Clumped CO Isotopes – New Tracers for Atmospheric Chemistry

Editors’ Highlights are summaries of recent papers by AGU’s journal editors.

Source: AGU Advances

The main sources of atmospheric carbon monoxide (CO) are incomplete combustion of organic or fossil fuels and the oxidation of atmospheric methane, while the main sink is oxidation to carbon dioxide (CO₂) by reaction with hydroxyl radicals (OH). By influencing the concentration of OH, CO impacts greenhouse gases like methane and tropospheric ozone. Thus, understanding how sources and OH reaction rates control CO concentrations is fundamental for atmospheric chemistry.

To date, investigations using stable isotopes have been limited to singly substituted (¹³C,¹⁸O) constituents. Henkes et al. [2024] develop and test a clever method to measure the ‘clumped’ isotope species ¹³C¹⁸O (mass 31). They report the first atmospheric measurements using samples collected near New York during 2019-2022. These indicate the kinetic isotope effect (KIE) for ¹³C¹⁸O is predictable from separately measured KIE for reaction of OH with ¹³C¹⁶O and ¹²C¹⁸O. The atmospheric lifetime of CO averages a few months – thus regions far from emissions sources would be expected to have low CO concentrations and Δ₃₁ values reflecting the degree of chemical ‘aging’. Regions with high fossil fuel emissions such as the Northeastern United States reflect both mixing of this aged ‘background’ with local CO sources having Δ₃₁ of about 0.

To better isolate mixing from OH reaction effects, the authors propose new measurements in the southern hemisphere. Importantly, the clumped isotope approaches developed here for CO can and likely will be applied to study other atmospheric gases.

Citation: Henkes, G. A., Place, P. F., & Mak, J. E. (2024). Large, negative atmospheric carbon monoxide clumped isotope values result from kinetic isotope fractionation, tracing OH• reactivity. AGU Advances, 5, e2023AV000922. https://doi.org/10.1029/2023AV000922

—Susan Trumbore, Editor-in-Chief, AGU Advances