Abstract
Abstract. We investigate the impact of the so-called energetic particle precipitation (EPP) indirect effect on lower stratospheric ozone, ClO, and ClONO2 in the Antarctic springtime. We use observations from the Microwave Limb Sounder (MLS) and Ozone Monitoring Instrument (OMI) on Aura, the Atmospheric Chemistry Experiment – Fourier Transform Spectrometer (ACE-FTS) on SCISAT, and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat, covering the period from 2005 to 2017. Using the geomagnetic activity index Ap as a proxy for EPP, we find consistent ozone increases with elevated EPP during years with an easterly phase of the quasi-biennial oscillation (QBO) in both OMI and MLS observations. While these increases are the opposite of what has previously been reported at higher altitudes, the pattern in the MLS O3 follows the typical descent patterns of EPP-NOx. The ozone enhancements are also present in the OMI total O3 column observations. Analogous to the descent patterns found in O3, we also found consistent decreases in springtime MLS ClO following winters with elevated EPP. To verify if this is due to a previously proposed mechanism involving the conversion of ClO to the reservoir species ClONO2 in reaction with NO2, we used ClONO2 observations from ACE-FTS and MIPAS. As ClO and NO2 are both catalysts in ozone destruction, the conversion to ClONO2 would result in an ozone increase. We find a positive correlation between EPP and ClONO2 in the upper stratosphere in the early spring and in the lower stratosphere in late spring, providing the first observational evidence supporting the previously proposed mechanism relating to EPP-NOx modulating Clx-driven ozone loss. Our findings suggest that EPP has played an important role in modulating 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-NOx will likely become a major contributor to Antarctic ozone loss.
Highlights
IntroductionOur understanding of the causes of the Antarctic stratospheric ozone hole (Farman et al, 1985) relies on half a century of discoveries about the Earth’s atmosphere: the Brewer–Dobson circulation (Brewer, 1949), which allows gases such as chlorofluorocarbons (CFCs) emitted in the tropical troposphere to be drawn into the southern polar atmosphere; the strong polar vortex in the Southern Hemisphere, which allows the polar stratosphere to become very cold, with a net downwelling effect pulling gases from the mesosphere and upper stratosphere into the lower stratosphere (Schoeberl and Hartmann, 1991); and polar stratospheric clouds (PSCs), forming in the very cold lower stratosphere which, with the reintroduction of sunlight in the early spring, enable the breakdown of chlorine reservoirs into simpler Clx (= Cl + ClO) molecules on the cloud surfaces (Solomon et al, 1986)
Ozone increases due to the so-called energetic particle precipitation (EPP) indirect effect had been previously suggested (Funke et al, 2014a), but, to our knowledge, this is the first time this has been shown in observations
Following the results of Gordon et al (2020), we propose that this is due to EPP-NOx which remains the lower stratosphere at least until November, having originally been transported from the mesosphere within the polar vortex
Summary
Our understanding of the causes of the Antarctic stratospheric ozone hole (Farman et al, 1985) relies on half a century of discoveries about the Earth’s atmosphere: the Brewer–Dobson circulation (Brewer, 1949), which allows gases such as chlorofluorocarbons (CFCs) emitted in the tropical troposphere to be drawn into the southern polar atmosphere; the strong polar vortex in the Southern Hemisphere, which allows the polar stratosphere to become very cold, with a net downwelling effect pulling gases from the mesosphere and upper stratosphere into the lower stratosphere (Schoeberl and Hartmann, 1991); and polar stratospheric clouds (PSCs), forming in the very cold lower stratosphere which, with the reintroduction of sunlight in the early spring, enable the breakdown of chlorine reservoirs into simpler Clx (= Cl + ClO) molecules on the cloud surfaces (Solomon et al, 1986). This mechanism for EPPNOx descent is well-documented (see e.g. Siskind et al, 2000; Randall et al, 2005; Seppälä et al, 2007; Randall et al, 2007; Funke et al, 2014a, b; Gordon et al, 2020), and the depleting effect on ozone has been reported by a number of studies; for example, Randall et al (2005) used observations from HALOE (HALogen Occultation Experiment), SAGE (Stratospheric Aerosol and Gas Experiment) II and III, POAM (Polar Ozone and Aerosol Measurement) II and III, MIPAS (Michelson Interferometer for Passive Atmospheric Sounding), and OSIRIS (Optical Spectrograph and InfraRed Imager System) to detect the NOx increases in the Northern Hemisphere in January to March 2004 and reported ozone loss in March 2004 in the polar stratosphere This was attributed to the combination of geomagnetic activity occurring in the winter and the reformation of the polar vortex following a sudden stratospheric warming (SSW) earlier in the winter.
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