Past Variations in Carbon Monoxide and Atmospheric Chemistry
Chemical and photo-chemical processes in the atmosphere control the lifetimes of most atmospheric trace species (e.g., methane, aerosols, ozone-destroying CFCs) and have a profound influence on the Earth’s climate system. Current understanding of atmospheric chemistry is imperfect, and state-of-the-art models need realistic training data sets that describe atmospheric conditions that are very different from today’s, such as the pre-industrial atmosphere.
One aspect of our research focuses on reconstructing past concentrations and stable isotopic composition of carbon monoxide (CO). CO is a reactive trace gas that plays a key role in atmospheric chemistry by being the main sink of tropospheric hydroxyl radicals (OH). OH is the main atmospheric oxidant and is responsible for removal of most atmospheric trace species. Because of its strong interaction with OH, CO is a must-have component of any data set that attempts to fully characterize the chemical state of the atmosphere. Our recent work succeeded for the first time in providing a reliable record of CO concentrations for the last 60 years for the Northern Hemisphere. Our reconstructions showed that CO was actually slightly higher in the Northern Hemisphere in 1950 than it is today. CO peaked around 1980, with a subsequent strong decrease. This CO trend was driven mainly by CO emissions from fossil fuel combustion, and the large post-1980 CO decrease was driven by the widespread introduction of catalytic converters in road transportation. Our CO reconstruction was in strong disagreement with predictions from a state-of-the-art climate-chemistry model, pointing out important errors in the modeling work.
We are continuing our work on past CO in the Northern Hemisphere, with the hope of extending the CO concentration and stable isotope records to ≈1750 AD, before the start of the Industrial Period. We are also studying CO in in Antarctic ice with the aim to explore CO changes in the natural atmosphere as far back in time as 20,000 years.
This research direction is being supported by NSF Polar awards ARC-1406236 and PLR-1443267, as well as by the Packard Foundation.
Helmig, D., V.V. Petrenko, P. Martinerie, E. Witrant, T. Roeckmann, A. Zuiderweg, R. Holzinger, J. Hueber, C. Stephens, J. White, W. Sturges, A. Baker, T. Blunier, D. Etheridge, M. Rubino and P. Tans. 2014. Reconstruction of Northern Hemisphere 1950 – 2010 atmospheric non-methane hydrocarbons. Atmospheric Chemistry and Physics, 14, 1463–1483. http://www.atmos-chem-phys.net/14/1463/2014/
Petrenko, V.V., P. Martinerie, P. Novelli, D. M. Etheridge, I. Levin, Z. Wang, G. Petron, T. Blunier, J. Chappellaz, J. Kaiser, P. Lang, L. P. Steele, F. Vogel, M. A. Leist, J. Mak, R. L. Langenfelds, J. Schwander, J. P. Severinghaus, G. Forster, W. Sturges, M. Rubino, J.W.C. White. 2013. A 60 yr record of atmospheric carbon monoxide reconstructed from Greenland firn air. Atmospheric Chemistry and Physics, 13, 7567 - 7585. http://www.atmos-chem-phys.net/13/7567/2013/
Wang, Z., J. Chappellaz, P. Martinerie, K. Park, V.V. Petrenko, E. Witrant, T. Blunier, C. A. M. Brenninkmeijer, J. E. Mak. 2012. The isotopic record of Northern Hemisphere atmospheric carbon monoxide since 1950, implications for the CO budget. Atmospheric Chemistry and Physics, 12, 4365–4377. http://www.atmos-chem-phys.net/12/4365/2012/
Reconstructing Past OH Concentrations with 14CO
Our research into cosmogenic 14C production in glacial firn and ice allows us to better isolate desired atmospheric 14C signals, which can provide valuable information about past atmospheric chemistry and climate dynamics.
Atmospheric Carbon-14 of carbon monoxide, or 14CO, is produced mainly by cosmic rays and largely (~90%) removed in the troposphere by hydroxyl radicals (OH). Because we know the past cosmic ray flux well from sunspot observations over the last several hundred years, we can use 14CO to constrain past OH concentrations.
To minimize the effect of complicating in-situ cosmogenic production of 14CO in glacial ice, we hope to work collaboratively with Australian scientists and travel to a high snow-accumulation site (1.2 meters per year, ice equivalent) known as DE-08 on Law Dome in Antarctica. The uniquely-high snowfall rate at this site rapidly buries snow and ice layers, shielding them from in-situ cosmogenic 14CO production.
Map of Preciptation at Law Dome, Antarctica
Additionally, such quick snow burial results in relatively “young” air being trapped between the lock-in (73 m) and close-off (83 m) depths at bottom of the firn column. This air is the same age as atmospheric 14CO measurements taken at Baring Head, New Zealand and Scott Base, Antarctica, allowing for a direct determination of the atmospheric versus cosmogenic component of 14CO.
Retrieving ice cores to a depth of ~220 m, we plan to provide the first observations of atmospheric OH concentration since ~1880 AD. This is important because OH concentration controls the lifetime of atmospheric species including greenhouse gases (e.g. methane), aerosols, and some ozone-depleting substances, all with implications for radiative forcing.