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. During transport to measurement locations at the Earth's surface 14CO is oxidized by OH. In this paper, the sensitivity of 14CO mixing ratio measurements to the 3-D OH distribution is assessed with the TM5 model. Simulated 14CO mixing ratios compare reasonably well with atmospheric measurements taken at five locations worldwide. As a next step, the sensitivity of 14CO measurements to OH is calculated with the adjoint TM5 model. For our sensitivity calculations the adjoint methodology outlined in the paper offers computational advantages compared to forward model calculations. The results indicate that 14CO measurements, especially those taken in the tropics, are sensitive to OH in a spatially confined region. Moreover, the OH sensitivity at a certain location varies strongly over time due to meteorological variability. On average, 14CO measurements are about 5 times more sensitive to OH at high latitudes than to OH in the tropics. Moreover, the measurements are sensitive to OH in the main 14CO source region in the upper atmosphere. It will therefore be difficult to assign model-measurement discrepancies to either the 14CO source uncertainty or to the OH sink. Nevertheless, the large distance between the 14CO source region and the measurement locations should allow the retrieval of information on OH. Specifically, the sensitivity to OH in the lower atmosphere during a relatively short time span may offer the possibility to constrain local OH. These efforts will strongly depend on the number of measurements available and on our ability to accurately model the 14CO transport.