Browsing by Author "Murray, LT"

Now showing 1 - 6 of 6
  • Thumbnail Image
    Item
    14-CO in glacial ice from Law Dome, Antarctica as a tracer of changes in atmospheric OH abundance from 1870 AD to present
    (Australian Nuclear Science and Technology Organisation, 2021-11-15) Smith, AM; Neff, PD; Petrenko, VV; Etheridge, DM; Crosier, EM; Hmiel, B; Thornton, DP; Jong, LM; Beaudette, R; Harth, CM; Langenfelds, RL; Mitrevski, B; Curran, MAJ; Buizert, C; Murray, LT; Trudinger, CM; Dyonisius, MN; Ng, J; Severinghaus, JP; Weiss, RF
    Hydroxyl, OH, is the main tropospheric oxidant and determines the lifetime of methane and most other trace gases in the atmosphere, thereby controlling the amount of greenhouse warming produced by these gases. Changes in OH concentration ([OH]) in response to large changes in reactive trace gas emissions (which may occur in the future) are uncertain. Measurements of 14C containing carbon monoxide (14CO) and other tracers such as methyl chloroform over the last ≈25 years have been successfully used to monitor changes in average [OH], but there are no observational constraints on [OH] further back in time. Reconstructions of 14CO from ice cores could in principle provide such constraints but are complicated by in-situ production of 14CO by cosmic rays directly in the ice. Recent work in Antarctica and Greenland shows that this in-situ component would be relatively small and can be accurately corrected for at sites with very high snow accumulation rates. A joint US and Australian team sampled and measured firn air and ice at Law Dome, Antarctica (2018-19 season, site DE08-OH, 1.2 m a-1 ice-equivalent snow accumulation), to a maximum depth of 240 m. Trapped air was extracted from the ice using an onsite large-volume ice melting system. Preliminary comparisons of methane measured in the samples to existing ice core records and atmospheric measurements suggest ice core air sample ages spanning from the 1870s to the early 2000s. Firn-air samples from the snow surface to 81 m depth capture air from the early 2000s to present. Analyses of [CO] and halocarbons in the samples show a relatively low and stable procedural CO blank and demonstrate that the samples are unaffected by ambient air inclusion. 14CO analyses in these firn and ice core air samples have been successfully completed. Corrections for in-situ 14CO production, validated against direct atmospheric measurements for the more recent samples, have allowed us to develop a preliminary 14CO history. This history will be interpreted with the aid of the GEOS-Chem chemistry-transport model to place the first observational constraints on the variability of Southern Hemisphere [OH] since ≈1870 AD. © The Authors
  • Thumbnail Image
    Item
    An improved method for atmospheric 14CO measurements
    (Copernicus Publications, 2021-03-15) Petrenko, VV; Smith, AM; Crosier, EM; Kazemi, R; Place, PF; Colton, A; Yang, B; Hua, Q; Murray, LT
    Important uncertainties remain in our understanding of the spatial and temporal variability of atmospheric hydroxyl radical concentration ([OH]). Carbon-14-containing carbon monoxide (14CO) is a useful tracer that can help in the characterization of [OH] variability. Prior measurements of atmospheric 14CO concentration ([14CO] are limited in both their spatial and temporal extent, partly due to the very large air sample volumes that have been required for measurements (500–1000 L at standard temperature and pressure, L STP) and the difficulty and expense associated with the collection, shipment, and processing of such samples. Here we present a new method that reduces the air sample volume requirement to ≈90 L STP while allowing for [14CO] measurement uncertainties that are on par with or better than prior work (≈3 % or better, 1σ). The method also for the first time includes accurate characterization of the overall procedural [14CO] blank associated with individual samples, which is a key improvement over prior atmospheric 14CO work. The method was used to make measurements of [14CO] at the NOAA Mauna Loa Observatory, Hawaii, USA, between November 2017 and November 2018. The measurements show the expected [14CO] seasonal cycle (lowest in summer) and are in good agreement with prior [14CO] results from another low-latitude site in the Northern Hemisphere. The lowest overall [14CO] uncertainties (2.1 %, 1σ) are achieved for samples that are directly accompanied by procedural blanks and whose mass is increased to ≈50 µgC (micrograms of carbon) prior to the 14C measurement via dilution with a high-CO 14C-depleted gas. © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 Licence.
  • No Thumbnail Available
    Item
    A preliminary record of changes in Southern Hemisphere atmospheric OH abundance from 14CO in glacial ice (Law Dome, Antarctica, 1870 AD to present)
    (American Geophysical Union (AGU), 2021-12-17) Neff, PD; Petrenko, VV; Etheridge, DM; Smith, AM; Crosier, EM; Hmiel, B; Thornton, DP; Jong, LM; Beaudette, R; Harth, CM; Langenfelds, RL; Mitrevski, B; Curran, MAJ; Buizert, C; Murray, LT; Trudinger, CM; Dyonisius, MN; Ng, J; Severinghaus, JP; Weiss, RF
    Hydroxyl, OH, is the main tropospheric oxidant and determines the lifetime of methane and most other trace gases in the atmosphere, thereby controlling the amount of greenhouse warming produced by these gases. Changes in OH concentration ([OH]) in response to large changes in reactive trace gas emissions (which may occur in the future) are uncertain. Measurements of 14C-containing carbon monoxide (14CO) and other tracers such as methyl chloroform over the last ≈25 years have been successfully used to monitor changes in average [OH], but there are no observational constraints on [OH] further back in time. Reconstructions of 14CO from ice cores at sites with very high snow accumulation rates can provide such constraints, as rapid snow burial limits in-situ production of 14CO by cosmic rays directly in the ice. A joint US and Australian team sampled and measured firn air and ice at Law Dome, Antarctica (2018-19 season, site DE08-OH, 1.2 m a-1 ice-equivalent snow accumulation), to a maximum depth of 240 m. Trapped air was extracted from the ice using an on-site large-volume ice melting system. Preliminary comparisons of methane measured in the samples to existing ice core records and atmospheric measurements suggest ice core air sample ages spanning from the 1870s to the early 2000s. Firn-air samples from the snow surface to 81 m depth capture air from the early 2000s to present. Analyses of [CO] and halocarbons in the samples show a relatively low and stable procedural CO blank and demonstrate that the samples are unaffected by ambient air inclusion. 14CO analyses in these firn and ice core air samples have been successfully completed. Corrections for in-situ 14CO production, validated against direct atmospheric measurements for the more recent samples, have allowed us to develop a preliminary 14CO history. This history will be interpreted with the aid of the GEOS-Chem chemistry-transport model to place the first observational constraints on the variability of Southern Hemisphere [OH] since ≈1870 AD.
  • No Thumbnail Available
    Item
    Using atmospheric 14CO to provide additional constraints for global OH: first results from a new approach and potential for future measurements
    (American Geophysical Union, 2018-12-13) Petrenko, VV; Murray, LT; Smith, AM; Crosier, EM; Colton, A; Hua, Q; Yang, B; Kazemi, R; Usoskin, IG; Poluianov, S
    The primary source of 14C-containing carbon monoxide (14CO) in the atmosphere is via 14C production from 14N by secondary cosmic rays, and the primary sink is removal by hydroxyl radicals (OH). Variations in the global abundance of 14CO that are not explained by variations in 14C production are mainly driven by variations in the global abundance of OH. Monitoring OH variability via methyl chloroform is becoming increasingly difficult as methyl chloroform abundance is continuing to decline. Measurements of atmospheric 14CO have previously been successfully used to infer OH variability. However, these measurements are currently only continuing at one location (Baring Head, New Zealand), which is insufficient to infer global trends. We propose to restart global 14CO monitoring with the aim of providing an additional constraint on OH variability. A new analytical system for 14CO sampling and measurements has been developed, allowing for a ten-fold reduction in the required sample air volumes and simplified field logistics. The first 14CO measurements from Mauna Loa Observatory show good agreement with prior measurements in the same latitude band. Preliminary work with a state-of-the-art chemical transport model is exploring sensitivity of 14CO at potential sampling locations to changes in production rates and OH.
  • Thumbnail Image
    Item
    Using atmospheric 14CO to provide additional constraints for global OH: results from a new approach and potential for future measurements
    (Copernicus GmbH, 2019-04-10) Petrenko, VV; Murray, LT; Crosier, EM; Colton, A; Hua, Q; Smith, AM; Yang, B; Kazemi, R; Neff, PD; Etheridge, DM; Usoskin, IG; Poluianov, S
    The primary source of 14C-containing carbon monoxide (14CO) in the atmosphere is via 14C production from 14N by secondary cosmic rays, and the primary sink is removal by hydroxyl radicals (OH). Variations in the global abundance of 14CO that are not explained by variations in 14C production are mainly driven by variations in the global abundance of OH. Monitoring OH variability via methyl chloroform is becoming increasingly difficult as methyl chloroform abundance is continuing to decline. Measurements of atmospheric 14CO have previously been successfully used to infer OH variability. However, these measurements have only continued at one location (Baring Head, New Zealand), which is insufficient to infer global trends. We propose to restart global 14CO monitoring with the aim of providing an additional constraint on OH variability. A new analytical system for 14CO sampling and measurements has been developed, allowing for a ten-fold reduction in the required sample air volumes and simplified field logistics. The first 14CO measurements from Mauna Loa Observatory show good agreement with prior measurements in the same latitude band. Preliminary work with a state-of-the-art chemical transport model is exploring sensitivity of 14CO at potential sampling locations to changes in production rates and OH. This presentation will also provide an update on a project which aims to improve the understanding of long-term OH variability via reconstructing a 150-year history of atmospheric 14CO from ice cores at Law Dome, Antarctica. Sampling of the ice and on-site extractions of large volumes of ancient air were in progress during December 2018 – January 2019. © Author(s) 2019. CC Attribution 4.0 license
  • No Thumbnail Available
    Item
    Using measurements of atmospheric 14CO in a global network to improve understanding of OH spatial and temporal variability
    (American Geophysical Union (AGU), 2022-12-16) Petrenko, VV; Crosier, EM; Smith, AM; Yang, B; Scholer, M; McCrea, K; Murray, LT; Colton, A; Thomas, B; Talamoa, G; Musick, R; Schaefer, H; Moss, RC; Spain, G; Yann, H; Hernandez, P; Blades, E; Chewitt-Lucas, RM; Kazemi, R; Stock, MP
    The primary source of 14C-containing carbon monoxide (14CO) in the atmosphere is via 14C production from 14N by secondary cosmic rays, and the primary sink is removal by hydroxyl radicals (OH). Variations in the abundance of 14CO that are not explained by variations in 14CO production or transport are mainly driven by variations in the abundance of OH. Because of its relatively short atmospheric lifetime (≈2 months on average), 14CO abundance is sensitive to spatial and seasonal OH variability. 14CO measurements in a new global network were made for one calendar year in 2021. Simulations in the GEOS-Chem chemical transport model (CTM) indicate that our 14CO network has good sensitivity to variations in both regional and global OH. In this presentation, we will show the new measurements as well as the CTM results available to-date.