Abstract
Abstract. Results from simulations performed for the Atmospheric Chemistry and Climate Modeling Intercomparison Project (ACCMIP) are analysed to examine how OH and methane lifetime may change from present day to the future, under different climate and emissions scenarios. Present day (2000) mean tropospheric chemical lifetime derived from the ACCMIP multi-model mean is 9.8 ± 1.6 yr (9.3 ± 0.9 yr when only including selected models), lower than a recent observationally-based estimate, but with a similar range to previous multi-model estimates. Future model projections are based on the four Representative Concentration Pathways (RCPs), and the results also exhibit a large range. Decreases in global methane lifetime of 4.5 ± 9.1% are simulated for the scenario with lowest radiative forcing by 2100 (RCP 2.6), while increases of 8.5 ± 10.4% are simulated for the scenario with highest radiative forcing (RCP 8.5). In this scenario, the key driver of the evolution of OH and methane lifetime is methane itself, since its concentration more than doubles by 2100 and it consumes much of the OH that exists in the troposphere. Stratospheric ozone recovery, which drives tropospheric OH decreases through photolysis modifications, also plays a partial role. In the other scenarios, where methane changes are less drastic, the interplay between various competing drivers leads to smaller and more diverse OH and methane lifetime responses, which are difficult to attribute. For all scenarios, regional OH changes are even more variable, with the most robust feature being the large decreases over the remote oceans in RCP8.5. Through a regression analysis, we suggest that differences in emissions of non-methane volatile organic compounds and in the simulation of photolysis rates may be the main factors causing the differences in simulated present day OH and methane lifetime. Diversity in predicted changes between present day and future OH was found to be associated more strongly with differences in modelled temperature and stratospheric ozone changes. Finally, through perturbation experiments we calculated an OH feedback factor (F) of 1.24 from present day conditions (1.50 from 2100 RCP8.5 conditions) and a climate feedback on methane lifetime of 0.33 ± 0.13 yr K−1, on average. Models that did not include interactive stratospheric ozone effects on photolysis showed a stronger sensitivity to climate, as they did not account for negative effects of climate-driven stratospheric ozone recovery on tropospheric OH, which would have partly offset the overall OH/methane lifetime response to climate change.
Highlights
Oxidation processes remove a range of environmentally important species from the atmosphere
Present day (2000) mean tropospheric chemical lifetime derived from the ACCMIP multi-model mean is 9.8 ± 1.6 yr (9.3 ± 0.9 yr when only including selected models)
RCP8.5 is the only scenario in which nitrogen oxides (NOx) emissions and global OH are related in terms of sign of change
Summary
Oxidation processes remove a range of environmentally important species from the atmosphere. Tropospheric oxidation heavily depends on the levels of the hydroxyl radical (OH) and its geographical distribution (Levy, 1971). Oxidation by OH is the primary loss mechanism for methane (CH4), the second most important anthropogenic greenhouse gas in the climate system (Forster et al, 2007; Shindell et al, 2009), and an important precursor of tropospheric ozone (O3) (Logan et al, 1981). Besides its role in methane oxidation, OH is involved in removing trace gases from the atmosphere, through oxidation of atmospheric pollutants such as nitrogen oxides (NOx), carbon monoxide (CO) and non-methane volatile organic compounds (NMVOCs), and removes ozone-depleting substances such as hydrofluorocarbons (HFCs) from the atmosphere (DeMore et al, 1996). OH participates in the formation of atmospheric aerosols such as sulfate, nitrate and secondary organics OH participates in the formation of atmospheric aerosols such as sulfate, nitrate and secondary organics (e.g. Koch et al, 2006)
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