Abstract
<p>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.</p>
Highlights
It is well established that the Antarctic ozone hole (Farman et al, 1985; Jones & Shanklin, 1995) is caused by chemical ozone depletion in austral spring through catalytic cycles driven by chlorine and bromine compounds (e.g., Solomon, 1999; WMO, 2018)
ClOx/Cly averaged over the polar vortex core (Φe > 75°N) exceeded 50% at a rather early stage on 14 December at a potential temperature between about 490 and 540 K
Arctic winter/spring 2019/2020 was exceptionally cold in the polar stratosphere and the polar vortex was stable for an unusually long period until early April, which led to significant chemical ozone depletion through the end of March and the conservation of this depleted ozone in the polar vortex until early April 2020
Summary
It is well established that the Antarctic ozone hole (Farman et al, 1985; Jones & Shanklin, 1995) is caused by chemical ozone depletion in austral spring through catalytic cycles driven by chlorine and bromine compounds (e.g., Solomon, 1999; WMO, 2018) For these cycles to run efficiently, chlorine needs to be activated from the reservoir compounds HCl and ClONO2 by heterogeneous reactions. These heterogeneous reactions only take place at low temperatures typically present in polar winter and spring, occurring on the surfaces of Polar Stratospheric Clouds (PSCs) and on cold sulfate aerosol (e.g., Drdla & Müller, 2012; Portmann et al, 1996; Solomon, 1999; Tritscher et al, 2021; WMO, 2018). The reported simulations indicate that the major causes of the severe ozone depletion reported in the Arctic in 2020 are the low stratospheric temperatures and the exceptionally stable polar vortex extending into spring (Lawrence et al, 2020)
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