Abstract
Abstract. We have analysed time-slice simulations from 17 global models, participating in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), to explore changes in present-day (2000) hydroxyl radical (OH) concentration and methane (CH4) lifetime relative to preindustrial times (1850) and to 1980. A comparison of modeled and observation-derived methane and methyl chloroform lifetimes suggests that the present-day global multi-model mean OH concentration is overestimated by 5 to 10% but is within the range of uncertainties. The models consistently simulate higher OH concentrations in the Northern Hemisphere (NH) compared with the Southern Hemisphere (SH) for the present-day (2000; inter-hemispheric ratios of 1.13 to 1.42), in contrast to observation-based approaches which generally indicate higher OH in the SH although uncertainties are large. Evaluation of simulated carbon monoxide (CO) concentrations, the primary sink for OH, against ground-based and satellite observations suggests low biases in the NH that may contribute to the high north–south OH asymmetry in the models. The models vary widely in their regional distribution of present-day OH concentrations (up to 34%). Despite large regional changes, the multi-model global mean (mass-weighted) OH concentration changes little over the past 150 yr, due to concurrent increases in factors that enhance OH (humidity, tropospheric ozone, nitrogen oxide (NOx) emissions, and UV radiation due to decreases in stratospheric ozone), compensated by increases in OH sinks (methane abundance, carbon monoxide and non-methane volatile organic carbon (NMVOC) emissions). The large inter-model diversity in the sign and magnitude of preindustrial to present-day OH changes (ranging from a decrease of 12.7% to an increase of 14.6%) indicate that uncertainty remains in our understanding of the long-term trends in OH and methane lifetime. We show that this diversity is largely explained by the different ratio of the change in global mean tropospheric CO and NOx burdens (ΔCO/ΔNOx, approximately represents changes in OH sinks versus changes in OH sources) in the models, pointing to a need for better constraints on natural precursor emissions and on the chemical mechanisms in the current generation of chemistry-climate models. For the 1980 to 2000 period, we find that climate warming and a slight increase in mean OH (3.5 ± 2.2%) leads to a 4.3 ± 1.9% decrease in the methane lifetime. Analysing sensitivity simulations performed by 10 models, we find that preindustrial to present-day climate change decreased the methane lifetime by about four months, representing a negative feedback on the climate system. Further, we analysed attribution experiments performed by a subset of models relative to 2000 conditions with only one precursor at a time set to 1860 levels. We find that global mean OH increased by 46.4 ± 12.2% in response to preindustrial to present-day anthropogenic NOx emission increases, and decreased by 17.3 ± 2.3%, 7.6 ± 1.5%, and 3.1 ± 3.0% due to methane burden, and anthropogenic CO, and NMVOC emissions increases, respectively.
Highlights
The hydroxyl radical (OH) is the dominant oxidizing agent in the global troposphere as it reacts with most pollutants (Levy, 1971; Logan et al, 1981; Thompson, 1992), thereby controlling their atmospheric abundance and lifetime
Since the atmospheric oxidizing capacity is sensitive to both the OH amount and its spatial distribution (Lawrence et al, 2000), we examine the regional OH distribution in the models as depicted in Fig. 1a showing the multi-model mean OH concentrations in the different atmospheric sub-domains for the 2000 time-slice
We find that intermodel variation in the sign and magnitude of the preindustrial to present-day change in global mean OH, whether positive or negative, correlates strongly with carbon monoxide (CO)/ NOx (Fig. 4; coefficient of determination (r2) = 0.7)
Summary
The hydroxyl radical (OH) is the dominant oxidizing agent in the global troposphere as it reacts with most pollutants (Levy, 1971; Logan et al, 1981; Thompson, 1992), thereby controlling their atmospheric abundance and lifetime. Any changes in OH affect the tropospheric chemical lifetime of methane (CH4), the most abundant organic species in the atmosphere and a potent greenhouse gas, since reaction with OH is the primary mechanism by which it is removed from the atmosphere. Complex series of chemical reactions involving tropospheric ozone (O3), methane, carbon monoxide (CO), non-methane volatile organic compounds (NMVOCs), and nitrogen oxides (NOx = NO + NO2), as well as the levels of solar radiation and humidity, determine the tropospheric abundance of OH (Logan et al, 1981; Thompson, 1992). We analyse results from global chemistry-climate models participating in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP; see www.giss.nasa.gov/projects/ accmip) to investigate the changes in tropospheric OH abundance and its drivers, and methane lifetime over the historical period between 1850 and 2000.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.