The sensitivity of stratospheric ozone changes through the 21st century to N<sub>2</sub>O and CH<sub>4</sub>

Abstract

Abstract. Through the 21st century, anthropogenic emissions of the greenhouse gases N 2 O and CH 4 are projected to increase, thus increasing their atmospheric concentrations. Consequently, reactive nitrogen species produced from N 2 O and reactive hydrogen species produced from CH 4 are expected to play an increasingly important role in determining stratospheric ozone concentrations. Eight chemistry-climate model simulations were performed to assess the sensitivity of stratospheric ozone to different emissions scenarios for N 2 O and CH 4 . Global-mean total column ozone increases through the 21st century in all eight simulations as a result of CO 2 -induced stratospheric cooling and decreasing stratospheric halogen concentrations. Larger N 2 O concentrations were associated with smaller ozone increases, due to reactive nitrogen-mediated ozone destruction. In the simulation with the largest N 2 O increase, global-mean total column ozone increased by 4.3 DU through the 21st century, compared with 10.0 DU in the simulation with the smallest N 2 O increase. In contrast, larger CH 4 concentrations were associated with larger ozone increases; global-mean total column ozone increased by 16.7 DU through the 21st century in the simulation with the largest CH 4 concentrations and by 4.4 DU in the simulation with the lowest CH 4 concentrations. CH 4 leads to ozone loss in the upper and lower stratosphere by increasing the rate of reactive hydrogen-mediated ozone loss cycles, however in the lower stratosphere and troposphere, CH 4 leads to ozone increases due to photochemical smogtype chemistry. In addition to this mechanism, total column ozone increases due to H 2 O-induced cooling of the stratosphere, and slowing of the chlorine-catalyzed ozone loss cycles due to an increased rate of the CH 4 + Cl reaction. Stratospheric column ozone through the 21st century exhibits a near-linear response to changes in N 2 O and CH 4 surface concentrations, which provides a simple parameterization for the ozone response to changes in these gases.

Drivers of changes in stratospheric and tropospheric ozone between year 2000 and 2100

“…Haigh and Pyle, 1982;Jonsson et al, 2004;Austin et al, 2010;Eyring et al, 2013;Meul et al, 2014). Insight into the impact of methane changes, which are not explored here, can also be garnered from previous literature (Randeniya et al, 2002;Stenke and Grewe, 2005;Portmann and Solomon, 2007;Fleming et al, 2011;Revell et al, 2012). These studies conclude that the stratospheric ozone response to increased methane results from a combination of increased HO x -catalysed destruction (upper stratosphere), enhanced production through smoglike chemistry (lower stratosphere), and reduced losses due to water-vapour-induced cooling and reductions in [ClO x ].…”

“…The analysis focuses on changes between 2000 and 2100 under the RCP4.5 and 8.5 climate forcing scenarios. Note that future methane emissions are highly uncertain and changes in its abundance, particularly at RCP8.5, will likely have large tropospheric and stratospheric impacts (Randeniya et al, 2002;Fleming et al, 2011;Revell et al, 2012Revell et al, , 2015Young et al, 2013) that are not the focus of this study. Instead, we wish to isolate other drivers of ozone changes, in particular, the role of a change in mean climate state at RCP8.5, without the assumption of a large increase in methane abundance.…”

Daily ozone cycle in the stratosphere: global, regional and seasonal behaviour modelled with the Whole Atmosphere Community Climate Model

“…Recent studies show that unregulated anthropogenic N 2 O emissions are the dominant ozone-depleting emission in the 21st century (Ravishankara et al, 2009). The increasing N 2 O fluxes from the troposphere and CH 4 play an increasing role in determining stratospheric ozone and potentially delay the recovery of the ozone layer (Revell et al, 2012;Shindell et al, 1998).…”

Chemical and climatic drivers of radiative forcing due to changes in stratospheric and tropospheric ozone over the 21<sup>st</sup> century

“…The chemical impacts of N 2 O are not investigated in this study although its radiative effects on climate is implicitly contained in (i). However, we note that changing concentrations of N 2 O within the RCP scenarios is also expected to impact on ozone, and hence be associated with an indirect RF in the stratosphere (Butler et al, 2016;Fleming et al, 2011;Portmann and Solomon, 2007;Revell et al, 2012). Most of the model studies addressing 10 future indirect RFs due to ozone conducted thus far have contained comprehensive chemistry in either the stratosphere or in the troposphere, but not both, which partly motivates this study.…”

“…In ΔCH4, this occurs 5 through an increase in the conversion of active chlorine to its reservoir, HCl, via the reaction CH 4 + Cl → HCl + CH 3 . There are further drivers of stratospheric ozone changes in this experiment (although we do not quantify their separate effects on ozone or the stratospheric RF): increases in lower stratospheric ozone (and hence the LW forcing) occur through NO xmediated production and transport of relatively high ozone amounts from the troposphere; increases in ozone through production of stratospheric water vapor and the consequent cooling; and reductions in ozone through greater HO x -catalysed 10 loss (Fleming et al, 2011;Portmann and Solomon, 2007;Revell et al, 2012;Wayne, 1991).…”