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

Abstract 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 13 CO and C 18 O undergo isotopic fractionation during its main chemical loss reaction, CO + OH•. The kinetic isotope effect (KIE) for 13 CO is mass dependent, with a value of ∼5‰; 12 CO reacts faster than 13 CO with OH. Whereas C 18 O + OH• exhibits an inversely mass dependent KIE ∼−10‰. We hypothesize these KIEs result in a relative depletion of 13 C 18 O, 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, Δ 31 , relative to a random distribution of 13 C and 18 O in CO. A clear negative trend between [CO] and Δ 31 is driven by two factors: (a) the atmospheric addition of CO from either a primary or secondary source with a Δ 31 of ∼0‰ and (b) the continuing reaction of CO with OH•, leaving the remaining CO pool relatively depleted in 13 C 18 O. This is analogous to the mechanism that determines CO Δ 17 O 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 Δ 31 observations motivate future experimental and observational studies targeted at characterizing the clumped isotopes of CO sources, background CO, and experimentally fractionated CO.