A large, negative clumped isotope ratio observed in atmospheric carbon monoxide
Abstract
Carbon monoxide (CO) is an important gas in the chemistry of the troposphere, principally because of the reaction between CO and hydroxyl radicals (OH). CO is also involved in atmospheric methanecycling, both directly by reaction and indirectly by controlling hydroxyl radical concentration (which readily oxidizes CH4). The C and O stable isotope ratios of CO are controlled by the oxidation of hydrocarbons (including methane), biomass burning, fossil fuel combustion, and the CO + OH reaction. Isotope fractionation associated with the CO+OH reaction is controlled by kinetics and is noteworthy because the kinetic isotope effect differs for C and O isotopes; the KIE is mass dependent for 13CO (13k/12k<1) but inverse mass dependent for C18O (18k/16k>1). A model of this reaction for all CO isotopologues predicts large, negative excursions in the isotope ratios for 13C18O as [CO+OH] continues, forming a basis for interpreting measurements of 13C18O in the atmosphere. We have collected CO from air at Stony Brook University on Long Island, NY, and oxidized that gas to CO2 using Schütze reagent on a vacuum extraction manifold. The sample workflow and oxidation has been characterized by analysis of CO equilibrated at 650 to 1000 C over Pt wire catalyst and by generating CO from the decomposition of calcite in the presence of Zn metal, yielding D47 values that are indistinguishable from theoretical predictions of CO clumped isotope temperature dependence. Air CO collected during the 2019-2020 winter had typical d13C and d 18O values, but D47 values that ranged from -2.36 to -2.08 per mil. There was a positive trend in [CO] and D47 that scales with our CO + OH modeling, thus we contend that the magnitude CO `anti-clumping' scales with the extent of atmospheric oxidation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMGC0380010H
- Keywords:
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- 3305 Climate change and variability;
- ATMOSPHERIC PROCESSES;
- 3339 Ocean/atmosphere interactions;
- ATMOSPHERIC PROCESSES;
- 3362 Stratosphere/troposphere interactions;
- ATMOSPHERIC PROCESSES;
- 0454 Isotopic composition and chemistry;
- BIOGEOSCIENCES