Large, Negative Atmospheric Carbon Monoxide Clumped Isotope Values Result From Kinetic Isotope Fractionation, Tracing OH• Reactivity

Because of its global abundance and reactivity with hydroxyl radicals (OH•), tropospheric carbon monoxide indirectly impacts the lifetimes of other OH•-reactive gases, in particular methane and reactive hydrocarbons. The origin and chemistry of atmospheric CO have been studied using stable isotopes. Both ¹³CO and C¹⁸O undergo isotopic fractionation during its main chemical loss reaction, CO + OH•. The kinetic isotope effect (KIE) for ¹³CO is mass dependent, with a value of ∼5‰; ¹²CO reacts faster than ¹³CO with OH. Whereas C¹⁸O + OH• exhibits an inversely mass dependent KIE ∼−10‰. We hypothesize these KIEs result in a relative depletion of ¹³C¹⁸O, a CO clumped isotope. To test this, we collected CO from air samples on Long Island, NY, and discovered a −3 to −8‰ difference in the clumped isotope ratio, Δ₃₁, relative to a random distribution of ¹³C and ¹⁸O in CO. A clear negative trend between [CO] and Δ₃₁ is driven by two factors: (a) the atmospheric addition of CO from either a primary or secondary source with a Δ₃₁ of ∼0‰ and (b) the continuing reaction of CO with OH•, leaving the remaining CO pool relatively depleted in ¹³C¹⁸O. This is analogous to the mechanism that determines CO Δ¹⁷O values. This study is among the first to show clumped isotope fractionation resulting from atmospheric chemistry and not thermal equilibration, which may inform the identification of clumped isotope KIEs in other atmospheric trace gases. These first Δ₃₁ observations motivate future experimental and observational studies targeted at characterizing the clumped isotopes of CO sources, background CO, and experimentally fractionated CO.