A comprehensive investigation of polar stratospheric clouds was performed on 25 January 2000 with instruments onboard a balloon gondola flown from Kiruna, Sweden. Cloud layers were repeatedly encountered at altitudes between 20 and 24 kilometers over a wide range of atmospheric temperatures (185 to 197 kelvin). Particle composition analysis showed that a large fraction of the cloud layers was composed of nitric acid trihydrate (NAT) particles, containing water and nitric acid at a molar ratio of 3:1; this confirmed that these long-sought solid crystals exist well above ice formation temperatures. The presence of NAT particles enhances the potential for chlorine activation with subsequent ozone destruction in polar regions, particularly in early and late winter.

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 H 2 SO 4 /HNO 3 /H 2 O 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.

A climatology of polar stratospheric clouds (PSCs) based on lidar measurements performed at McMurdo Station, Antarctica (78°S, 167°E) from 1993 to 2001 is presented here. The observations are discussed in terms of occurrence and temporal and spatial variability of PSC types. The climatological analysis reveals that in McMurdo PSCs mainly occur between July and mid‐August. During this time their altitude changes from 22 to 14 km, following the stratospheric temperature minimum trend. At the beginning of the accounted period (1993–1994), volcanic aerosols from the Mt. Pinatubo eruption in 1991 were still present in the southern polar vortex. Therefore these 2 years have been corrected for the direct contribution of the volcanic aerosol to the backscatter signal. A close examination of the data set evidences that most PSCs appear either as rather thin layers (<1 km) or as layers with a considerably higher thickness. Therefore all observed PSCs have been divided into two classes, depending on the variation of the backscattering ratio with respect to the altitude (e.g., small‐scale variations, or SSV, and large‐scale variations, or LSV). The seasonal behavior and the occurrence of PSC types under each class have been studied, keeping 1993 and 1994 separated to better highlighting the effect of the volcanic aerosol load on cloud properties. Finally, in order to shed some light on PSC formation, back‐trajectory analysis has been performed for retrieving thermal histories of opportunely selected PSCs.

Different definitions for estimating the degree of changes in signal polarization measured by lidar measurements are used both to detect the presence of nonspherical aerosol particles and to estimate their shape and density. Our aim is to provide a tool for calculation and interpretation of changes in polarization that are due to aerosol backscatter measured by the lidar technique. An overview of several techniques used to calculate linear depolarization from two-channel lidar measurements is given. Advantages and disadvantages of each method are analyzed when we apply them on a lidar vertical profile. Systematic errors are also discussed. First, an overview of different estimations of polarizability of atmospheric molecules is given. The presence of signal with orthogonal polarization in each channel (cross talk) is a source of error in depolarization estimation. It is calculated at various degrees of contamination, and the total uncertainty on depolarization definition is retrieved.

In situ measurements of ice crystal size distributions in tropical upper troposphere/lower stratosphere (UT/LS) clouds were performed during the SCOUT-AMMA campaign over West Africa in August 2006. The cloud properties were measured with a Forward Scattering Spectrometer Probe (FSSP-100) and a Cloud Imaging Probe (CIP) operated aboard the Russian high altitude research aircraft M-55 Geophysica with the mission base in Ouagadougou, Burkina Faso. A total of 117 ice particle size distributions were obtained from the measurements in the vicinity of Mesoscale Convective Systems (MCS). Two to four modal lognormal size distributions were fitted to the average size distributions for different potential temperature bins. The measurements showed proportionately more large ice particles compared to former measurements above maritime regions. With the help of trace gas measurements of NO, NO_y, CO₂, CO, and O₃ and satellite images, clouds in young and aged MCS outflow were identified. These events were observed at altitudes of 11.0 km to 14.2 km corresponding to potential temperature levels of 346 K to 356 K. In a young outflow from a developing MCS ice crystal number concentrations of up to (8.3 ± 1.6) cm^&minus;3 and rimed ice particles with maximum dimensions exceeding 1.5 mm were found. A maximum ice water content of 0.05 g m^&minus;3 was observed and an effective radius of about 90 &mu;m. In contrast the aged outflow events were more diluted and showed a maximum number concentration of 0.03 cm^&minus;3, an ice water content of 2.3 &times; 10^&minus;4 g m^&minus;3, an effective radius of about 18 &mu;m, while the largest particles had a maximum dimension of 61 &mu;m. <br><br> Close to the tropopause subvisual cirrus were encountered four times at altitudes of 15 km to 16.4 km. The mean ice particle number concentration of these encounters was 0.01 cm^&minus;3 with maximum particle sizes of 130 &mu;m, and the mean ice water content was about 1.4 &times; 10^&minus;4 g m^&minus;3. All known in situ measurements of subvisual tropopause cirrus are compared and an exponential fit on the size distributions is established for modelling purposes. <br><br> A comparison of aerosol to ice crystal number concentrations, in order to obtain an estimate on how many ice particles may result from activation of the present aerosol, yielded low ratios for the subvisual cirrus cases of roughly one cloud particle per 30 000 aerosol particles, while for the MCS outflow cases this resulted in a high ratio of one cloud particle per 300 aerosol particles