Observational Evidence for the Modulation of Antarctic Springtime Ozone Depletion by Energetic Particle Precipitation

Abstract

Energetic particles of solar origin raining on the atmosphere is known as energetic particle precipitation (EPP) and causes NOx (NO + NO2) increases in the polar atmosphere. When this occurs during the Southern polar winter, the lack of sunlight and stable atmospheric conditions allow for the NOx to be transported down to the stratosphere, where it lingers until early spring.

Using NO2 observations from the Ozone Monitoring Instrument (OMI) and Ap index as a proxy for EPP, we show that NOx increases from EPP in the winter (EPP-NOx) are detectable in the NO2 stratospheric column, and when the polar vortex is accounted for, EPP-NO$_x$ is traceable until the polar vortex break-up in late November. We show that the easterly phase of the Quasi-Biennial Oscillation (QBO) strengthens the EPP-NOx signal. We postulate that the QBO modulates transport of equatorial N2O, the principle NOx source, while also affecting levels of NOx in the lower polar atmosphere. This is supported by observations of HNO3 and N2O from the Microwave Limb Sounder (MLS).

NOx is notable in the stratosphere because it is a catalyst in O3 depletion reactions. We use O3 observations from both OMI and MLS to find the effect of EPP-NOx on O3 in the springtime stratosphere. We find O3 increases in the mid and lower stratosphere that are positively correlated with EPP levels, and occur simultaneously with the already observed NO2 increases. We also find that the easterly phase of the QBO results in higher correlation between O3 and EPP.

It has been previously hypothesised that EPP-NOx in the stratosphere reacts with the chlorine species responsible for the ozone hole in the Antarctic stratosphere in spring, forming ClONO2, a reservoir species that is unavailable for O3 destruction. We test this with ClO observations from MLS, and ClONO2 observations from a combination of Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS), and Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) We find statistically significant decreases in ClO, suggesting that EPP is associated with ClO reduction in the Antarctic springtime stratosphere. We also find evidence of simultaneous increases in ClONO2 providing further proof that indeed ClO is being buffered to ClONO2. We show that the easterly phase of the QBO results in less chlorine activation and associated increases in ClONO2, suggesting that easterly QBO inhibits ClO activation from ClONO2.