Liquid Polar Stratospheric Clouds Research Articles

Lagrangian analysis of microphysical and chemical processes in the Antarctic stratosphere: a case study

Abstract. We investigated chemical and microphysical processes in the late winter in the Antarctic lower stratosphere, after the first chlorine activation and initial ozone depletion. We focused on a time interval when both further chlorine activation and ozone loss, but also chlorine deactivation, occur. We performed a comprehensive Lagrangian analysis to simulate the evolution of an air mass along a 10-day trajectory, coupling a detailed microphysical box model to a chemistry model. Model results have been compared with in situ and remote sensing measurements of particles and ozone at the start and end points of the trajectory, and satellite measurements of key chemical species and clouds along it. Different model runs have been performed to understand the relative role of solid and liquid polar stratospheric cloud (PSC) particles for the heterogeneous chemistry, and for the denitrification caused by particle sedimentation. According to model results, under the conditions investigated, ozone depletion is not affected significantly by the presence of nitric acid trihydrate (NAT) particles, as the observed depletion rate can equally well be reproduced by heterogeneous chemistry on cold liquid aerosol, with a surface area density close to background values. Under the conditions investigated, the impact of denitrification is important for the abundances of chlorine reservoirs after PSC evaporation, thus stressing the need to use appropriate microphysical models in the simulation of chlorine deactivation. We found that the effect of particle sedimentation and denitrification on the amount of ozone depletion is rather small in the case investigated. In the first part of the analyzed period, when a PSC was present in the air mass, sedimentation led to a smaller available particle surface area and less chlorine activation, and thus less ozone depletion. After the PSC evaporation, in the last 3 days of the simulation, denitrification increases ozone loss by hampering chlorine deactivation.

A-train CALIOP and MLS observations of early winter Antarctic polar stratospheric clouds and nitric acid in 2008

Abstract. A-train Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and Microwave Limb Sounder (MLS) observations are used to investigate the development of polar stratospheric clouds (PSCs) and the gas-phase nitric acid distribution in the early 2008 Antarctic winter. Observational evidence of gravity-wave activity is provided by Atmospheric Infrared Sounder (AIRS) radiances and infrared spectroscopic detection of nitric acid trihydrate (NAT) in PSCs is obtained from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). Goddard Earth Observing System Data Assimilation System (GEOS-5 DAS) analyses are used to derive Lagrangian trajectories and to determine temperature-time histories of air parcels. We use CALIOP backscatter and depolarization measurements to classify PSCs and the MLS measurements to determine the corresponding gas-phase HNO3 as a function of temperature. For liquid PSCs the uptake of HNO3 follows the theoretical equilibrium curve for supercooled ternary solutions (STS), but at temperatures about 1 K lower as determined from GEOS-5. In the presence of solid phase PSCs, above the ice frost-point, the HNO3 depletion occurs over a wider range of temperatures (+2 to −7 K) distributed about the NAT equilibrium curve. Rapid gas-phase HNO3 depletion is first seen by MLS from from 23–25 May 2008, consisting of a decrease in the volume mixing ratio from 14 ppbv (parts per billion by volume) to 7 ppbv on the 46–32 hPa (hectopascal) pressure levels and accompanied by a 2–3 ppbv increase by renitrification at the 68 hPa pressure level. The observed region of depleted HNO3 is substantially smaller than the region bounded by the NAT existence temperature threshold. Temperature-time histories of air parcels demonstrate that the depletion is more clearly correlated with prior exposure to temperatures a few kelvin above the frost-point. From the combined data we infer the presence of large-size NAT particles with effective radii >5–7 μm and low NAT number densities <1 × 10−3 cm−3. This denitrification event is observed close to the pole in the Antarctic vortex before synoptic temperatures first fall below the ice frost point and before the widespread occurrence of large-scale NAT PSCs. An episode of mountain wave activity detected by AIRS on 28 May 2008 led to wave-ice formation in the rapid cooling phases over the Antarctic Peninsula and Ellsworth Mountains, seeding an outbreak of NAT PSCs that were detected by CALIOP and MIPAS. The NAT clouds formed at altitudes of 18–26 km in a polar freezing belt and appear to be composed of relatively small particles with estimated effective radii of around 1 μm and high NAT number densities >0.2 cm−3. This NAT outbreak is similar to an event previously reported from MIPAS observations in mid-June 2003.

Statistical estimation of stratospheric particle size distribution by combining optical modelling and lidar scattering measurements

Abstract. A method for estimating the stratospheric particle size distribution from multiwavelength lidar measurements is presented. It is based on matching measured and model-simulated backscatter coefficients. The lidar backscatter coefficients measured at the three commonly used wavelengths 355, 532 and 1064 nm are compared to a precomputed look-up table of model-calculated values. The optical model assumes that particles are spherical and that their size distribution is unimodal. This inverse problem is not trivial because the optical model is highly non-linear with a strong sensitivity to the size distribution parameters in some cases. The errors in the lidar backscatter coefficients are explicitly taken into account in the estimation. The method takes advantage of the statistical properties of the possible solution cluster to identify the most probable size distribution parameters. In order to discard model-simulated outliers resulting from the strong non-linearity of the model, solutions farther than one standard deviation of the median values of the solution cluster are filtered out, because the most probable solution is expected to be in the densest part of the cluster. Within the filtered solution cluster, the estimation algorithm minimizes a cost function of the misfit between measurements and model simulations. Two validation cases are presented on Polar Stratospheric Cloud (PSC) events detected above the ALOMAR observatory (69° N – Norway). A first validation is performed against optical particle counter measurements carried out in January 1996. In non-depolarizing regions of the cloud (i.e. spherical particles), the parameters of an unimodal size distribution and those of the optically dominant mode of a bimodal size distribution are quite successfully retrieved, especially for the median radius and the geometrical standard deviation. As expected, the algorithm performs poorly when solid particles drive the backscatter coefficient. A small bias is identified in modelling the refractive index when compared to previous works that inferred PSC type Ib refractive indices. The accuracy of the size distribution retrieval is improved when the refractive index is set to the value inferred in the reference paper. Our results are then compared to values retrieved with another similar method that does not account for the effect of the measurements errors and the non-linearity of the optical model on the likelihood of the solution. The case considered is a liquid PSC observed over northern Scandinavia on January 2005. An excellent agreement is found between the two methods when our algorithm is applied without any statistical filtering of the solution cluster. However, the solution for the geometrical standard deviation appears to be rather unlikely with a value close to unity (σ≈1.04). When our algorithm is applied with solution filtering, a more realistic value of the standard deviation (σ≈1.27) is found. This highlights the importance of taking into account the non linearity of the model together with the lidar errors, when estimating particle size distribution parameters from lidar measurements.

Climatology of Arctic polar stratospheric clouds as measured by lidar in Ny‐Ålesund, Spitsbergen (79°N, 12°E)

Polar stratospheric clouds (PSCs) are observed by lidar at the Koldewey Station in Ny‐Ålesund, Spitsbergen (79°N, 12°E). Here we present a PSC climatology for the period 1995/1996–2003/2004, when the Arctic stratosphere was not disturbed by the presence of volcanic aerosols. The study is restricted to only five of these winters, i.e., when the stratospheric temperatures over Spitsbergen were cold enough for PSC formation. The typical altitude range where PSCs are found is 20–24 km, with a downward shifting of the cloud altitude level occurring during the season. Liquid PSCs, the majority of the observations, are usually detected from mid‐January onward; nitric acid trihydrate (NAT) is more frequent in early winter. Furthermore, NAT is generally located at lower altitudes than the liquid clouds. In this data set we rarely observe NAT enhanced PSCs, and ice clouds are entirely absent. In addition to the traditional PSC classification scheme (types Ia/Ib and II) the detected clouds are distinguished according to their vertical thickness, and they are grouped in two categories (thick and thin PSCs), as in a previous study performed on the Antarctic McMurdo PSC data set. The analysis reveals that in Ny‐Ålesund the spatial‐temporal distribution of thick and thin PSCs corresponds to NAT and liquid cloud signatures, respectively. Comparison between the Ny‐Ålesund and the McMurdo PSC climatologies is also reported. In both records, thick and thin PSCs show similar altitude distribution, while discrepancies between the Arctic and Antarctic cases exist for the appearance of NAT and liquid PSCs during the polar winter.

Differences in Arctic and Antarctic PSC occurrence as observed by lidar in Ny-Ålesund (79° N, 12° E) and McMurdo (78° S, 167° E)

Abstract. The extent of springtime Arctic ozone loss does not reach Antarctic ``ozone hole'' dimensions because of the generally higher temperatures in the northern hemisphere vortex and consequent less polar stratospheric cloud (PSC) particle surface for heterogeneous chlorine activation. Yet, with increasing greenhouse gases stratospheric temperatures are expected to further decrease. To infer if present Antarctic PSC occurrence can be applied to predict future Arctic PSC occurrence, lidar observations from McMurdo station (78° S, 167° E) and NyÅlesund (79° N, 12° E) have been analysed for the 9 winters between 1995 (1995/1996) and 2003 (2003/2004). Although the statistics may not completely cover the overall hemispheric PSC occurrence, the observations are considered to represent the main synoptic cloud features as both stations are mostly situated in the centre or at the inner edge of the vortex. Since the focus is set on the occurrence frequency of solid and liquid particles, the analysis has been restricted to volcanic aerosol free conditions. In McMurdo, by far the largest part of PSC observations is associated with NAT PSCs. The observed persistent background of NAT particles and their potential ability to cause denoxification and irreversible denitrification is presumably more important to Antarctic ozone chemistry than the scarcely observed ice PSCs. Meanwhile in Ny-Ålesund, ice PSCs have never been observed, while solid NAT and liquid STS clouds both occur in large fraction. Although they are also found solely, the majority of observations reveals solid and liquid particle layers in the same profile. For the Ny-Ålesund measurements, the frequent occurrence of liquid PSC particles yields major significance in terms of ozone chemistry, as their chlorine activation rates are more efficient. The relationship between temperature, PSC formation, and denitrification is nonlinear and the McMurdo and Ny-Ålesund PSC observations imply that for predicted stratospheric cooling it is not possible to directly apply current Antarctic PSC occurrence to the Arctic stratosphere. Future Arctic PSC occurrence, and thus ozone loss, is likely to depend on the shape and barotropy of the vortex rather than on minimum temperature alone.

A condensed-mass advection based model for the simulation of liquid polar stratospheric clouds

Abstract. We present a condensed-mass advection based model (MADVEC) designed to simulate the condensation/evaporation of liquid polar stratospheric cloud (PSC) particles. A (Eulerian-in-radius) discretization scheme is used, making the model suitable for use in global or mesoscale chemistry and transport models (CTMs). The mass advection equations are solved using an adaption of the weighted average flux (WAF) scheme. We validate the numerical scheme using an analytical solution for multicomponent aerosols. The physics of the model are tested using a test case designed by Meilinger et al. (1995). The results from this test corroborate the composition gradients across the size distribution under rapid cooling conditions that were reported in earlier studies.

Lidar Observations of Polar Stratospheric Clouds over Ny-Aalesund in the Winters of 1994/95-1996/97: Impact of the Temperature and the Temperature History on the PSC Structure

Lidar observations of polar stratospheric clouds (PSCs) were made during three winter campaigns from 1994/95 to 1996/97 at Ny-Aalesund, Svalbard. PSCs composed mainly of solid particles were found at the beginning of most PSC events. They appeared when the temperature became lower than the assumed equilibrium temperature of nitric acid trihydrate (T NAT ) but sometimes appeared even at temperature a little higher than T NAT . As the stratospheric temperature became very low, close to the frost point of ice (T ice ), PSCs composed mainly of liquid particles appeared. The behaviors of liquid PSCs observed are consistent with the expected ones, which were estimated based on the current formation theory of super cooled ternary solution (STS) particles. Results of backward trajectory analysis show that some solid PSCs experienced the temperature below T NAT at least for more than 10 hours after passing through the melting point of sulfuric acid tetrahydrate. The solid PSCs which appeared at temperatures higher than T NAT experienced the temperature below T NAT for more than 1 day in the past. Almost no PSCs experienced the temperature lower than T ice during 10 days prior to the observation. Results of trajectory analysis also suggest that the increase of the depolarization ratio depends strongly on the degree to which air parcels of PSCs cooled below T NAT and on the period during which the air parcels experienced the temperature lower than the condensation point, but not on the experience of lower temperature than T ice . Some liquid PSCs with a large scattering ratio (4-5) and a low depolarization ratio (0-0.005) appeared after they experienced the temperature close to T ice for a few days prior to the observation.

Microphysical properties of wave polar stratospheric clouds retrieved from lidar measurements during SOLVE/THESEO 2000

Mountain wave induced ice clouds in the stratosphere are known to cause the formation of nitric acid hydrate particles downwind. Understanding the microphysical properties of these solid particles is important because they may contribute to a general background of solid polar stratospheric clouds (PSCs) that is observed but whose origin is not understood. Based on the limited set of observations of PSCs directly attributable to mountain waves, it has not been possible to determine their general microphysical properties. Here we analyze lidar observations from the SOLVE/THESEO 2000 campaign. Between December 1999 and March 2000, seven of the twelve flights of the DC‐8 aircraft showed clear signs of mountain wave induced clouds, with nitric acid hydrate clouds often extending many hundreds of kilometers downwind of mountains. On the basis of T‐matrix calculations, we have developed a technique to estimate the microphysical properties of spherical and nonspherical particles from multiwavelength backscatter and depolarization data from the Goddard Space Flight Center/Langley Research Center (GSFC/LaRC) aerosol lidar. The technique allows particle radius, condensed mass, and number densities of ice, nitric acid trihydrate, and liquid PSCs to be estimated. Ice clouds were found to contain approximately 1–3 ppmv of condensed water with a number density from 1 to 10 cm−3 and a narrow size distribution width with mode radii from 1 to 1.5 μm. Nitric acid hydrate particles downwind of the ice clouds were consistent with 1–5 ppbv condensed HNO3, a number density from 0.5 to 2 cm−3, and mode radius around 0.5 μm. These hydrate clouds are characterized by high aerosol backscatter and depolarization and are distinct from type 1a clouds that are observed away from mountains, which have low aerosol backscatter.

Characteristics of Arctic polar stratospheric clouds in the winter of 1996/1997 inferred from ILAS measurements

The Improved Limb Atmospheric Spectrometer (ILAS) captured many polar stratospheric cloud (PSC) events in the Northern Hemisphere during the winter and early spring of 1997. Simultaneous measurements of nitric acid and aerosols by ILAS made it possible to infer PSC composition. The aerosol extinction coefficient and nitric acid data were compared with the theoretically predicted values for supercooled ternary solution (STS), nitric acid dihydrate (NAD), and nitric acid trihydrate (NAT) at thermodynamic equilibrium to classify PSC types. The observations showed that in 1997, both nitric‐acid‐containing solid and liquid PSCs formed over the Arctic during winter and early spring, until mid‐March. The STS PSCs were observed early in the PSC season, in mid‐January. Most of the PSCs observed late in the PSC season had features of nitric‐acid‐containing hydrates. An intensive analysis of the temperature histories suggested that most of the STS events observed in January had experienced the thermal conditions necessary for the formation of liquid PSCs. The nitric‐acid‐containing hydrates observed in March seemed not to have been influenced by any mountain‐induced lee waves. The process of nitric‐acid‐containing hydrate formation based on synoptic scale temperature change is discussed.

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.

Optical classification, existence temperatures, and coexistence of different polar stratospheric cloud types

Multispectral lidar measurements of polar stratospheric clouds (PSCs) from two winter campaigns in 1994/1995 and 1996'1997 at Sodankylä, Finland, have been evaluated together with temperature data from local radiosondes to find optical parameters for a PSC classification of different particle types and their existence temperatures. Precise depolarization measurements show that both solid and liquid particles exist below the NAT (nitric acid trihydrate) temperature. A comparison of temperatures at the PSC base and at the cloud top shows a good agreement with the NAT‐existence temperature for solid type Ia clouds and a 3–4 K lower temperature for liquid type Ib clouds. The two particle families are therefore consistent with solid NAT particle formation and condensational growth of HNO3, H2O and H2SO4 liquid ternary solutions. The coexistence of solid and liquid particles has been observed by means of the temporal development of parallel and perpendicular polarized lidar signals. These time series of subsequent lidar measurements show stronger and faster fluctuations in the liquid particle mode compared to the solid particles and thus indicate a higher sensitivity toward temperature fluctuations for the liquid PSCs. While the optical properties of most observations are consistent with the definition of PSC type Ia (solid) and type Ib (liquid) clouds, a third type has been observed which does not fit into the current type Ia/Ib optical classification. This cloud type consists of solid particles but has a higher backscatter than type Ia PSC.

Polar stratospheric clouds observed by lidar over Spitsbergen in the winter of 1994/1995: Liquid particles and vertical “sandwich” structure

Polar stratospheric clouds (PSCs) were observed by lidar at Ny‐Ålesund, Spitsbergen, in December 1994 and January 1995. The backscattering coefficient at wavelengths of 1064 and 532 nm and the depolarization ratio at 532 nm of PSCs were measured by the lidar system. The stratospheric temperature was below the estimated frost point of nitric acid tri‐hydrate (NAT) in the winter of 1994/1995. PSCs were observed more frequently in this low‐temperature period than in previous winters since 1991. The characteristics of the PSCs were very variable but had a noticeable vertical “sandwich” structure in January in which a layer of liquid PSC particles at the altitude around 20 km existed between the two solid particle layers. The wavelength dependence of the backscattering shows that the size of both liquid and solid particles was larger than the average size of background stratospheric aerosols. Lidar observations of the liquid layer particles show characteristics in qualitative agreement with those expected from model PSC particles grown in ternary solutions of H2SO4, HNO3, and H2O with a temperature decrease. However, the observed backscattering coefficient and its wavelength dependence indicate that PSC particles require further growth than that predicted by the ternary solution model at temperature where most HNO3 molecules in the surrounding atmosphere are considered to be condensed on PSCs.