Trends in global tropospheric hydroxyl radical and methane lifetime since 1850 from AerChemMIP

David S Stevenson,Guang Zeng,Louisa Emmons,Alcide Zhao,Paul T Griffiths,Sungbo Shim,Lori T Sentman,Vaishali Naik,Fiona M O'Connor,Lee T Murray,Simone Tilmes,William J Collins,Larry W Horowitz

doi:10.5194/acp-20-12905-2020

Abstract

Abstract. We analyse historical (1850–2014) atmospheric hydroxyl (OH) and methane lifetime data from Coupled Model Intercomparison Project Phase 6 (CMIP6)/Aerosols and Chemistry Model Intercomparison Project (AerChemMIP) simulations. Tropospheric OH changed little from 1850 up to around 1980, then increased by around 9 % up to 2014, with an associated reduction in methane lifetime. The model-derived OH trends from 1980 to 2005 are broadly consistent with trends estimated by several studies that infer OH from inversions of methyl chloroform and associated measurements; most inversion studies indicate decreases in OH since 2005. However, the model results fall within observational uncertainty ranges. The upward trend in modelled OH since 1980 was mainly driven by changes in anthropogenic near-term climate forcer emissions (increases in anthropogenic nitrogen oxides and decreases in CO). Increases in halocarbon emissions since 1950 have made a small contribution to the increase in OH, whilst increases in aerosol-related emissions have slightly reduced OH. Halocarbon emissions have dramatically reduced the stratospheric methane lifetime by about 15 %–40 %; most previous studies assumed a fixed stratospheric lifetime. Whilst the main driver of atmospheric methane increases since 1850 is emissions of methane itself, increased ozone precursor emissions have significantly modulated (in general reduced) methane trends. Halocarbon and aerosol emissions are found to have relatively small contributions to methane trends. These experiments do not isolate the effects of climate change on OH and methane evolution; however, we calculate residual terms that are due to the combined effects of climate change and non-linear interactions between drivers. These residual terms indicate that non-linear interactions are important and differ between the two methodologies we use for quantifying OH and methane drivers. All these factors need to be considered in order to fully explain OH and methane trends since 1850; these factors will also be important for future trends.

Highlights

The hydroxyl radical (OH) is a highly reactive, and very short-lived, component of the Earth’s atmosphere that lies at the heart of atmospheric chemistry
This study presents results from multiple transient 1850– 2014 simulations performed for Coupled Model Intercomparison Project Phase 6 (CMIP6) (Eyring et al, 2016) and the associated AerChemMIP (Collins et al, 2017), and it is organized as follows
We find very similar results between the fully coupled and the atmosphereonly experiments

Summary

Introduction

The hydroxyl radical (OH) is a highly reactive, and very short-lived, component of the Earth’s atmosphere that lies at the heart of atmospheric chemistry. It is often referred to as the cleansing agent of the atmosphere, as it is the main oxidant of many important trace gases, including methane (CH4), carbon monoxide (CO), and nonmethane volatile organic compounds (NMVOCs). Holmes et al, 2013; Turner et al, 2019). Hydroxyl controls the removal rates of these species and their atmospheric residence times Because of this key role in determining the trace gas composition of the atmosphere, it is important to understand what controls OH’s global distribution, its temporal evolution, and drivers of changes Oxidation of CO and CH4 (and other NMVOCs) consumes OH and generates HO2 and RO2: CO + OH(+O2) → CO2 + HO2,

Methods

Results

Conclusion