Observational evidence of energetic particle precipitation NO<sub><i>x</i></sub> (EPP-NO<sub><i>x</i></sub>) interaction with chlorine curbing Antarctic ozone loss

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.

Influence of energetic particle precipitation on Antarctic stratospheric chlorine and ozone over the 20th century

“…Interestingly, the relative 190 difference between EXP and REF (Figure 10b) follows the overall level of ClO (and CFC emissions), and is a very different than in the mid-stratosphere in Figure 7f. Positive ClO response in the lower stratosphere due to EPP was also found by Gordon et al (2021). Damiani et al (2016) showed negative ozone response at same altitudes related to EPP, albeit they were related to regression analysis with the geomagnetic activity.…”

“…This occurs at altitudes where polar stratospheric clouds (PSC) form in the Antarctic (Pitts et al, 2018), likely being a consequence of reservoir gases breaking on PSC (Webster et al, 1993). This positive ClO response in the lower stratosphere was also seen in satellite data by Gordon et al (2021) during August-October.…”

“…Reaction R9 couples catalytic ozone destruction cycles of chlorine and NO x (Brasseur and Solomon, 2005). A recent study showed that satellite observations of Antarctic ClO correlated negatively with the geomagnetic activity during springtime (Gordon et al, 2021). This implies that ozone depletion by polar NO x is modulated by chlorine loading, especially during the CFC era.…”

Stratospheric ozone, UV radiation, and climate interactions

“…One mechanism is mediated through photochemical processes in the upper atmosphere that are modified by changes in solar UV-C (100–280 nm) radiation. The other process is driven by changes in the rate of energetic particle 30 precipitation (EPP), which mainly affect ozone over the polar regions [ 191 , 192 ].…”

Impact of EMIC‐Wave Driven Electron Precipitation on the Radiation Belts and the Atmosphere

“…Once in the stratosphere this EEP‐NO x can contribute to long‐term ozone variability in complex ways: recent observational evidence has shown that in addition to directly causing ozone loss, EEP‐NO x can also cause indirect increases in ozone at the main ozone layer altitudes by binding harmful, ozone hole causing halogen compounds, thus preventing them from contributing to springtime polar ozone loss (Gordon, Seppälä, Funke, et al., 2020). The ability to correctly estimate and model atmospheric ozone levels is critical for climate simulations as, for example, ozone provides a critical source for heating and cooling in the atmosphere linking it to dynamical patterns and regional climate variability (Matthes et al., 2017).…”