Tropospheric OH and stratospheric OH and Cl concentrations determined from CH4, CH3Cl, and SF6 measurements

Mengze Li,Einar Karu,Carl Brenninkmeijer,Jonathan Williams,Horst Fischer,Jos Lelieveld

doi:10.1038/s41612-018-0041-9

Mengze Li, Einar Karu + Show 4 more

Open Access

https://doi.org/10.1038/s41612-018-0041-9

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Abstract

The hydroxyl (OH) radical is the key oxidant in the global atmosphere as it controls the concentrations of toxic gases like carbon monoxide and climate relevant gases like methane. In some regions, oxidation by chlorine (Cl) radical is also important, and in the stratosphere both OH and Cl radicals impact ozone. An empirical method is presented to determine effective OH concentrations in the troposphere and lower stratosphere, based on CH4, CH3Cl, and SF6 data from aircraft measurements (IAGOS-CARIBIC) and a ground-based station (NOAA). Tropospheric OH average values of 10.9 × 105 (σ = 9.6 × 105) molecules cm−3 and stratospheric OH average values of 1.1 × 105 (σ = 0.8 × 105) molecules cm−3 were derived over mean ages derived from SF6. Using CH4 led to higher OH estimates due to the temperature dependence of the CH4 + OH reaction in the troposphere and due to the presence of Cl in the stratosphere. Exploiting the difference in effective OH calculated from CH3Cl and CH4 we determine the main altitude for tropospheric CH4 oxidation to be 4.5 ~ 10.5 km and the average Cl radical concentration in the lower stratosphere to be 1.1 × 104 (σ = 0.6 × 104) molecules cm−3 (with a 35% measurement uncertainty). Furthermore, the data are used to examine the temporal trend in annual average stratospheric OH and Cl radical concentrations between 2010 and 2015. The year 2013 showed highest stratospheric OH and lowest Cl but no clear temporal trend was observed in the data in this period. These data serve as a baseline for future studies of stratospheric circulation changes.

Highlights

The hydroxyl radical (OH) is the most important oxidant in the troposphere and lower stratosphere
We have developed and applied an empirical databased method to estimate the “effective OH concentration” that has acted in the troposphere and lower stratosphere over longer timescales between 2008–2015
The rationale is that an air sample collected in the upper troposphere—lower stratosphere (UTLS) region can be regarded as a mixture of two major largescale airflows.[26,27,28]

Summary

Introduction

The hydroxyl radical (OH) is the most important oxidant in the troposphere and lower stratosphere. It initiates removal from the atmosphere of toxic gases such as carbon monoxide (CO), radiatively active gases such as methane (CH4), tropospheric ozone precursors such as volatile organic compounds (VOCs), and NOx (NO + NO2), and stratospheric ozone-depleting compounds such as hydrochlorofluorocarbons (HCFCs).[1,2] it plays a key role in the atmospheric oxidation capacity, air quality, and climate.[3]. One source of atmospheric OH is the reaction of O1D, a minor product in the photolysis of ozone, with H2O.4,5. An even larger source in terms of gross OH formation is recycling from its reaction products, which maintains the atmospheric oxidation efficiency.[6]. These in-situ measurements are broadly consistent with regional scale indirect

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