What do ¹⁴CO measurements tell us about OH?

M. C. Krol (1, 2, 3) , J. Fokke Meirink (2) , P. Bergamaschi (4) , J. E. Mak (5) , D. Lowe (6) , P. Jöckel (7) , S. Houweling (1, 2) , T. Röckmann (2)

1 SRON - SRON Netherlands Institute for Space Research
2 IMAU - Institute for Marine and Atmospheric Research [Utrecht]
3 WUR - Wageningen University and Research [Wageningen]
4 IES - JRC Institute for Environment and Sustainability
5 ITPA - Institute for Terrestrial and Planetary Atmospheres [Stony Brook]
6 NIWA - National Institute of Water and Atmospheric Research [Wellington]
7 MPIC - Max Planck Institute for Chemistry

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.

Domaines: Océan, Atmosphère

M. C. Krol, J. Fokke Meirink, P. Bergamaschi, J. E. Mak, D. Lowe, et al.. What do ¹⁴CO measurements tell us about OH?. Atmospheric Chemistry and Physics Discussions, 2007, 7 (4), pp.10405-10438. ⟨hal-00302982⟩