Backscatter Sonde Research Articles

Toward Rapid Balloon Experiments for Sudden Aerosol Injection in the Stratosphere (REAS) by Volcanic Eruptions and Wildfires

Abstract Stratospheric aerosols are greatly influenced by medium-to-large volcanic eruptions. Over the last few years, extreme wildfires have been identified as new sources of stratospheric particles, in the form of carbonaceous aerosols injected by pyrocumulonimbus (pyroCb) events in the upper troposphere and lower stratosphere, associated with significant impacts on climate and ozone chemistry. To assess the impact of wildfires and volcanic eruptions on stratospheric aerosol loadings in the Northern Hemisphere, the Rapid Balloon Experiments for Sudden Aerosol Injection in the Stratosphere (REAS) project has been initiated. REAS is an international initiative that aims to respond to sudden events impacting stratospheric aerosol composition. Seventeen balloons were launched from Reims, eastern France, between November 2021 and January 2022 to quantify the atmospheric content for both aerosols and trace/greenhouse gases from the ground up to stratospheric levels. The main measurements concerned trace gases (CO/CO2 as tracers of smoke) and aerosol together with ozone using instruments such as a gas collector, optical particle counters, backscatter sondes, an aerosol sampler, an aerosol impactor, and ozonesondes. The Groupe de Spectrométrie Moléculaire et Atmosphérique (GSMA) launch facility provided unique possibilities of combining multiple measurements in one flight thanks to medium flights (corresponding to a 6 kg payload). While no major event impacted the stratosphere during the campaign, we particularly discuss the influence of the aged volcanic plume from La Soufrière volcano (Saint Vincent island) and smoke particles from series of pyroCb events that took place in North America. The burden as well as the optical and microphysical properties of the observed aerosols are quantified from these in situ observations in association with various satellite data.

Synergy processing of diverse ground-based remote sensing and in situ data using the GRASP algorithm: applications to radiometer, lidar and radiosonde observations

Abstract. The exploration of aerosol retrieval synergies from diverse combinations of ground-based passive Sun-photometric measurements with collocated active lidar ground-based and radiosonde observations using versatile Generalized Retrieval of Atmosphere and Surface Properties (GRASP) algorithm is presented. Several potentially fruitful aspects of observation synergy were considered. First, a set of passive and active ground-based observations collected during both day- and nighttime was inverted simultaneously under the assumption of temporal continuity of aerosol properties. Such an approach explores the complementarity of the information in different observations and results in a robust and consistent processing of all observations. For example, the interpretation of the nighttime active observations usually suffers from the lack of information about aerosol particles sizes, shapes and complex refractive index. In the realized synergy retrievals, the information propagating from the nearby Sun-photometric observations provides sufficient constraints for reliable interpretation of both day- and nighttime lidar observations. Second, the synergetic processing of such complementary observations with enhanced information content allows for optimizing the aerosol model used in the retrieval. Specifically, the external mixture of several aerosol components with predetermined sizes, shapes and composition has been identified as an efficient approach for achieving reliable retrieval of aerosol properties in several situations. This approach allows for achieving consistent and accurate aerosol retrievals from processing stand-alone advanced lidar observations with reduced information content about aerosol columnar properties. Third, the potential of synergy processing of the ground-based Sun-photometric and lidar observations, with the in situ backscatter sonde measurements was explored using the data from KAUST.15 and KAUST.16 field campaigns held at King Abdullah University of Science and Technology (KAUST) in the August of 2015 and 2016. The inclusion of radiosonde data has been demonstrated to provide significant additional constraints to validate and improve the accuracy and scope of aerosol profiling. The results of all retrieval setups used for processing both synergy and stand-alone observation data sets are discussed and intercompared.

VolKilau: Volcano Rapid Response Balloon Campaign during the 2018 Kilauea Eruption

AbstractAfter nearly 35 years of stable activity, the Kilauea volcanic system in Hawaii went through sudden changes in May 2018 with the emergence of 20 volcanic fissures along the Lower Eastern Rift Zone (LERZ), destroying 700 homes in Leilani Estates and forcing more than 2,000 people to evacuate. Elevated volcanic emissions lasted for several months between May and September 2018, leading to low visibility and poor air quality in Hawaii and across the western Pacific. The NASA-funded VolKilau mission was rapidly mounted and conducted between 11 and 18 June 2018 to (i) profile volcanic emissions with SO2 and aerosol measurements, (ii) validate satellite observations, and (iii) increase readiness for the next large volcanic eruption. Through a series of balloon-borne measurements with tethered and free-released launches, we measured SO2 concentration, aerosol concentration, and optical properties 60–80 km downwind from the volcanic fissures using gas sensors, optical particle counters, backscatter sondes, and an aerosol impactor. While most of the measurements made during the Kilauea eruption were ground based, the VolKilau mission represented a unique opportunity to characterize plume properties, constrain emission profiles, study early chemistry involving the conversion of SO2 into sulfuric acid, and understand the influence of water clouds in the removal of SO2. This unprecedented combination of measurements has significantly improved our team’s ability to assess the atmospheric and human impacts of a major event such as this.

Far‐Ranging Impact of Mountain Waves Excited Over Greenland on Stratospheric Dehydration and Rehydration

AbstractIn situ observations of reduced stratospheric water vapor combined with those of ice particle formation are rarely conducted. On the one hand, they are essential to broaden our knowledge about the formation of polar stratospheric clouds (PSCs). On the other hand, the observed profiles allow the comparison with global circulation models. Here we report about a balloon‐borne observation above Sodankylä, Finland on 26 January 2005. The frostpoint hygrometer detected layers of reduced water vapor by up to 2 ppmv from 18.5 to 23 km. Beneath, a 1‐km‐deep layer of increased water vapor was identified. An aerosol backscatter sonde measured the presence of stratospheric ice clouds. According to meteorological analysis the PSCs were formed upstream above the east coast of Greenland due to mountain wave‐induced cooling. The inertia‐gravity waves generated a large and persistent stratospheric wake far downstream of Greenland and led to the observed dehydration. Comparing the most recent ERA5 data with operational analyses from 2005, we find an improved representation of mesoscale internal gravity waves, dehydration and PSC formation for this particular event.

Polarimetric Backscatter Sonde Observations of Southern Ocean Clouds and Aerosols

Balloon-borne polarimetric backscatter sonde (polarsonde) observations of aerosol and cloud during the approach of a cold front at Macquarie Island (54.499 S 158.937 E) are described. The polarsonde captures vertical profiles of cloud occurrence and phase. The cloud base and cloud top heights from the backscatter sonde compare favourably with observations made by a co-located cloud radar and ceilometer. An estimate of the total scatter probability from a liquid cloud layer at 1000 m height is used with a Monte Carlo model of the instrument to obtain cloud particle concentration, and this is compared to a measurement of cloud condensation nucleus concentration made at sea level. Backscatter from aerosol, as well as cloud, is significant. A high aerosol loading in part of the pre-frontal airmass is observed at altitudes up to 6 km. Below the melting level, the high cross-polarised return, relative to the co-polarised, indicates a substantial concentration of solid, non-spherical aerosol particles, which due to the high humidity cannot be sea salt or sulphate. A back trajectory analysis indicates that the observed aerosol includes continental dust.

Optical design of low-cost polarimetric back-scatter sondes.

The optical design of a simple low-cost polarimetric balloon-borne backscatter sonde for detecting cloud thermodynamic phase is described. Simulations are performed to determine the best choice of polarization as well as the layout of the detectors with respect to the emitter. The variety of scattering angles of the detected photons, due to finite size detectors, has at least as great an effect as multiple scattering in determining the ratio of cross-polarized to co-polarized signal. The problem with aligned flat-plate ice crystals giving similar depolarization to spherical scatterers, noted in the context of cloud lidar, also occurs here even with a cloud of randomly oriented large plates. Nevertheless, we find that configurations of the instrument that can distinguish liquid droplets from a wide variety of ice crystal habits, including plates, are possible.

In situ and space-based observations of the Kelud volcanic plume: The persistence of ash in the lower stratosphere.

Volcanic eruptions are important causes of natural variability in the climate system at all time scales. Assessments of the climate impact of volcanic eruptions by climate models almost universally assume that sulfate aerosol is the only radiatively active volcanic material. We report satellite observations from the Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) on board the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite after the eruption of Mount Kelud (Indonesia) on 13 February 2014 of volcanic materials in the lower stratosphere. Using these observations along with in situ measurements with the Compact Optical Backscatter AerosoL Detector (COBALD) backscatter sondes and optical particle counters (OPCs) made during a balloon field campaign in northern Australia, we find that fine ash particles with a radius below 0.3 µm likely represented between 20 and 28% of the total volcanic cloud aerosol optical depth 3 months after the eruption. A separation of 1.5–2 km between the ash and sulfate plumes is observed in the CALIOP extinction profiles as well as in the aerosol number concentration measurements of the OPC after 3 months. The settling velocity of fine ash with a radius of 0.3 µm in the tropical lower stratosphere is reduced by 50% due to the upward motion of the Brewer‐Dobson circulation resulting a doubling of its lifetime. Three months after the eruption, we find a mean tropical clear‐sky radiative forcing at the top of the atmosphere from the Kelud plume near −0.08 W/m2 after including the presence of ash; a value ~20% higher than if sulfate alone is considered. Thus, surface cooling following volcanic eruptions could be affected by the persistence of ash and should be considered in climate simulations.

Tropical deep convective life cycle: Cb-anvil cloud microphysics from high-altitude aircraft observations

Abstract. The case study presented here focuses on the life cycle of clouds in the anvil region of a tropical deep convective system. During the SCOUT-O3 campaign from Darwin, Northern Australia, the Hector storm system has been probed by the Geophysica high-altitude aircraft. Clouds were observed by in situ particle probes, a backscatter sonde, and a miniature lidar. Additionally, aerosol number concentrations have been measured. On 30 November 2005 a double flight took place and Hector was probed throughout its life cycle in its developing, mature, and dissipating stage. The two flights were four hours apart and focused on the anvil region of Hector in altitudes between 10.5 and 18.8 km (i.e. above 350 K potential temperature). Trajectory calculations, satellite imagery, and ozone measurements have been used to ensure that the same cloud air masses have been probed in both flights. The size distributions derived from the measurements show a change not only with increasing altitude but also with the evolution of Hector. Clearly different cloud to aerosol particle ratios as well as varying ice crystal morphology have been found for the different development stages of Hector, indicating different freezing mechanisms. The development phase exhibits the smallest ice particles (up to 300 μm) with a rather uniform morphology. This is indicative for rapid glaciation during Hector's development. Sizes of ice crystals are largest in the mature stage (larger than 1.6 mm) and even exceed those of some continental tropical deep convective clouds, also in their number concentrations. The backscatter properties and particle images show a change in ice crystal shape from the developing phase to rimed and aggregated particles in the mature and dissipating stages; the specific shape of particles in the developing phase cannot be distinguished from the measurements. Although optically thin, the clouds in the dissipating stage have a large vertical extent (roughly 6 km) and persist for at least 6 h. Thus, the anvils of these high-reaching deep convective clouds have a high potential for affecting the tropical tropopause layer by modifying the humidity and radiative budget, as well as for providing favourable conditions for subvisible cirrus formation. The involved processes may also influence the amount of water vapour that ultimately reaches the stratosphere in the tropics.

Microphysical properties of synoptic-scale polar stratospheric clouds: in situ measurements of unexpectedly large HNO<sub>3</sub>-containing particles in the Arctic vortex

Abstract. In January 2010 and December 2011, synoptic-scale polar stratospheric cloud (PSC) fields were probed during seven flights of the high-altitude research aircraft M-55 Geophysica within the RECONCILE (Reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and its climate interaction) and the ESSenCe (ESSenCe: ESA Sounder Campaign) projects. Particle size distributions in a diameter range between 0.46 and 40μm were recorded by four different optical in situ instruments. Three of these particle instruments are based on the detection of forward-scattered light by single particles. The fourth instrument is a grayscale optical array imaging probe. Optical particle diameters of up to 35μm were detected with particle number densities and total particle volumes exceeding previous Arctic measurements. Also, gas-phase and particle-bound NOy was measured, as well as water vapor concentrations. The optical characteristics of the clouds were measured by the remote sensing lidar MAL (Miniature Aerosol Lidar) and by the in situ backscatter sonde MAS (Multiwavelength Aerosol Scatterometer), showing the synoptic scale of the encountered PSCs. The particle mode below 2μm in size diameter has been identified as supercooled ternary solution (STS) droplets. The PSC particles in the size range above 2μm in diameter are considered to consist of nitric acid hydrates, and the particles' high HNO3 content was confirmed by the NOy instrument. Assuming a particle composition of nitric acid trihydrate (NAT), the optically measured size distributions result in particle-phase HNO3 mixing ratios exceeding available stratospheric values. Therefore the measurement uncertainties concerning probable overestimations of measured particle sizes and volumes are discussed in detail. We hypothesize that either a strong asphericity or an alternate particle composition (e.g., water ice coated with NAT) could explain our observations. In particular, with respect to the denitrification by sedimentation of large HNO3-containing particles, generally considered to be NAT, our new measurements raise questions concerning composition, shape and nucleation pathways. Answering these would improve the numerical simulation of PSC microphysical processes like cloud particle formation, growth and denitrification, which is necessary for better predictions of future polar ozone losses, especially under changing global climate conditions. Generally, it seems that the occurrence of large NAT particles – sometimes termed "NAT rocks" – are a regular feature of synoptic-scale PSCs in the Arctic.

Arctic stratospheric dehydration – Part 1: Unprecedented observation of vertical redistribution of water

Abstract. We present high-resolution measurements of water vapour, aerosols and clouds in the Arctic stratosphere in January and February 2010 carried out by in situ instrumentation on balloon sondes and high-altitude aircraft combined with satellite observations. The measurements provide unparalleled evidence of dehydration and rehydration due to gravitational settling of ice particles. An extreme cooling of the Arctic stratospheric vortex during the second half of January 2010 resulted in a rare synoptic-scale outbreak of ice polar stratospheric clouds (PSCs) remotely detected by the lidar aboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) satellite. The widespread occurrence of ice clouds was followed by sedimentation and consequent sublimation of ice particles, leading to vertical redistribution of water inside the vortex. A sequence of balloon and aircraft soundings with chilled mirror and Lyman- α hygrometers (Cryogenic Frostpoint Hygrometer, CFH; Fast In Situ Stratospheric Hygrometer, FISH; Fluorescent Airborne Stratospheric Hygrometer, FLASH) and backscatter sondes (Compact Optical Backscatter Aerosol Detector, COBALD) conducted in January 2010 within the LAPBIAT (Lapland Atmosphere-Biosphere Facility) and RECONCILE (Reconciliation of Essential Process Parameters for an Enhanced Predictability of Arctic Stratospheric Ozone Loss and its Climate Interactions) campaigns captured various phases of this phenomenon: ice formation, irreversible dehydration and rehydration. Consistent observations of water vapour by these independent measurement techniques show clear signatures of irreversible dehydration of the vortex air by up to 1.6 ppmv in the 20–24 km altitude range and rehydration by up to 0.9 ppmv in a 1 km thick layer below. Comparison with space-borne Aura MLS (Microwave Limb Sounder) water vapour observations allow the spatiotemporal evolution of dehydrated air masses within the Arctic vortex to be derived and upscaled.

Particle backscatter and relative humidity measured across cirrus clouds and comparison with microphysical cirrus modelling

Abstract. Advanced measurement and modelling techniques are employed to estimate the partitioning of atmospheric water between the gas phase and the condensed phase in and around cirrus clouds, and thus to identify in-cloud and out-of-cloud supersaturations with respect to ice. In November 2008 the newly developed balloon-borne backscatter sonde COBALD (Compact Optical Backscatter and AerosoL Detector) was flown 14 times together with a CFH (Cryogenic Frost point Hygrometer) from Lindenberg, Germany (52° N, 14° E). The case discussed here in detail shows two cirrus layers with in-cloud relative humidities with respect to ice between 50% and 130%. Global operational analysis data of ECMWF (roughly 1° × 1° horizontal and 1 km vertical resolution, 6-hourly stored fields) fail to represent ice water contents and relative humidities. Conversely, regional COSMO-7 forecasts (6.6 km × 6.6 km, 5-min stored fields) capture the measured humidities and cloud positions remarkably well. The main difference between ECMWF and COSMO data is the resolution of small-scale vertical features responsible for cirrus formation. Nevertheless, ice water contents in COSMO-7 are still off by factors 2–10, likely reflecting limitations in COSMO's ice phase bulk scheme. Significant improvements can be achieved by comprehensive size-resolved microphysical and optical modelling along backward trajectories based on COSMO-7 wind and temperature fields, which allow accurate computation of humidities, homogeneous ice nucleation, resulting ice particle size distributions and backscatter ratios at the COBALD wavelengths. However, only by superimposing small-scale temperature fluctuations, which remain unresolved by the numerical weather prediction models, can we obtain a satisfying agreement with the observations and reconcile the measured in-cloud non-equilibrium humidities with conventional ice cloud microphysics. Conversely, the model-data comparison provides no evidence that additional changes to ice-cloud microphysics – such as heterogeneous nucleation or changing the water vapour accommodation coefficient on ice – are required.

Polar stratospheric clouds observed during the Airborne Polar Experiment–Geophysica Aircraft in Antarctica (APE‐GAIA) campaign

In this work, observations of polar stratospheric clouds (PSC) carried out during one flight of the stratospheric research aircraft Geophysica deployed in the Airborne Polar Experiment–Geophysica Aircraft in Antarctica (APE‐GAIA) campaign in September–October 1999 are presented. An analysis of data from the two lidars and a backscatter sonde on board the Geophysica aircraft is presented, coupled with temperature measurements. The observations are analyzed with the aid of a mesoscale model to provide air mass thermal histories and a microphysical box model to simulate PSC formation. The results obtained from this comparison are discussed in view of the theories for PSC formation processes.

Chemical, microphysical, and optical properties of polar stratospheric clouds

A balloonborne gondola for a comprehensive study of polar stratospheric clouds (PSCs) was launched on 25 January 2000 near Kiruna/Sweden. Besides an aerosol composition mass spectrometer, the gondola carried optical particle counters, two backscatter sondes, a hygrometer, and several temperature and pressure sensors. A mountain wave induced PSC was sampled between 20 and 23 km altitude. Strongly correlated PSC particle properties were detected with the different instruments. A large variability of particle types was measured in numerous PSC layers, and PSC development was followed for about two hours. Liquid ternary PSC layers were found at temperatures near the ice frost point. A large fraction of the sampled cloud layers consisted of nitric acid trihydrate (NAT) particles with a molar ratio H2O:HNO3 close to 3 at temperatures near and below the equilibrium temperature TNAT. The median radius of the NAT particle size distribution was between 0.5 and 0.75 μm at concentrations around 0.5 cm−3. Below the NAT layers and above TNAT, thin cloud layers containing a few large particles with radii up to 3.5 μm coexisted with smaller solid or liquid particles. The molar ratio in this region was found to be close to two.

Observations of vertically thick polar stratospheric clouds and record low temperature in the Arctic Vortex

On 12 and 13 January 2001 backscatter sondes launched at Sodankylä, Finland (67°N, 27°E) detected an extraordinarily thick polar stratospheric cloud layer of more than 8 km vertical extent. On these days the polar vortex passed over northern Scandinavia. This provided synoptic‐scale low stratospheric temperatures leading to the formation of both liquid and solid phase particles. Two days later, on 15 January 2001, a regular radiosonde measured record low temperature of 176.7 K at an altitude of 25.2 km at the vortex edge. High vertical resolution radiosonde profiles and meteorological analyses indicate strong mountain wave activity on this day. This provides further evidence that the coldest temperatures in the Arctic lower stratosphere occur as a consequence of mountain wave cooling under cold synoptic‐scale background conditions.

Evolution of the Arctic stratospheric aerosol mixing ratio measured with balloon‐borne aerosol backscatter sondes for years 1988–2000

Balloon‐borne aerosol backscatter measurements were made at 12 Arctic stations as part of a polar stratospheric cloud study. The record starts in 1988, which is well before the eruption of Mount Pinatubo in the beginning of June 1991, and continues to 2000. These measurements provide absolutely calibrated in situ detection of atmospheric aerosols with simultaneous measurements of pressure, temperature, relative humidity, and O3 partial pressure. The instrument is also capable of operating during cloudy conditions, which may be considered as an advantage compared with lidar measurements. Even though backscatter soundings represent the state of the atmosphere at the sounding time and site, we demonstrate here that with a limited, homogeneous set of measurements it is possible to effectively study the time development of atmospheric aerosol loading. The initial aim of the study has been to define the general features of aerosol distribution in the Arctic winter troposphere and stratosphere and then to document the perturbation in the lower stratospheric aerosols caused by the eruption of Mount Pinatubo and, in addition, to infer the background state of lower stratospheric aerosol loading during the pre‐ and post‐Pinatubo conditions. Our measurements suggest that the e‐folding time for the decaying volcanic aerosol intrusion was ∼0.7 year and the full recovery of the Arctic lower stratosphere from the Mount Pinatubo perturbation was roughly 5 years.

A Lidar and Backscatter Sonde Measurement Campaign at Table Mountain during February–March 1997: Observations of Cirrus Clouds

Results from a measurement campaign performed at Table Mountain Facility/Jet Propulsion Laboratory/ California Institute of Technology (34.388N, 117.688W, 2280 m ASL) are presented. Between 19 February and 18 March 1997 more than 400 h worth of lidar data were acquired and four backscatter sondes were launched. About 50% of the observations show the presence of cirrus clouds at altitudes close to and below the tropopause. Of these clouds 80% are characterized as subvisual with optical depths below 0.03 at a wavelength of 532 nm. Simple geometrical considerations lead to cloud spatial scales of 0.31 km vertically and 7.5 km horizontally, respectively. Deviations from color ratio values derived on the basis of geometrical optics are interpreted as small particle signatures. Comparing backscatter ratio profiles observed concurrently by three aerosol lidars, mean deviations of about 10% are found.

Comparison of chemical and optical in situ measurements of polar stratospheric cloud particles

The chemical composition of polar stratospheric cloud (PSC) particles has been measured in a balloon‐borne mass spectrometer experiment [Schreiner et al., 1999]. The cloud particles have simultaneously been observed by a backscatter sonde flown on the same gondola. Taken separately, the two data sets indicate that the PSC particles are composed of liquid supercooled ternary solutions (STS). The chemical analysis shows that the particles contain relatively low concentrations of nitric acid, indicating a not fully developed type 1b PSC. Assuming the particles to be composed of STS, it appears that the calculated particle volumes are too small to generate the large observed backscatter ratios. Possible explanations for the apparent discrepancy are discussed, based on STS equilibrium model and microphysical/optical model calculations and including a meteorological mesoscale analysis of the mountain lee wave situation during the flight. Assuming a relatively large value of the real part refractive index of STS particles around 1.50 at 940 nm wavelength would apparently bring the two data sets in agreement. However, this large value seems to be in conflict with previous laboratory measurements and theoretical predictions, although in better agreement with recent balloon‐borne observations. The limited observational data set does not allow us to assess if the particles could be composed of a metastable dilute solid solution or a higher nitric acid hydrate.

Chemical analysis of polar stratospheric cloud particles

A balloon-borne gondola carrying a particle analysis system, a backscatter sonde, and pressure and temperature sensors was launched from Kiruna, Sweden, on 25 January 1998. Measurements within polar stratospheric cloud layers inside the Arctic polar vortex show a close correlation between large backscatter ratios and enhanced particle-related water and nitric acid signals at low temperatures. Periodic structures in the data indicate the presence of lee waves. The H2O/HNO3 molar ratios are consistently found to be above 10 at atmospheric temperatures between 189 and 192 kelvin. Such high ratios indicate ternary solution particles of H2O, HNO3, and H2SO4 rather than the presence of solid hydrates.

Temperature histories in liquid and solid polar stratospheric cloud formation

Polar stratospheric clouds (PSCs) have been observed by balloonborne backscatter sondes from Alert, Thule, Heiss Island, Scoresbysund, Sodankylä, Søndre Strømfjord, and Ny Ålesund during winters 1989, 1990, 1995, and 1996 in 30 flights. The observations can be categorized into two main groups: type 1a and type 1b PSC particles. Type 1b PSCs show the characteristics expected from liquid ternary solution (HNO3/H2SO4/H2O) particles, consistent with model simulations. Type 1a PSCs are observed at all temperatures below the condensation temperature TNAT of nitric acid trihydrate (NAT), consistent with solid NAT composition. Air parcel trajectories have been calculated for all observations to provide synoptic temperature histories of the observed particles. A number of cases have been identified, where the particles have experienced temperatures close to or above the sulfuric acid tetrahydrate melting temperatures within 20 days prior to observation. This assures a knowledge of the physical phase (liquid) of the particles at this time, prior to observation. The subsequent synoptic temperature histories, between melting and the time of observation, show pronounced differences for type 1a and type 1b PSC particles, indicating the qualitative temperature conditions, necessary to generate solid type 1a PSCs. The temperature histories of type 1b particles show smoothly, in most cases monotonie, decreasing temperatures. The temperature can apparently decrease to the frost point without causing the particles to freeze. The type 1b PSC particles are mostly observed shortly after entering a cold region. The observed type 1a particles have spent several days at temperatures close to or below TNAT prior to observation, often associated with several synoptic temperature oscillations around TNAT, and the particles are observed in aged clouds. It appears that the PSC particles may freeze, if they experience synoptic temperatures below TNAT with a duration of at least 1 day, possibly accompanied by several temperature oscillations. However, liquid particles that experience a smooth cooling, even to very low temperatures, or single smooth cooling/heating below TNAT without synoptic temperature fluctuations do not seem to freeze.

Dehydration and sedimentation of ice particles in the Arctic stratospheric vortex

Balloon borne frost‐point hygrometers and backscatter sondes were launched at Sodankylä, Finland in January and February of 1996. These instruments measure water vapor and the backscatter ratio of light due to polar stratospheric clouds in the Arctic stratospheric vortex. Here we report the results of a hygrometer sonde and a backscatter sonde launched within 3.5 hours of each other on January 22/23. Together these soundings show a strong loss of water vapor due to the formation of ice clouds as a result of record cold temperatures in the Arctic stratosphere. The separation of the upper edge of the layer showing water vapor loss and the upper edge of the PSC layer indicates sedimentation of the ice particle layer, possibly leading to a permanent dehydration in the upper part of the layer exhibiting water vapor loss.