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

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 CH 4 , CH 3 Cl, and SF 6 data from aircraft measurements (IAGOS-CARIBIC) and a ground-based station (NOAA). Tropospheric OH average values of 10.9 × 10 5 ( σ = 9.6 × 10 5 ) molecules cm −3 and stratospheric OH average values of 1.1 × 10 5 ( σ = 0.8 × 10 5 ) molecules cm −3 were derived over mean ages derived from SF 6 . Using CH 4 led to higher OH estimates due to the temperature dependence of the CH 4 + OH reaction in the troposphere and due to the presence of Cl in the stratosphere. Exploiting the difference in effective OH calculated from CH 3 Cl and CH 4 we determine the main altitude for tropospheric CH 4 oxidation to be 4.5 ~ 10.5 km and the average Cl radical concentration in the lower stratosphere to be 1.1 × 10 4 ( σ = 0.6 × 10 4 ) 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. Atmospheric measurements: OH radical where art thou? A method to ascertain the effective concentration of OH in certain regions of the atmosphere by measuring ‘‘proxy’’ species has been developed. Knowing how much of the radical OH is present in the atmosphere is important because of its role in breaking down polluting gases such as CO, NO x and volatile organic compounds. Now, a team led by Jonathan Williams at the Max Planck Institute for Chemistry, in Mainz, Germany, has shown how long-term aircraft and ground-based measurements of CH 4 , CH 3 Cl and SF 6 can be used to determine the effective concentration of OH in the troposphere and the stratosphere. The method is used to estimate the OH concentration over the time period 2008–2015 and—as long as the same ‘‘proxy’’ gases are measured—will allow the impact of future atmospheric events to be assessed.