separated from the first phase by a minor warming event. Finally there was a third phase from February to the end of March where only sporadic and mostly weak PSC events took place. The composition of PSCs during the winter period has also been examined, exploiting in particular an infra-red spectral signature which is probably characteristic of NAT. The MIPAS observations show the presence of these particles on a number of occasions in December but very rarely in January. The PSC type differentiation from MIPAS indicates that future comparisons of PSC observations with microphysical and denitrification models might be revealing about aspects of solid particle existence and location.

Polar stratospheric clouds during SOLVE/THESEO: Comparison of lidar observations with in situ measurements

Polar stratospheric clouds (PSCs) were observed on several flights during the 1999/2000 SOLVE/THESEO mission. Here we present an analysis of PSC size distribution, composition, and particle phase based on near coincident measurements using the Multiangle Aerosol Spectrometer Probe (MASP) and Focused Cavity Aerosol Spectrometer (FCAS III) on board the ER‐2 as well as lidar instruments on board the DC‐8 and Falcon aircrafts. We calculate the aerosol backscatter ratios and aerosol depolarization ratios at infrared and visible wavelengths based on the particle size distributions measured by the MASP and FCAS III instruments. We then compare our calculations to observed lidar measurements taken from on board the DC‐8 and Falcon aircraft, which flew the same flight paths as the ER‐2 on 20 January and 3 February 2000, respectively. Our comparison shows that calculations based on the mixed clouds containing small (submicrometer) spherical particles and large (2.5–22 μm diameter) nonspherical particles seen by the FCAS III and MASP are consistent with lidar observations. Our analysis shows that the infrared aerosol depolarization ratio must be measured to detect and identify mixed PSC clouds and that relying only on measurements of depolarization in the visible causes incorrect identification of mixed clouds as solution droplets. This study suggests that mixed clouds of supercooled ternary solutions were observed in coexistence with larger solid nitric acid hydrate particles.

Analysis of ozone loss in the Arctic stratosphere during the late winter and spring of 1997 using the Chemical Species Mapping on Trajectories (CSMT) technique

A scheme to create synoptic maps of stratospheric minor species from asynoptic satellite measurements using a photochemical box model and trajectory analysis was developed and named Chemical Species Mapping on Trajectories (CSMT). CSMT combined with Improved Limb Atmospheric Spectrometer (ILAS) data were used to study the Arctic ozone loss mechanism in the late winter and early spring of 1997. Long‐ and short‐lived species in the stratosphere were successfully mapped by the CSMT initialized with ILAS‐observed ozone, nitric acid, and nitrous oxide. Comparisons of CSMT‐derived data and ozonesonde data, and/or other satellite data, validated the scheme. The comparisons showed the reliability of the scheme in mapping long‐ and short‐lived species. A chemical ozone loss amount was estimated using the chemical model; the maximum ozone loss rate was about 34 ppbv/day in late February. The integrated ozone loss from 13 January to 31 March was 41%, averaged over the entire polar vortex. The effects of differences in polar stratospheric cloud composition and a possible warm bias in the temperature data set were examined, and only minor differences were found in the ozone loss amount. The derived ozone loss rates and integrated ozone loss are consistent with results from other studies. The Arctic ozone loss in the late winter and early spring of 1997 depended significantly on latitude and showed complex features: ozone loss occurred mainly at lower latitudes until late February; the region of significant ozone loss shifted to higher latitudes in March. The CSMT scheme shows good potential for various applications including detailed analyses of chemical mechanisms in the atmosphere.