Future changes in the stratosphere-to-troposphere ozone mass flux and the contribution from climate change and ozone recovery

Stefanie Meul,Philipp Kröger,Patrick Jöckel,Sophie Oberländer-Hayn,Ulrike Langematz

doi:10.5194/acp-18-7721-2018

Abstract

Abstract. Using a state-of-the-art chemistry–climate model we investigate the future change in stratosphere–troposphere exchange (STE) of ozone, the drivers of this change, as well as the future distribution of stratospheric ozone in the troposphere. Supplementary to previous work, our focus is on changes on the monthly scale. The global mean annual influx of stratospheric ozone into the troposphere is projected to increase by 53 % between the years 2000 and 2100 under the RCP8.5 greenhouse gas scenario. The change in ozone mass flux (OMF) into the troposphere is positive throughout the year with maximal increase in the summer months of the respective hemispheres. In the Northern Hemisphere (NH) this summer maximum STE increase is a result of increasing greenhouse gas (GHG) concentrations, whilst in the Southern Hemisphere(SH) it is due to equal contributions from decreasing levels of ozone depleting substances (ODS) and increasing GHG concentrations. In the SH the GHG effect is dominating in the winter months. A large ODS-related ozone increase in the SH stratosphere leads to a change in the seasonal breathing term which results in a future decrease of the OMF into the troposphere in the SH in September and October. The resulting distributions of stratospheric ozone in the troposphere differ for the GHG and ODS changes due to the following: (a) ozone input occurs at different regions for GHG- (midlatitudes) and ODS-changes (high latitudes); and (b) stratospheric ozone is more efficiently mixed towards lower tropospheric levels in the case of ODS changes, whereas tropospheric ozone loss rates grow when GHG concentrations rise. The comparison between the moderate RCP6.0 and the extreme RCP8.5 emission scenarios reveals that the annual global OMF trend is smaller in the moderate scenario, but the resulting change in the contribution of ozone with stratospheric origin (O3s) to ozone in the troposphere is of comparable magnitude in both scenarios. This is due to the larger tropospheric ozone precursor emissions and hence ozone production in the RCP8.5 scenario.

Highlights

IntroductionOzone (O3) in the troposphere has two sources: photochemical production involving ozone precursor species such as nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (e.g., methane (CH4)) and the transport of ozone from the stratosphere into the troposphere (i.e., stratosphere– troposphere exchange, stratosphere–troposphere exchange (STE)) (IPCC, 2001)
Ozone (O3) in the troposphere has two sources: photochemical production involving ozone precursor species such as nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (e.g., methane (CH4)) and the transport of ozone from the stratosphere into the troposphere (IPCC, 2001)
Besides the temperature effect of the greenhouse gases (GHG) on the ozone chemistry, the increasing abundances of CH4 and N2O have an impact on the net production of stratospheric ozone: while higher N2O concentrations are associated with enhanced ozone loss in the stratosphere due to reactive nitrogen, a CH4 increase causes larger ozone loss in the lower and upper stratosphere and leads to increased ozone production in the lower stratosphere where it acts as an ozone precursor (e.g., Revell et al, 2012)

Summary

Introduction

Ozone (O3) in the troposphere has two sources: photochemical production involving ozone precursor species such as nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (e.g., methane (CH4)) and the transport of ozone from the stratosphere into the troposphere (i.e., stratosphere– troposphere exchange, STE) (IPCC, 2001). Mass can be exchanged between the stratosphere and the troposphere along isentropic surfaces which intersect the tropopause in the lowermost stratosphere (LMS) (Holton et al, 1995), where the chemical lifetime of ozone is larger than the transport timescale. Tropopause folds in the vicinity of the polar and the subtropical jets and cutoff lows are important structures for the effective transport of stratospheric air masses into the troposphere because of their large displacement of the tropopause on isentropic surfaces (Stohl et al, 2003). Radiative cooling of the stratosphere associated with the rising concentrations of well-mixed greenhouse gases (GHG) (i.e., carbon dioxide (CO2), nitrous oxide (N2O), and CH4) will lead to reduced ozone loss rates and an ozone increase in the stratosphere (e.g., Jonsson et al, 2004). The combined effect of increasing GHG concentrations (including the interactions between the chemical cycles) on the net stratospheric ozone production is positive (e.g., Meul et al, 2014)

Objectives

Methods

Findings

Conclusion