AS3.22

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.

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.

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.

We present the impact of the so-called energetic particle precipitation (EPP), part of natural solar forcing on the atmosphere, on polar stratospheric NO_x, 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 NO_x 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, 2021.

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.

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.

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.

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.

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.

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.

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.

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.

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.