Mass-Independent Oxygen Isotope Fractionation in Atmospheric CO as a Result of the Reaction CO + OH

Atmospheric carbon monoxide (CO) exhibits mass-independent fractionation in the oxygen isotopes. An $^{17}$O excess up to 7.5 per mil was observed in summer at high northern latitudes. The major source of this puzzling fractionation in this important trace gas is its dominant atmospheric removal reaction, CO + OH → CO$_2$ + H, in which the surviving CO gains excess $^{17}$O. The occurrence of mass-independent fractionation in the reaction of CO with OH raises fundamental questions about kinetic processes. At the same time the effect is a useful marker for the degree to which CO in the atmosphere has been reacting with OH.