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Polar Ozone and Polar Stratospheric Clouds

The session will cover all aspects of polar stratospheric ozone, other species in the polar regions as well as all aspects of polar stratospheric clouds. Special emphasis is given to results from recent polar campaigns, including observational and model studies.

We encourage contributions on chemistry, microphysics, radiation, dynamics, small and large scale transport phenomena, mesoscale processes and polar-midlatitudinal exchange. In particular, we encourage contributions on ClOx/BrOx chemistry, chlorine activation, NAT nucleation mechanisms and on transport and mixing of processed air to lower latitudes.

We welcome contributions on polar aspects of ozone/climate interactions, including empirical analyses and coupled chemistry/climate model results and coupling between tropospheric climate patterns and high latitude ozone as well as representation of the polar vortex and polar stratospheric ozone loss in global climate models.

We particularly encourage contributions from the polar airborne field campaigns as e.g. the POLSTRACC (Polar Stratosphere in a Changing Climate) and SouthTRAC (Southern Hemisphere - Transport Composition Dynamics) campaign as well as related activities, which aim at providing new scientific knowledge on the Arctic/Antarctic lowermost stratosphere and upper troposphere in a changing climate. Contributions from WMO's Global Atmosphere Watch (GAW) Programme and from the Network for the Detection of Atmospheric Composition Change (NDACC) are also encouraged.

Convener: Farahnaz Khosrawi | Co-conveners: Hideaki Nakajima, Michael Pitts, Ines TritscherECSECS
| Mon, 23 May, 10:20–11:50 (CEST)
Room 0.31/32

Mon, 23 May, 10:20–11:50

Chairpersons: Ines Tritscher, Michael Pitts, Farahnaz Khosrawi


Andrea Pazmino et al.

The amplitude and rate of ozone depletion in the Arctic is monitored every year since 1994 by comparison between SAOZ UV-Vis ground-based network from NDACC and Multi-Sensor Reanalysis 2 (MSR-2) total ozone measurements over 8 stations in the Arctic and 3-D chemical transport model simulations in which ozone is considered as a passive tracer. The passive ozone method allows determining the cumulative loss at the end of the winter. The amplitude of the destruction varies between 0-10% in relatively warm and short vortex duration years to 25-38% in colder and longer ones, which the record winters estimated in 2010/2011 and 2019/2020.

In this study, the interannual variability of 10-days average rate of 2021/2022 winter will be analyzed and compared to previous years. In addition, SAOZ NO2 data will be used to evaluate re-noxification in the Arctic. The long-term ozone loss series estimated from measurements will be compared to REPROBUS and SLIMCAT CTM simulations. Relationship with illuminated Polar Stratospheric Clouds will be also presented.

How to cite: Pazmino, A., Goutail, F., Pommereau, J.-P., Lefèvre, F., Godin-Beekmann, S., Hauchecorne, A., Lecouffe, A., Chipperfield, M., Feng, W., Van Roozendael, M., Jepsen, N., Hansen, G., Kivi, R., Alwarda, R., Strong, K., and Walker, K.: Total ozone loss during the 2021/22 Arctic winter and comparison to previous years, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7991, https://doi.org/10.5194/egusphere-egu22-7991, 2022.

Leonie Bernet et al.

Even though ozone-depleting substances have been substantially reduced due to the Montreal Protocol, it is still not possible to state with confidence that the total column amount of ozone (total ozone) recovers globally. A special focus lies on high latitudes, as they experienced strong stratospheric ozone depletion in the 1980s and 1990s. Especially at northern high latitudes, it is still challenging to detect significant total ozone trends. It is therefore important to use carefully homogenized and stable long-term ozone measurements and advanced trend models to derive ozone trends at northern high latitudes.

This study uses ground-based total ozone measurements in Norway and the Arctic to investigate total ozone trends at northern high latitudes. We present combined total ozone time series from Brewer Spectrophotometers at Oslo (60°N) and Andøya (69°N) in Norway, from 2000 to 2020. In addition, measurements from a SAOZ instrument and a Brewer at Ny-Ålesund in Svalbard are used. The combined Brewer time series consist of direct sun (DS) and global irradiance (GI) Brewer measurements and are complemented with measurements from ground-based ultraviolet radiometers (GUV). This makes it possible to obtain measurements during cloudy conditions and in winter and spring, where DS measurements cannot be retrieved due to large solar zenith angles and reduced direct sunlight.

We present total ozone trends at the three measurement stations using the LOTUS (Long-term Ozone Trends and Uncertainties in the Stratosphere) multilinear regression model. We test various explanatory variables and select a set of predictors to obtain the best possible regression fit. We found that besides the commonly used predictors QBO, ENSO, and solar cycle, tropopause pressure and stratospheric temperature are also important to improve the fit. We finally present annual total ozone trends and trends for different months at each station. Despite that the annual trends were generally found to be insignificant, we detected significant trends in some months.

We believe that our study contributes to a better understanding of long-term ozone changes at northern high latitudes, which is essential to assess how Arctic ozone responds to changes in ozone depleting substances and to climate change.

How to cite: Bernet, L., Svendby, T., Hansen, G., Orsolini, Y., Dahlback, A., Goutail, F., Pazmiño, A., and Petkov, B.: Total ozone trends and variability at three northern high-latitude stations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3708, https://doi.org/10.5194/egusphere-egu22-3708, 2022.

Peter Krizan

The aim of this presentation is to search for the occurrence of discontinuities in the total ozone data from the ERA-5 reanalyse, with the help of the Pettitt, Buishand and the Standard homogeneity tests.   This occurrence is important for trend analyses, because the presence of discontinuities influences the values of trends and their significance. Discontinuities arise from the changing in the assimilation procedure, introducing new observation to the reanalyse, and changing of data quality. We search for their spatial, temporal and geographical occurrence. There are dates which the occurrence of discontinuities is expected in: 2004- transition from SBUV to EOS Aura data and 2015-  the 4.2 MLS data were started to use instead of version 2.2. We search for discontinuities in the following classes of extremity: 1st, 10th, 25th, 50th,75th,90th and 99th percentile as well as the mean. Generally speaking, the discontinuities are occurred approximately from 30 to 60 % of all grid cells.  The results are slightly test dependent and the Pettitt test is not able to detect the discontinuities in 2015.The best performance in discontinuity detection in this year was obtained for the Standard homogeneity test. Ozone data with high occurrence of the discontinuities is not suitable for trend analyses.   

How to cite: Krizan, P.: Occurrence of discontinuities in the total ozone from ERA-5 reanalyse, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1460, https://doi.org/10.5194/egusphere-egu22-1460, 2022.

Janis Pukite et al.

Chlorine dioxide (OClO) is a by-product of the ozone depleting halogen chemistry in the stratosphere. Although being rapidly photolysed at low solar zenith angles (SZAs) it plays an important role as an indicator of the chlorine activation in polar regions during polar winter and spring at twilight conditions because of the nearly linear dependence of its formation on chlorine oxide (ClO).

The TROPOspheric Monitoring Instrument (TROPOMI) is an UV-VIS-NIR-SWIR instrument on board the Sentinel-5P satellite developed for monitoring the composition of the Earth’s atmosphere. It was launched on 13 October 2017 in a near polar orbit. It measures spectrally resolved earthshine radiances at an unprecedented spatial resolution of around 3.5x7.2 km2 (3.5x5.6 km2 starting from 6 Aug 2019) (near nadir) with a total swath width of ~2600 km on the Earth's surface providing daily global coverage and even higher temporal coverage in polar regions. From the measured spectra high resolved trace gas distributions can be retrieved by means of differential optical absorption spectroscopy (DOAS).

We compare slant column densities (SCDs) of chlorine dioxide (OClO) obtained from TROPOMI measurements with meteorological data and CALIPSO Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) polar stratospheric cloud (PSC) observations for both Antarctic and Arctic regions for the time period since TROPOMI launch in 2017 till now.

How to cite: Pukite, J., Borger, C., Dörner, S., Gu, M., and Wagner, T.: OClO as continuously observed by TROPOMI on Sentinel 5P, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9359, https://doi.org/10.5194/egusphere-egu22-9359, 2022.

Alexander James et al.

Nitric Acid Trihydrate (NAT) crystal formation in the absence of water ice is important for a subset of Polar Stratospheric Clouds (PSCs) and thereby Ozone. However, nucleation of these crystals is not understood.

It has been suggested previously that either fragmented meteoroids or meteoric smoke particles (MSPs), or possibly both, are important as heterogeneous nuclei. The role of H2SO4, which is present in liquid PSCs, in these nucleation processes has not been investigated. It is known that metal-containing Meteoric Smoke Particles (MSPs) are processed, partially dissolving whilst some components re-precipitate within H2SO4 droplets, producing silica and alumina particles which differ in size from the original MSPs. We recently found that analogues for nanoparticulate MSPs have a low ability to nucleate NAT relative to larger particles of similar material, suggesting that the size of particles may be a critical parameter for the nucleating ability of silica particles.  We previously showed experimentally that nano-particulate fumed silica is a poor promoter of nucleation, whilst micron scale fused quartz was found to be effective. Both materials have similar chemical and structural (crystallographically amorphous) properties.

In this study we developed a model using Classical Nucleation Theory (CNT) where we account for surface curvature of primary grains. This model is able to account for the discrepancy in nucleation effectiveness of fumed silica and fused quartz, by treating them as having the same nucleating ability (contact angle) but differing particle size (or equivalently surface curvature), assuming interfacial energies which are physically reasonable given literature measurements. We also performed new experiments which allowed us to refine our understanding of the H2SO4 sensitivity of NAT nucleation by meteoric fragments. Combining sedimentation modelling with our results and recent experiments on fragmentation of incoming meteoroids suggests that fragments are unlikely to be important as heterogeneous nuclei. However, the CNT model developed here provides evidence that nucleation of NAT on (10s nm) MSP analogues is effective enough to explain observed NAT crystal number concentrations in PSCs (without ice).

How to cite: James, A., Pace, F., Sikora, S. N. F., Murray, B. J., Mann, G. W., and Plane, J. M. C.: The Importance of Acid Processed Meteoric Smoke Relative to Meteoric Fragments for Crystal Nucleation in Polar Stratospheric Clouds, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4632, https://doi.org/10.5194/egusphere-egu22-4632, 2022.

Bianca Lauster et al.

Polar stratospheric clouds (PSCs) are an important component of the ozone stratospheric chemistry in polar regions. Although satellite observations nowadays provide high spatial coverage, continuous long-term spectroscopic measurements from the ground with high temporal resolution remain a valuable complement. Moreover, the presented method allows the detection of PSCs even in the presence of tropospheric clouds, while this is not possible with ground-based lidar measurements in such cases.

For a comprehensive interpretation of measurement data, the well-established radiative transfer model McArtim is used and spectra of scattered sunlight at different solar zenith angles are simulated for various atmospheric conditions. Investigating the ratio between observed intensities at two wavelengths, i.e. the so-called colour index (CI), enables the detection of PSCs during twilight. Due to the wavelength variability of scattering processes, the choice of the wavelength pair is determining the effect which PSCs exhibit in the spectra. Likewise, the optical properties, altitude and extent of the PSC layer are decisive parameters that are investigated in detail with the help of 3D simulations. In these, the PSC layer is not simulated as horizontally extended, but as a confined area with different sizes.

The findings are then compared to measured spectra from a MAX-DOAS (Multi AXis-Differential Optical Absorption Spectroscopy) instrument, which has been operating at the German research station Neumayer (70° S, 8° W) in Antarctica since 1999. While the simulations already provide insight into the sensitivity of ground-based spectroscopic measurements for the detection of PSCs, the comparison to measurement data confirms the good applicability of this method.

How to cite: Lauster, B., Dörner, S., Frieß, U., Gu, M., Pukite, J., and Wagner, T.: Detectability of polar stratospheric clouds using the colour index retrieved from ground-based spectroscopic measurements, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4880, https://doi.org/10.5194/egusphere-egu22-4880, 2022.

Michael Pitts and Lamont Poole

After more than three decades of research, the roles of polar stratospheric clouds (PSCs) in stratospheric ozone depletion is well established. Heterogeneous reactions on PSCs convert the stable chlorine reservoirs HCl and ClONO2 to chlorine radicals that destroy ozone catalytically. PSCs also prolong ozone depletion by delaying chlorine deactivation through the removal of gas-phase HNO3 and H2O by sedimentation of large nitric acid trihydrate (NAT) and ice particles. A substantial recovery of the ozone layer is expected by the middle of this century with reduced global production of ozone depleting substances in accordance with the Montreal Protocol and subsequent amendments. But as climate changes, leading to a colder and perhaps wetter stratosphere and upper troposphere, reliable model predictions of recovery of the Antarctic ozone hole and of potentially more severe ozone depletion in the Arctic are challenging. This is due both to a lack of detailed understanding of the underlying physics and the fact that many global models use simple parameterizations that do not accurately represent PSC processes.

A more complete picture of PSC processes on vortex-wide scales has emerged from the CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) instrument on the CALIPSO satellite that has been observing PSCs at latitudes up to 82 degrees in both hemispheres since June 2006. The CALIOP Version 2.0 (v2) PSC algorithm was recently developed to address known deficiencies in previous algorithms and includes additional refinements to increase the robustness of the inferred PSC composition. In this paper, we present an updated PSC reference data record and comprehensive climatology constructed by applying the v2 algorithm to the more than 16-year CALIOP spaceborne lidar dataset. In addition to showing 4-D (latitude, longitude, altitude, and time) information on the occurrence, composition, and variability of PSCs in both hemispheres, we also compare the post-Pinatubo CALIOP PSC data record with the 1979-1989 SAM II (Stratospheric Aerosol Measurement II) solar occultation PSC record to investigate possible long-term variability in PSC occurrence.

How to cite: Pitts, M. and Poole, L.: Updated PSC climatology based on CALIOP measurements from 2006-2022 , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5007, https://doi.org/10.5194/egusphere-egu22-5007, 2022.

Ines Tritscher et al.

Polar ozone loss in late winter and early spring is caused by enhanced concentrations of active chlorine. The surface necessary for heterogeneous reactions activating chlorine species is provided by cold stratospheric aerosols and by polar stratospheric clouds (PSCs). Moreover, sedimentation of PSC particles changes the chemical composition of the lower stratosphere and alters the ozone depleting process by irreversible redistribution of nitric acid and water vapor.

Over the past few years, the Chemical Lagrangian Model of the Stratosphere (CLaMS) has been further developed by the implementation of a microphysical PSC scheme. Within the sedimentation module of CLaMS, nitric acid trihydrate (NAT) and ice particles nucleate, grow, sediment, and evaporate along individual trajectories. Results from different Arctic and Antarctic winters have been compared to measurements from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), and the Microwave Limb Sounder (MLS). For this study, we focus on characteristics of different PSC formation routes: Are there typical meteorological conditions which promote certain nucleation pathways? Are there general hemispheric differences? Do different nucleation pathways contribute differently to the total PSC volume? Vice versa, is it possible to conclude from observations which kind of nucleation mechanism took place?

How to cite: Tritscher, I., Grooß, J.-U., Spang, R., Pitts, M., and Müller, R.: An update on polar stratospheric clouds within CLaMS, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6187, https://doi.org/10.5194/egusphere-egu22-6187, 2022.

Julien Jumelet et al.

Polar Stratospheric Clouds (PSCs) are precursors in the polar stratospheric ozone depletion processes. Aside from recent improvements in both spaceborne PSCs monitoring and classification as well as investigations on microphysics and modeling, there are still uncertainties associated to solid particle formation and their denitrification potential. Besides, complex pathways in PSC formation microphysics lead to mixtures of particles with different optical properties and chemical efficiencies. In that regard, groundbased instruments deliver detailed and valuable measurements that complement the global spaceborne coverage especially in areas near the vortex edge where spaceborne coverage is more difficult and PSC fields present finer structures, especially regarding altitude, similar to the Arctic.

Operated at the French antarctic station Dumont d’Urville (DDU) in the frame of the international Network for the Detection of Atmospheric Composition Change (NDACC), the Rayleigh/Mie/Raman stratospheric lidar provides a solid dataset to feed both process and classification studies, by monitoring cloud and aerosol occurrences in the upper troposphere and lower stratosphere. Located on the antarctic shore (66°S - 140°E), the station has a privileged access to polar vortex dynamics and also recorded persistent signatures of the 2019/2020 Australian originated wildfires.

We hereby present a consolidated dataset from 10 years of lidar measurements using the 532nm backscatter ratio, the aerosol depolarisation and local atmospheric conditions to help in building an aerosol/cloud classification based on existing works using 2008-2020 data.

Overall, the DDU PSC pattern is very consistent with expected typical temperature controlled microphysical calculations. Outside of background sulfate aerosols and anomalies related to volcanic activity (like in 2015), Supercooled Ternary Solution (STS) particles are the most observed particle type, closely followed by Nitric Acid Trihydrate (NAT). ICE clouds are less but regularly observed. ICE clouds also have to be cleary separated from cirrus clouds, raising the issue of accurate tropopause calculations.

Validation of the spaceborne classification schemes as well as careful speciation of the multiple signatures of volcanic or biomass originated aerosol plumes strengthens the need for groundbased monitoring especially in polar regions.

How to cite: Jumelet, J., Tencé, F., and Sarkissian, A.: 2008-2020 lidar measurements of Polar Stratospheric Clouds at the French antarctic station Dumont d’Urville, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11418, https://doi.org/10.5194/egusphere-egu22-11418, 2022.