Past and Present Variations in Atmospheric Hydroxyl Radical Abundance
Hydroxyl radicals (OH) are the main atmospheric oxidant and remove many important greenhouse gases (such as methane) as well as other pollutants (e.g., carbon monoxide, HCFCs) from the atmosphere. Because of this, there is a great deal of interest in understanding the variability in global OH abundance and how it responds to changes in emissions of reactive gases.
Our group currently has two projects that are addressing this important question. The first of these projects is attempting to provide the first robust reconstruction of OH changes between the preindustrial period and today via measurements of carbon-14 of carbon monoxide (14CO) in ice cores. 14CO is produced naturally in the atmosphere by cosmic rays, and is removed by OH. Because 14CO production rates for the preindustrial period are well understood, this presents an opportunity to use the record of past 14CO to examine variations in OH. In November 2018 – February 2019, we successfully obtained (very large!) ice samples from the Law Dome site in Antarctica. This site is special because it has a very high rate of snowfall, which results in rapid burial of the new ice. This rapid burial shields the ice from further 14C production by cosmic rays (which would present an interference to the atmospheric 14CO record). Air was extracted from the ice samples on site at Law Dome and we are now processing the samples and making measurements.
Another project is monitoring modern atmospheric 14CO to improve the understanding of OH spatial and seasonal variability. This project initially collected air samples at the Mauna Loa Observatory in Hawaii, and we have now expanded the measurements to more stations (adding Barrow, Alaska; Mace Head, Ireland; Ragged Point, Barbados; Cape Matatula, American Samoa; Maido, Reunion Island and Baring Head, New Zealand) to achieve global coverage – this was no small feat under COVID restrictions! We are also collaborating with Prof. Lee Murray in our department to use the 14CO results in a state-of-the-art climate-chemistry model to improve our understanding of modern OH.
These projects have been / are being supported by the Packard Foundation and by NSF awards OPP-1643669 and AGS-1920602.
Petrenko, V.V., A.M. Smith, E.M. Crosier, R. Kazemi, P. Place, A. Colton, B. Yang, Q. Hua, L.T. Murray. 2021. An improved method for atmospheric 14CO measurements. Atmospheric Measurement Techniques, 14, 2055–2063. https://amt.copernicus.org/articles/14/2055/2021/.