Dynamically controlled ozone decline in the tropical mid-stratosphere observed by SCIAMACHY

• Main Authors: E. Galytska, A. Rozanov, M. P. Chipperfield, Sandip. S. Dhomse, M. Weber, C. Arosio, W. Feng, J. P. Burrows
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-01-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/19/767/2019/acp-19-767-2019.pdf

<p>Despite the recently reported beginning of a recovery in global stratospheric ozone (<span class="inline-formula">O₃</span>), an unexpected <span class="inline-formula">O₃</span> decline in the tropical mid-stratosphere (around 30–35&thinsp;km altitude) was observed in satellite measurements during the first decade of the 21st century. We use SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) measurements for the period 2004–2012 to confirm the significant <span class="inline-formula">O₃</span> decline. The SCIAMACHY observations show that the decrease in <span class="inline-formula">O₃</span> is accompanied by an increase in <span class="inline-formula">NO₂</span>.</p> <p>To reveal the causes of these observed <span class="inline-formula">O₃</span> and <span class="inline-formula">NO₂</span> changes, we performed simulations with the TOMCAT 3-D chemistry-transport model (CTM) using different chemical and dynamical forcings. For the 2004–2012 time period, the TOMCAT simulations reproduce the SCIAMACHY-observed <span class="inline-formula">O₃</span> decrease and <span class="inline-formula">NO₂</span> increase in the tropical mid-stratosphere. The simulations suggest that the positive changes in <span class="inline-formula">NO₂</span> (around 7&thinsp;%&thinsp;decade<span class="inline-formula">⁻¹</span>) are due to similar positive changes in reactive odd nitrogen (<span class="inline-formula">NO_<i>y</i></span>), which are a result of a longer residence time of the source gas <span class="inline-formula">N₂O</span> and increased production via <span class="inline-formula">N₂O</span>&thinsp;+&thinsp;O(<span class="inline-formula">¹</span>D). The model simulations show a negative change of 10&thinsp;%&thinsp;decade<span class="inline-formula">⁻¹</span> in <span class="inline-formula">N₂O</span> that is most likely due to variations in the deep branch of the Brewer–Dobson Circulation (BDC). Interestingly, modelled annual mean “age of air” (AoA) does not show any significant changes in transport in the tropical mid-stratosphere during 2004–2012.</p> <p>However, further analysis of model results demonstrates significant seasonal variations. During the autumn months (September–October) there are positive AoA changes that imply transport slowdown and a longer residence time of <span class="inline-formula">N₂O</span> allowing for more conversion to <span class="inline-formula">NO_<i>y</i></span>, which enhances <span class="inline-formula">O₃</span> loss. During winter months (January–February) there are negative AoA changes, indicating faster <span class="inline-formula">N₂O</span> transport and less <span class="inline-formula">NO_<i>y</i></span> production. Although the variations in AoA over a year result in a statistically insignificant linear change, non-linearities in the chemistry–transport interactions lead to a statistically significant negative <span class="inline-formula">N₂O</span> change.</p>