AS3.22

EDI
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
vPICO presentations
| Mon, 26 Apr, 15:30–17:00 (CEST)

vPICO presentations: Mon, 26 Apr

Chairpersons: Michael Pitts, Hideaki Nakajima
15:30–15:35
15:35–15:37
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EGU21-12805
Florence Goutail, Andrea Pazmino, Jean-Pierre Pommereau, Franck Lefevre, Sophie Godin-Beekmann, Alain Hauchecorne, Audrey Lecouffe, Cathy Clerbaux, Anne Boynard, Juliette Hadji-Lazaro, Martyn Chipperfield, Wuhu Feng, Michel VanRoozendael, Nis Jepsen, Georg Hansen, Rigel Kivi, Kristof Bognar, Kimberly Strong, Kaley Walker, and Steve Colwell

Ozone depletion over Polar Regions is monitored each year by satellite and ground-based instruments. The first signs of healing of the ozone layer linked to the decrease of ozone destructive substances (ODSs) were observed in Antarctica using different metrics (ozone mean values, ozone mass deficit, area of the ozone hole) and simple or sophisticated models. Chemistry climate models predict that climate change will not affect expected ozone recovery over Antarctica but will accelerate recovery in the Arctic due to the possible enhancement of the Brewer Dobson circulation. However, ozone loss observations by SAOZ UV-Vis spectrometers do not show a clear sign of recovery in the latter region. In addition, a record of 38% ozone loss in 2010/2011 and 2019/2020 was estimated.

In this study, the vortex-averaged ozone loss in the last three decades will be evaluated for both Polar Regions using the passive ozone tracer of two chemical transport models (REPROBUS and SLIMCAT CTMs) and total ozone observations from SAOZ and satellite observations (IASI/METOP and Multi-Sensor Reanalysis (MSR-2)).

The tracer method allows us to determine the evolution of the daily rate of ozone destruction, and the amplitude of the cumulative loss at the end of the winter. The cumulative ozone destruction in the Artic varies between 0-10% in relatively warm winters with short vortex duration to up to 25-38% in colder winters with longer vortex persistence, while in Antarctica it is mostly stable, around 50%.

Interannual variability of 10-days average rate will be analyzed and compared between both hemispheres as well as the timing to reach different thresholds of absolute ozone loss values. Finally, linear trend of ozone loss and temperature since 2000 will be estimated in both Polar Regions in order to evaluate possible ozone recovery.

How to cite: Goutail, F., Pazmino, A., Pommereau, J.-P., Lefevre, F., Godin-Beekmann, S., Hauchecorne, A., Lecouffe, A., Clerbaux, C., Boynard, A., Hadji-Lazaro, J., Chipperfield, M., Feng, W., VanRoozendael, M., Jepsen, N., Hansen, G., Kivi, R., Bognar, K., Strong, K., Walker, K., and Colwell, S.: Evaluation of interannual variability of Arctic and Antarctic ozone loss since 1989, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12805, https://doi.org/10.5194/egusphere-egu21-12805, 2021.

15:37–15:39
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EGU21-11967
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ECS
Aliaksei Borisovets, Ilya Bruchkouski, Aliaksandr Svetashev, and Aliaksandr Krasouski

Regions with an expected low anthropogenic load are of particular interest for monitoring small gas components of the atmosphere. Under such conditions, the concentration of ozone is determined by natural processes. One of these regions is East Antarctica.

The experimental part of the research included:

- Complex of meteorological observations;

- Measurement of total ozone column (TOC) in the vertical column of the atmosphere;

- Monitoring of spectra, levels and doses of surface solar radiation;

The research was carried out in the areas where the Belarusian Antarctic expeditions were based: the stations “Mount Vechernaya” and “Progress”.

The measurements were carried out by a two-channel filter photometer PION-F and an ultraviolet spectroradiometer PION-UV, developed at the NOMREC. When determining the TOC values, the method was used, which consists in restoring the TOC values by analyzing the spectral distribution of the illumination density of the Earth's surface in the UV range. According to this method, the TOC values can be obtained using the ratio of illuminances at two wavelengths of the solar spectrum, one of which falls in the region of sufficiently strong absorption of atmospheric ozone, and the other is outside this region.

During the Belarusian Antarctic expeditions, a significant amount of experimental data was obtained (more than 100,000 spectra of energy illumination, more than 1,000 average daily values of TOC, etc.). The accumulated array of experimental data can be used to study theoretical problems and solve applied problems.

This paper presents a description of the dynamics of TOC in the atmosphere of East Antarctica during the period of seasonal expeditions 2014-2020.

How to cite: Borisovets, A., Bruchkouski, I., Svetashev, A., and Krasouski, A.: Results of measurements of the state of the ozone layer in East Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11967, https://doi.org/10.5194/egusphere-egu21-11967, 2021.

15:39–15:41
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EGU21-13012
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ECS
Lily Zhang, Susan Solomon, Kane Stone, John Burrows, Steve Colwell, Joshua Eveson, David Haffner, Anna Jones, Natalya Kramarova, Gordon Labow, Pieternel Levelt, Paul Newman, Jonathan Shanklin, Mark Weber, and Catherine Wilka

Measurements by the Dobson ozone spectrophotometer at the British Antarctic Survey’s (BAS) Halley research station form a record of Antarctic total column ozone that dates back to 1956. Due to its location, length, and completeness, the record has been, and continues to be, uniquely important for studies of long-term changes in Antarctic ozone. However, a crack in the ice shelf on which it resides forced the station to abruptly close for eight months and [SC-UB1]  led to a gap in its historic record.  We develop and test a method for filling in the record of Halley total ozone by combining and bias-correcting overpass data from a range of different satellite instruments. Tests suggest that our method reproduces the monthly ground-based Dobson total ozone values to within 20 Dobson units.  We show that our approach improves on the overall performance as compared to simply using the raw satellite average or an individual instrument. The method also provides a check on the consistency of the automated Dobson used at Halley after 2018 compared to earlier manual Dobson data, and suggests a significant difference between the two.  The filled Halley dataset provides further support that the Antarctic ozone hole is healing not only during September, but also in January.

How to cite: Zhang, L., Solomon, S., Stone, K., Burrows, J., Colwell, S., Eveson, J., Haffner, D., Jones, A., Kramarova, N., Labow, G., Levelt, P., Newman, P., Shanklin, J., Weber, M., and Wilka, C.: On the Use of Satellite Observations to Fill Gaps in the Halley Station Total Ozone Record, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13012, https://doi.org/10.5194/egusphere-egu21-13012, 2021.

15:41–15:43
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EGU21-157
Annika Seppälä, Emily Gordon, Bernd Funke, Johanna Tamminen, and Kaley Walker

We present the impact of the so-called energetic particle precipitation (EPP), part of natural solar forcing on the atmosphere, on polar stratospheric NOx, ozone, and chlorine chemistry in the Antarctic springtime, using multi-satellite observations covering the overall period of 2005–2017. We find consistent ozone increases when high solar activity occurs during years with easterly phase of the quasi biennial oscillation. These ozone enhancements are also present in total O3 column observations. We find consistent decreases in springtime active chlorine following winters of elevated solar activity. Further analysis shows that this is accompanied by increase of chemically inactive chlorine reservoir species, explaining the observed ozone increase. This provides the first observational evidence supporting the previously proposed mechanism relating to EPP modulating chlorine driven ozone loss. Our findings suggest that solar activity via EPP has played an important role in modulating Antarctic ozone depletion in the last 15 years. As chlorine loading in the polar stratosphere continues to decrease in the future, this buffering mechanism will become less effective and catalytic ozone destruction by EPP produced NOx will likely become a major contributor to Antarctic ozone loss.

How to cite: Seppälä, A., Gordon, E., Funke, B., Tamminen, J., and Walker, K.: Observational evidence of solar activity interaction with chlorine chemistry curbing Antarctic ozone loss, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-157, https://doi.org/10.5194/egusphere-egu21-157, 2020.

15:43–15:45
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EGU21-14507
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ECS
Jia Jia, Antti Kero, Niilo Kalakoski, Monika E. Szeląg, and Pekka T. Verronen

Recent studies reported up to a 10 % average decrease of lower stratospheric ozone at ∼ 20 km altitude following solar proton events (SPEs), based on superposed epoch analysis (SEA) of ozonesonde anomalies. Our study uses 49 SPEs that occurred after the launch of Aura MLS (2004–now) and 177 SPEs that occurred in the WACCM-D (Whole Atmosphere Community Climate Model with D-region ion chemistry) simulation period (1989–2012) to evaluate Arctic polar atmospheric ozone changes following SPEs. At the mesospheric altitudes a statistically significant ozone depletion is present. At the lower stratosphere (<25 km), SEA of the satellite dataset provides no solid evidence of any average direct SPE impact on ozone. In the individual case studies, we find only one potential case (January 2005) in which the lower-stratospheric ozone level was significantly decreased after the SPE onset (in both model simulation and MLS observation data). However, similar decreases could not be identified in other SPEs of similar or larger magnitude. We find a very good overall consistency between WACCM-D simulations and MLS observations of SPE-driven ozone anomalies both on average and for the individual cases, including case in January 2005.

How to cite: Jia, J., Kero, A., Kalakoski, N., E. Szeląg, M., and T. Verronen, P.: Investigation of direct solar proton impact on Arctic stratospheric ozone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14507, https://doi.org/10.5194/egusphere-egu21-14507, 2021.

15:45–15:47
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EGU21-4189
Janis Pukite, Christian Borger, Steffen Dörner, and Myojeong Gu

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 relation of its formation to 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).

Here we present retrieved time series of OClO slant column densities (SCDs) for the period 2017 - 2020, compare them with ground based zenith sky measurements and correlate them with meteorological data for both Antarctic and Arctic regions.

How to cite: Pukite, J., Borger, C., Dörner, S., and Gu, M.: OClO as observed by TROPOMI on Sentinel 5P, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4189, https://doi.org/10.5194/egusphere-egu21-4189, 2021.

15:47–15:49
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EGU21-8987
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ECS
Myojeong Gu, Carl-Fredrik Enell, Janis Pukite, Ulrich Platt, Uwe Raffalski, and Thomas Wagner

Recent research on stratospheric ozone indicates signs of ozone recovery, but on the other hand, ozone recovery is also expected to be delayed by many aspects (e.g climate change). Therefore, it is important to monitor continuously stratospheric trace gases to predict the future evolution of the Arctic ozone and other trace gases which are involved in the ozone depletion chemistry. OClO is well known as an indicator of the stratospheric chlorine activation and can be measured using remote sensing techniques.

In this study, we present long-term measurements of OClO slant column densities at Kiruna, Sweden (67.84°N, 20.41°E) which were obtained from the ground-based zenith sky DOAS instruments since 1997. The measurement site is located north of the polar circle in which the variability of the OClO abundance depends on the state of stratospheric chlorine activation but also whether the polar vortex is located above the measurement site.

The aim of this study is to give an overview of the measured stratospheric OClO abundance for 19 years, and to investigate the dominant parameters affecting ozone and OClO during periods of stratospheric chlorine activation. One particular focus is on the parameters which trigger the activation and de-activation at the beginning and the end of the polar winter.

To do so, we compare the general dependencies of OClO on other trace gases and meteorological conditions.

How to cite: Gu, M., Enell, C.-F., Pukite, J., Platt, U., Raffalski, U., and Wagner, T.: Stratospheric OClO observed with ground-based DOAS over Kiruna in the Arctic winters 1996/1997 – 2019/2020, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8987, https://doi.org/10.5194/egusphere-egu21-8987, 2021.

15:49–15:51
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EGU21-12227
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ECS
Andrea Rau, Valentin Lauther, Fridolin Hader, Svetlana Cvetkova, C. Michael Volk, Peter Hoor, Vera Bense, and Heiko Bozem

In September 2019 a rare sudden stratospheric warming occurred in the Antarctic region. During the course of this event the airborne campaign SouthTRAC (Transport and composition of the Southern Hemisphere UTLS) was conducted with the main goal of studying the impact of the Antarctic vortex on the southern hemisphere upper troposphere / lower stratosphere (UTLS). SouthTRAC deployed the German High Altitude and LOng range research aircraft (HALO) in two phases (September/early October and November) based in Rio Grande, Argentina. The mission comprised 23 scientific flights including transfer flights to/from Argentina and local flights from Rio Grande. During several of these flights HALO flew over the Antarctic Peninsula and adjacent regions, thus probing the bottom of the Antarctic vortex, and crossing vortex streamers and thin filaments.

We present and analyse in situ measurements of CO2 and various other long-lived tracers obtained by the University of Wuppertal’s 5-channel High Altitude Gas AnalyzeR (HAGAR-V) along with N2O measured by the University of Mainz's UMAQS (University of Mainz Airborne QCL Spectrometer) using laser absorption techniques. For our analysis we use mixing ratios of CO2, SF6, CFC-11, CFC-12, N2O, and age of air (AoA) derived from CO2 and SF6.

Vertical and meridional distributions as well as tracer correlations show differences between phase 1 and phase 2 of the mission. During September the distributions at mid-latitudes indicate stronger isentropic transport of vortex and subtropical air than during November. The CO2-N2O correlation also changed between September and November due to isentropic mixing at 330-400 K potential temperature. The oldest observed AoA as derived from CO2 was about 4.5 years at 390 K, while significantly older AoA is derived from SF6, but is presumably an overestimate due to mesospheric loss of SF6. We have compared the tracer distributions and AoA during SouthTRAC with those of the undisturbed 1999 Antarctic vortex sampled by the M55 Geophysica aircraft during the Antarctic campaign APE-GAIA. For September/October we find similar distributions and age values in both years, which would suggest that net tracer descent trough isentropes in the disturbed 2019 lower Antarctic vortex was not substantially different from that occurring in a typical undisturbed winter.

How to cite: Rau, A., Lauther, V., Hader, F., Cvetkova, S., Volk, C. M., Hoor, P., Bense, V., and Bozem, H.: Airborne in situ tracer and age of air observations in the UTLS during the rare Antarctic sudden stratospheric warming 2019, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12227, https://doi.org/10.5194/egusphere-egu21-12227, 2021.

15:51–15:53
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EGU21-2699
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Highlight
Ingo Wohltmann, Peter von der Gathen, Ralph Lehmann, Marion Maturilli, Holger Deckelmann, Gloria Manney, Jonathan Davies, David Tarasick, Nis Jepsen, Rigel Kivi, Norrie Lyall, and Markus Rex

In the Antarctic ozone hole, ozone mixing ratios have been decreasing to extremely low values of 0.01–0.1 ppm in nearly all spring seasons since the late 1980s, corresponding to 95–99% local chemical loss. In contrast, Arctic ozone loss has been much more limited and mixing ratios have never before fallen below 0.5 ppm. In Arctic spring 2020, however, ozonesonde measurements in the most depleted parts of the polar vortex show a highly depleted layer, with ozone loss averaged over sondes peaking at 93% at 18 km. Typical minimum mixing ratios of 0.2 ppm were observed, with individual profiles showing values as low as 0.13 ppm (96% loss). The reason for the unprecedented chemical loss was an unusually strong, long-lasting, and cold polar vortex, showing that for individual winters the effect of the slow decline of ozone-depleting substances on ozone depletion may be counteracted by low temperatures.

How to cite: Wohltmann, I., von der Gathen, P., Lehmann, R., Maturilli, M., Deckelmann, H., Manney, G., Davies, J., Tarasick, D., Jepsen, N., Kivi, R., Lyall, N., and Rex, M.: Near‐Complete Local Reduction of Arctic Stratospheric Ozone by Severe Chemical Loss in Spring 2020, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2699, https://doi.org/10.5194/egusphere-egu21-2699, 2021.

15:53–15:55
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EGU21-8892
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ECS
Ramina Alwarda, Kristof Bognar, Kimberly Strong, Martyn Chipperfield, Sandip Dhomse, James Drummond, Wuhu Feng, Vitali Fioletov, Florence Goutail, Beatriz Herrera, Gloria Manney, Emily McCullough, Luis Millan, Andrea Pazmino, Kaley Walker, Tyler Wizenberg, and Xiaoyi Zhao

The Arctic winter of 2019-2020 was characterized by an unusually persistent polar vortex and temperatures in the lower stratosphere that were consistently below the threshold for the formation of polar stratospheric clouds (PSCs). These conditions led to ozone loss that is comparable to the Antarctic ozone hole. Ground-based measurements from a suite of instruments at the Polar Environment Atmospheric Research Laboratory (PEARL) in Eureka, Canada (80.05°N, 86.42°W) were used to investigate chemical ozone depletion. The vortex was located above Eureka longer than in any previous year in the 20-year dataset and lidar measurements provided evidence of polar stratospheric clouds (PSCs) above Eureka. Additionally, UV-visible zenith-sky Differential Optical Absorption Spectroscopy (DOAS) measurements showed record ozone loss in the 20-year dataset, evidence of denitrification along with the slowest increase of NO2 during spring, as well as enhanced reactive halogen species (OClO and BrO). Complementary measurements of HCl and ClONO2 (chlorine reservoir species) from a Fourier transform infrared (FTIR) spectrometer showed unusually low columns that were comparable to 2011, the previous year with significant chemical ozone depletion. Record low values of HNO3 in the FTIR dataset are in accordance with the evidence of PSCs and a denitrified atmosphere. Estimates of chemical ozone loss were derived using passive ozone from the SLIMCAT offline chemical transport model to account for dynamical contributions to the stratospheric ozone budget.

How to cite: Alwarda, R., Bognar, K., Strong, K., Chipperfield, M., Dhomse, S., Drummond, J., Feng, W., Fioletov, V., Goutail, F., Herrera, B., Manney, G., McCullough, E., Millan, L., Pazmino, A., Walker, K., Wizenberg, T., and Zhao, X.: Record springtime stratospheric ozone depletion at 80°N in 2020, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8892, https://doi.org/10.5194/egusphere-egu21-8892, 2021.

15:55–15:57
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EGU21-2429
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Highlight
Jens-Uwe Grooß and Rolf Müller

In Arctic winter/spring 2019/2020, the stratospheric temperatures  were exceptionally low until early April and the polar vortex was  very stable.  As a consequence, significant chemical ozone depletion  occurred in Northern polar regions in spring 2020.  Here, we present  simulations by the Chemical Lagrangian Model of the Stratosphere  (CLaMS) that address the development of chlorine compounds and  ozone in the polar stratosphere in 2020.  The simulation reproduces  relevant observations of ozone and chlorine compounds, as shown by  comparisons with data from Microwave Limb Sounder (MLS), Atmospheric  Chemistry Experiment - Fourier Transform Spectrometer (ACE-FTS),  in-situ ozone sondes and the Ozone Monitoring Instrument (OMI).  Although the concentration of chlorine and bromine compounds in the  polar stratosphere has decreased by more than 10% compared to the  peak values around the year 2000, the meteorological conditions in  winter/spring 2019/2020 caused an unprecedented ozone depletion. The  simulated lowest ozone mixing ratio was around 0.05 ppmv and the  calculated partial ozone column depletion in the vortex core in the  lower stratosphere reached 141 Dobson Units between 350 and 600 K  potential temperature, which is more than the  loss in the years 2011 and 2016 which until 2020 had seen the  largest Arctic ozone depletion on record.

How to cite: Grooß, J.-U. and Müller, R.: Simulation of the record Arctic stratospheric ozone depletion in 2020, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2429, https://doi.org/10.5194/egusphere-egu21-2429, 2021.

15:57–15:59
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EGU21-6643
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ECS
Ines Tritscher, Michael C. Pitts, Lamont R. Poole, and Thomas Peter and the SPARC PSCi team

The important role of polar stratospheric clouds (PSCs) in stratospheric ozone depletion during winter and spring at high latitudes has been known since the 1980s. However, contemporary observations by the spaceborne instruments MIPAS (Michelson Interferometer for Passive Atmospheric Sounding), MLS (Microwave Limb Sounder), and CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) have brought about a comprehensive and clearer understanding of PSC spatial and temporal distributions, their conditions of existence, and the processes through which they impact polar ozone. Within the SPARC (Stratosphere-troposphere Processes And their Role in Climate) PSC initiative (PSCi), those datasets have been synthesized and discussed in depth with the result of a new vortex-wide climatology of PSC occurrence and composition. We will present our results within this vPICO together with a review of the significant progress that has been made in our understanding of PSC nucleation, related dynamical processes, and heterogeneous chlorine activation. Moreover, we have compiled different techniques for parameterizing PSCs and we will show their effects in global models.

How to cite: Tritscher, I., Pitts, M. C., Poole, L. R., and Peter, T. and the SPARC PSCi team: Polar Stratospheric Clouds: Satellite Observations, Processes, and Role in Ozone Depletion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6643, https://doi.org/10.5194/egusphere-egu21-6643, 2021.

15:59–16:01
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EGU21-4560
Francesco Cairo, Marcel Snels, Luca Di Liberto, Terry Deshler, Andrea Scoccione, and Marco Bragaglia

Mie scattering codes have long been used to study the optical properties of Polar Stratospheric Clouds, once the particle size distribution (PSD) is known and a suitable refractive index is assumed. However, PSCs are often composed as external mixtures of STS and NAT, making questionable the use of Mie theory with a single refractive index. Furthermore, the NAT particles are non-spherical, while strictly speaking the applicability of Mie theory is limited to particles with circular symmetry along the direction of propagation of the incident light. 
Here we consider a set of 15 coincident measurements of polar stratospheric clouds above McMurdo Station, Antarctica, by ground-based lidar (backscatter and depolarization) and balloon-borne Optical Particle Counters (PSD), and apply Mie theory to the measured PSD, to seek matching with the observed optical parameters.
In our model, we consider the PSD particles as STS if their radius is below a certain threshold value R and NAT if above it, assuming the corresponding refractive indexes known from literature. Moreover, we reduce the Mie calculation for the NAT part of the PSD by multiplying it by a factor C <1, which takes into account the backscattering depression expected from aspheric particles. Finally, we consider the fraction X of the backscattering contribution of the NAT part of the PSD as polarized, and the remaining (1-X) as depolarized.
The three parameters R, C and X of our model are then chosen to provide the best match with the observed lidar backscattering and depolarization.

How to cite: Cairo, F., Snels, M., Di Liberto, L., Deshler, T., Scoccione, A., and Bragaglia, M.: A study of Mie scattering modelling for external mixtures of NAT and STS Polar Stratospheric Clouds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4560, https://doi.org/10.5194/egusphere-egu21-4560, 2021.

16:01–16:03
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EGU21-2591
Matthias Tesche, Peggy Achtert, and Michael Pitts

Spaceborne observations of Polar Stratospheric Clouds (PSCs) with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite provide a comprehensive picture of the occurrence of Arctic and Antarctic PSCs as well as their microphysical properties. However, advances in understanding PSC microphysics also require measurements with ground-based instruments, which are often superior to CALIOP in terms of, e.g. time resolution, measured parameters, and signal-to-noise ratio. This advantage is balanced by the location of ground-based PSC observations and their dependence on tropospheric cloudiness. CALIPSO observations during the boreal winters from December 2006 to February 2018 and the austral winters 2012 and 2015 are used to assess the effect of tropospheric cloudiness and other measurement-inhibiting factors on the representativeness of ground-based PSC observations with lidar in the Arctic and Antarctic, respectively. Information on tropospheric and stratospheric clouds from the CALIPSO Cloud Profile product (05kmCPro version 4.10) and the PSC mask version 2, respectively, is combined on a profile-by-profile basis to identify conditions under which a ground-based lidar is likely to perform useful measurements for the analysis of PSC occurrence. It is found that the location of a ground-based measurement together with the related tropospheric cloudiness can have a profound impact on the derived PSC statistics and that these findings are rarely in agreement with polar-wide results from CALIOP observations. Considering the current polar research infrastructure, it is concluded that the most suitable sites for the expansion of capabilities for ground-based lidar observations of PSCs are Summit and Villum in the Arctic and Mawson, Troll, and Vostok in the Antarctic.

How to cite: Tesche, M., Achtert, P., and Pitts, M.: On the best locations for ground-based PSC observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2591, https://doi.org/10.5194/egusphere-egu21-2591, 2021.

16:03–16:05
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EGU21-13362
Marcel Snels, Francesco Colao, Francesco Cairo, Ilir Shuli, Andrea Scoccione, Mauro De Muro, Michael Pitts, Lamont Poole, and Luca Di Liberto

Polar stratospheric clouds have been observed at Dome C by a ground-based lidar from 2014 up to the present, possibly in coincidence with nearby overpasses of the CALIPSO satellite, with the CALIOP lidar on board.

A thorough study has been made in terms of detection efficiency and composition classification of near coincident lidar observations, with the goal to identify the main biases between the two lidars.

When comparing ground-based lidar observations with nearby CALIOP overpasses, several biases might occur, due to the distance between ground-based lidar and nearest overpass, observation geometry and integration times and different algorithms used for data analysis.

The bias resulting from different data analysis has been reduced by applying an algorithm for PSC detection and composition classification to the ground-based data which is very similar to the V2 algorithm used for CALIOP.

By comparing 5 years of PSC observations at Dome C, considering both detection efficiency and composition of the observed PSCs, the impact of all biases will be discussed and possibly quantified. 

How to cite: Snels, M., Colao, F., Cairo, F., Shuli, I., Scoccione, A., De Muro, M., Pitts, M., Poole, L., and Di Liberto, L.: Comparison of polar stratospheric cloud detection and composition as observed by ground-based lidar and CALIOP at Dome C., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13362, https://doi.org/10.5194/egusphere-egu21-13362, 2021.

16:05–16:07
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EGU21-5385
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ECS
Bianca Lauster, Steffen Dörner, Udo Frieß, Myojeong Gu, Janis Pukite, and Thomas Wagner

Polar Stratospheric Clouds (PSCs) favour heterogeneous reactions and thus are an important component of ozone depletion processes in polar regions. Although satellite observations already yield high spatial coverage, the sampling frequency of a specific air volume depends on the measurement method. Here, continuous ground-based measurements with high temporal resolution can be a valuable complement.

Since 1999, a MAX-DOAS (Multi AXis-Differential Optical Absorption Spectroscopy) instrument has been operating at the German research station Neumayer (70° S, 8° W), Antarctica. Primarily, slant column densities of trace gases such as NO2, BrO and OClO are retrieved. However, in this study the so-called colour index (CI), i.e. the colour of the zenith sky, is investigated. Defined as the ratio between the observed intensities of scattered sun light at two wavelengths, it enables to monitor the occurrence of polar stratospheric clouds during twilight even in the presence of tropospheric clouds.

Using the radiative transfer model McArtim, the CI changes in the presence of polar stratospheric clouds can be analysed. Especially the height of the PSC layer affects the retrieved signal, but also the choice of the wavelengths has a strong impact. Here, it is advantageous that measurements are available in the UV and visible spectral range which allows a more extensive comparison of different CI choices. In order to assess the application of the colour index method, meteorological data are used to identify PSC cases in the data set.

The aim is to improve and evaluate the potential of this method. It is then used to infer the occurrence of PSCs throughout the measurement time series of more than 20 years.

How to cite: Lauster, B., Dörner, S., Frieß, U., Gu, M., Pukite, J., and Wagner, T.: Occurrence of Polar Stratospheric Clouds using ground-based DOAS observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5385, https://doi.org/10.5194/egusphere-egu21-5385, 2021.

16:07–16:09
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EGU21-12508
Julien Jumelet, Florent Tencé, Alain Sarkissian, Slimane Bekki, and Philippe Keckhut

Polar Stratospheric Clouds (PSCs) play a primary role in polar stratospheric ozone depletion processes. Aside from recent improvements in both spaceborne monitoring as well as investigations on microphysics and modeling, there are still caveats on building a comprehensive picture of the PSC particle population, especially considering the fine optical signatures of some particles. In that regard, groundbased instruments provide fine and long term reference measurements that complement the global spaceborne coverage.

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 lidar provides over the years 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 antarctic shore (66°S - 140°E), the station has a privileged access to polar vortex dynamics. Measurements are weather-dependent with a yearly average of 130 nights of monitoring. Expected PSC formation temperatures are used to evaluate the whole PSC season occurrence statistics.

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. Overall, the DDU PSC pattern is very consistent with expected typical temperature controlled thresholds. Supercooled Ternary Solution (STS) particles are the most observed particle type, closely followed by Nitric Acid Trihydrate (NAT). ICE clouds are more rarely observed. The measurements also feature significant and detailed signatures of various aerosols events having reached the polar antarctic stratosphere, like the Calbuco eruption (2015) or the 2 australian wildfires episodes (2009 and 2019). We aim at refining the identification of those aerosols to include their impact in the scope of the scientific questions studied at DDU.

How to cite: Jumelet, J., Tencé, F., Sarkissian, A., Bekki, S., and Keckhut, P.: 10 years of Polar Stratospheric Clouds lidar measurements at the French antarctic station Dumont d’Urville, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12508, https://doi.org/10.5194/egusphere-egu21-12508, 2021.

16:09–16:11
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EGU21-8547
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Highlight
Peter von der Gathen, Rigel Kivi, Ingo Wohltmann, Ross Salawitch, and Markus Rex

The chemical loss of ozone during Arctic winter and spring due to anthropogenic halogens is driven by temperature at high latitudes, with more loss occurring during cold years with meteorological conditions that are favourable for formation of polar stratospheric clouds (PSCs). We show that a positive, statistically significant rise in the local maxima of PSC formation potential (PFPLM), i.e. seasonal integrals of the fraction of the vortex volume below the formation temperature of PSCs, within the Northern Hemisphere polar vortex over the past four decades is apparent in data from four meteorological centres. Output from numerous General Circulation Models (GCMs) that submitted results to the CMIP5 and CMIP6 archives also exhibits positive trends in PFPLM over 1950 to 2100, with the highest values occurring at end of century for model runs driven by increasing radiative forcing of climate due to greenhouse gases (GHGs) (i.e., the RCP 8.5 scenario for CMIP5 and the SSP5-8.5 scenario for CMIP6). We combine projections of the future decline in stratospheric halogen loading and possible future increases in stratospheric humidity with GCM-based forecasts of PFP to suggest that conditions favourable for large, seasonal loss of Arctic column O3 could persist until the end of this century, especially for GCM simulations constrained by either the RCP 8.5 or SSP5-8.5 GHG scenario. Conversely, if future GHG loading follow the SSP1-2.6 scenario, conditions favourable for chemical loss of Arctic O3 are projected to decline throughout the rest of this century.

How to cite: von der Gathen, P., Kivi, R., Wohltmann, I., Salawitch, R., and Rex, M.: The sensitivity of chemical loss of Arctic ozone to future levels of GHGs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8547, https://doi.org/10.5194/egusphere-egu21-8547, 2021.

16:11–17:00