What can ¹⁴CO measurements tell us about OH?

Abstract. The possible use of ¹⁴CO measurements to constrain hydroxyl radical (OH) concentrations in the atmosphere is investigated. ¹⁴CO is mainly produced in the upper atmosphere from cosmic radiation. Measurements of ¹⁴CO at the surface show lower concentrations compared to the upper atmospheric source region, which is the result of oxidation by OH. In this paper, the sensitivity of ¹⁴CO mixing ratio surface measurements to the 3-D OH distribution is assessed with the TM5 model. Simulated ¹⁴CO mixing ratios agree within a few molecules ¹⁴CO cm⁻³ (STP) with existing measurements at five locations worldwide. The simulated cosmogenic ¹⁴CO distribution appears mainly sensitive to the assumed upper atmospheric ¹⁴C source function, and to a lesser extend to model resolution. As a next step, the sensitivity of ¹⁴CO measurements to OH is calculated with the adjoint TM5 model. The results indicate that ¹⁴CO measurements taken in the tropics are sensitive to OH in a spatially confined region that varies strongly over time due to meteorological variability. Given measurements with an accuracy of 0.5 molecules ¹⁴CO cm⁻³ STP, a good characterization of the cosmogenic ¹⁴CO fraction, and assuming perfect transport modeling, a single ¹⁴CO measurement may constrain OH to 0.2–0.3×10⁶ molecules OH cm⁻³ on time scales of 6 months and spatial scales of 70×70 degrees (latitude×longitude) between the surface and 500 hPa. The sensitivity of ¹⁴CO measurements to high latitude OH is about a factor of five higher. This is in contrast with methyl chloroform (MCF) measurements, which show the highest sensitivity to tropical OH, mainly due to the temperature dependent rate constant of the MCF–OH reaction. A logical next step will be the analysis of existing ¹⁴CO measurements in an inverse modeling framework. This paper presents the required mathematical framework for such an analysis.