Ozonolysis of nonmethane hydrocarbons as a source of the observed mass independent oxygen isotope enrichment in tropospheric CO

Combined 17 O and 18 O measurements of tropospheric CO from two northern hemisphere (NH) sites reveal systematically enhanced 17 O levels, confirming the existence of significant mass independent oxygen isotope enrichment in this important trace gas. When CO levels increase in the NH winter, the mass independent enrichment decreases proportionally. A possible source of this rare isotope effect in CO is transfer of the mass independent enrichment from O 3 to the CO pool via ozonolysis of unsaturated hydrocarbons such as isoprene and terpenes. Laboratory ozonolysis experiments indeed confirm that this process is a potentially important source of mass independently enriched CO. The extra 17 O enrichment found in ozonolysis‐derived CO is similar to the mass independent fractionation measured on ozone. If ozonolysis is the only source of mass independent enrichment in CO, it is estimated that about 10% of all CO in middle to high latitude winter, at ground level, originates from the ozonolysis of unsaturated hydrocarbons. While the laboratory experiments prove that ozonolysis is a source of mass independent enrichment in CO, a further problem unfolds. Because tropospheric O 3 is strongly enriched in 18 O as well, this enrichment is also transferred to the CO inventory. When 10% of the CO inventory has the strong 18 O enrichment, the measured low 18 O abundance of atmospheric CO requires for compensation the existence of a strongly 18 O depleted CO source, or a selective isotopic modification due to isotope fractionation. No such source has been identified to date, and thus ozonolysis may not be the only source of mass independent fractionation in atmospheric CO. Because the mass independent isotopic enrichment is by its nature indestructible by common (i.e., mass dependent) fractionation processes, it is an ideal tracer.