What can (CO)-C-14 measurements tell us about OH?

The possible use of (CO)-C-14 measurements to constrain hydroxyl radical (OH) concentrations in the atmosphere is investigated. (CO)-C-14 is mainly produced in the upper atmosphere from cosmic radiation. Measurements of (CO)-C-14 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 (CO)-C-14 mixing ratio surface measurements to the 3-D OH distribution is assessed with the TM5 model. Simulated (CO)-C-14 mixing ratios agree within a few molecules (CO)-C-14 cm(-3) (STP) with existing measurements at five locations worldwide. The simulated cosmogenic (CO)-C-14 distribution appears mainly sensitive to the assumed upper atmospheric C-14 source function, and to a lesser extend to model resolution. As a next step, the sensitivity of (CO)-C-14 measurements to OH is calculated with the adjoint TM5 model. The results indicate that (CO)-C-14 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 (CO)-C-14 cm(-3) STP, a good characterization of the cosmogenic (CO)-C-14 fraction, and assuming perfect transport modeling, a single (CO)-C-14 measurement may constrain OH to 0.2-0.3 x 10(6) molecules OH cm(-3) on time scales of 6 months and spatial scales of 70 x 70 degrees (latitude x longitude) between the surface and 500 hPa. The sensitivity of (CO)-C-14 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,