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 several projects that are addressing this important question. The first set of 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 processed and measured for 14CO. We are now in the process of interpreting these results. We are also preparing for a Greenland ice coring campaign in 2026 that would provide a past 14CO record and information about past OH for the Northern Hemisphere.
Another set of projects, including the recently initiated large international collaboration called , 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; Izaña, Spain; Ragged Point, Barbados; Cape Matatula, American Samoa; Maido, Reunion Island and Baring Head, New Zealand). We are collaborating with Prof. Lee Murray's research group in our department to use the 14CO measurements from these projects in a state-of-the-art climate-chemistry model to improve our understanding of past and modern OH.
These projects have been or are being supported by and by NSF awards OPP-1643669 and AGS-1920602, by Schmidt Sciences and by the David and Lucile Packard Foundation.
Related Publications:
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.