This study investigates the origin of the zonal asymmetry in stratospheric ozone trends at northern high latitudes, identified in satellite limb observations over the past two decades. We use a merged dataset consisting of ozone profiles retrieved at the University of Bremen from SCIAMACHY and OMPS-LP measurements to derive ozone trends. We also use TOMCAT chemical transport model (CTM) simulations, forced by ERA5 reanalyses, to investigate the factors which determine the asymmetry observed in the long-term changes. By studying seasonally and longitudinally resolved observation-based ozone trends, we find, especially during spring, a well-pronounced asymmetry at polar latitudes, with values up to +6 % per decade over Greenland and -5 % per decade over western Russia. The control CTM simulation agrees well with these observed trends, whereas sensitivity simulations indicate that chemical mechanisms, involved in the production and removal of ozone, or their changes, are unlikely to explain the observed behaviour. The decomposition of TOMCAT ozone time series and of ERA5 geopotential height into the first two wavenumber components shows a clear correlation between the two variables in the middle stratosphere and demonstrates a weakening and a shift in the wavenumber-1 planetary wave activity over the past two decades. Finally, the analysis of the polar vortex position and strength points to a decadal oscillation with a reversal pattern at the beginning of the century, also found in the ozone trend asymmetry. This further stresses the link between changes in the polar vortex position and the identified ozone trend pattern.

In the winter and spring of 2019/2020, the unusually cold, strong, and stable polar vortex created favorable conditions for ozone depletion in the Arctic. Chemical ozone loss started earlier than in any previous year in the satellite era, and continued until the end of March, resulting in the unprecedented reduction of the ozone column. The vortex was located above the Polar Environment Atmospheric Research Laboratory in Eureka, Canada (80 °N, 86 °W) from late February to the end of April, presenting an excellent opportunity to examine ozone loss from a single ground station. Measurements from a suite of instruments show that total column ozone in 2020 was at an all-time low in the 20-year dataset, 22 to 102 DU below previous records set in 2011. Ozone minima (<200 DU), enhanced OClO and BrO slant columns, and unusually low HCl, ClONO2 , and HNO3 columns were observed in March. Polar stratospheric clouds were present as late as 20 March, and ozonesondes show unprecedented depletion in the March and April ozone profiles (to <0.2 ppmv). While both chemical and dynamical factors lead to reduced ozone when the vortex is cold, the contribution of chemical depletion was exceptional in spring 2020 when compared to typical Arctic winters. The mean chemical ozone loss over Eureka was estimated to be 111-127 DU (27-31%) using April measurements and passive ozone from the SLIMCAT chemical transport model. While absolute ozone loss was generally smaller in 2020 than in 2011, percentage ozone loss was greater in 2020.

Satellite observations of relevant trace gases, together with meteorological data from ERA5, were used to describe the dynamics and chemistry of the spectacular Arctic 2019/20 winter/spring season. Exceptionally low total ozone values of slightly less than 220 DU were observed in mid March within an unusually large stratospheric polar vortex. This was associated with very low temperatures and extensive polar stratospheric cloud formation, a prerequisite for substantial springtime ozone depletion. Very high OClO and very low NO2 column amounts observed by GOME-2A are indicative of unusually large active chlorine levels and significant denitrification, which likely contributed to large chemical ozone loss. Using results from the TOMCAT chemical transport model (CTM) and ozone observations from S5P/TROPOMI, GOME-2 (total column), SCIAMACHY and OMPS-LP (vertical profiles) chemical ozone loss was evaluated and compared with the previous record Arctic winter 2010/11. The polar-vortex-averaged total column ozone loss in 2019/20 reached 88 DU (23%) and 106~DU (28%) based upon observations and model, respectively, by the end of March, which was similar to that derived for 2010/11. The total column ozone loss is in agreement with OMPS-LP-derived partial column loss between 350 K and 550 K to within the uncertainty. The maximum ozone loss (~80%) observed by OMPS-LP was near the 450 K potential temperature level (~18 km altitude). Because of the larger polar vortex area in March 2020 compared to March 2011 (about 25% at 450 K), ozone mass loss was larger in Arctic winter 2019/20.

In this presentation I will explain an analysis of three different recovered remote-sensing measurements of the 1960s Northern Hemisphere mid-latitude stratospheric aerosol layer. Two of the datasets were recovered within student projects on the Leeds MRes in Climate and Atmospheric Science, the 3rd following a collaboration with Dr. Juan-Carlos Antuna Marrero (Univ. Valladolid, Spain) as part of a “data rescue activity” within the World Climate Research Program activity on stratospheric sulphur, SSiRC: http://www.sparc-ssirc.org/data/datarescueactivity.html Two of the datasets are for the 1963-1965 period when the tropical stratospheric reservoir was highly elevated following the two March 1963 Agung major eruptions (e.g. Niemeier et al., 2019): a series of searchlight measurements from White Sands, New Mexico during 1963 and 1964 (Elterman and Campbell, 1964; Elterman, 1966; Elterman et al., 1973), and the first ever multi-annual stratospheric aerosol dataset from the MIT lidar at Lexington, Massachussetts (Grams, 1966; Grams & Fiocco, 1967; Antuna Marrero et al., 2020). The 3rd dataset, from the 1966-67 period (after the Agung aerosol cloud had fully dispersed) is from two types of balloon measurements: a dust-sonde OPC (Rosen, 1964; Rosen, 1968) and solar-extinction-sounder (Rosen, 1969; Pepin, 1970) both balloon instruments measuring during a Sep 1966 field campaign in the tropics (Panama City, Panama) and a sustained set of NH mid-latitude measurements from Minneapolis, Minnesota in 1963-1967. The observations will be compared to interactive stratospheric aerosol model simulations in GA4 UM-UKCA of the Agung aerosol cloud (Dhomse et al., 2020) and new model experiments seeking to constrain the aerosol clouds from two VEI4 eruptions in Sep 1965 (Taal, Phillipines) and Aug 1966 (Awu, Indonesia).

The widespread presence of meteoric smoke particles (MSPs) within a distinct class of stratospheric aerosol particles has become clear from in-situ measurements in the Arctic, Antarctic and at mid-latitudes. We apply an adapted version of the interactive stratosphere aerosol configuration of the composition-climate model UM-UKCA, to predict the global distribution of meteoric-sulphuric particles nucleated heterogeneously on MSP cores. We compare the UM-UKCA results to new MSP-sulphuric simulations with the European stratosphere-troposphere chemistry-aerosol modelling system IFS-CB05-BASCOE-GLOMAP. The simulations show a strong seasonal cycle in meteoric-sulphuric particle abundance results from the winter-time source of MSPs transported down into the stratosphere in the polar vortex. Coagulation during downward transport sees high latitude MSP concentrations reduce from ~500 per cm3 at 40km to ~20 per cm3 at 25km, the uppermost extent of the stratospheric aerosol particle layer (the Junge layer). Once within the Junge layer’s supersaturated environment, meteoric-sulphuric particles form readily on the MSP cores, growing to 50-70nm dry-diameter (Dp) at 20-25km. Further inter-particle coagulation between these non-volatile particles reduces their number to 1-5 per cc at 15-20km, particle sizes there larger, at Dp ~100nm. The model predicts meteoric-sulphurics in high-latitude winter comprise >90% of Dp > 10nm particles above 25km, reducing to ~40% at 20km, and ~10% at 15km. These non-volatile particle fractions are slightly less than measured from high-altitude aircraft in the lowermost Arctic stratosphere (Curtius et al., 2005; Weigel et al., 2014), and consistent with mid-latitude aircraft measurements of lower stratospheric aerosol composition (Murphy et al., 1998), total particle concentrations also matching in-situ balloon measurements from Wyoming (Campbell and Deshler, 2014). The MSP-sulphuric interactions also improve agreement with SAGE-II observed stratospheric aerosol extinction in the quiescent 1998-2002 period. Simulations with a factor-8-elevated MSP input form more Dp>10nm meteoric-sulphurics, but the increased number sees fewer growing to Dp ~100nm, the increased MSPs reducing the stratospheric aerosol layer’s light extinction.