OZONOLYSIS OF NONMETHANE HYDROCARBONS AS A SOURCE OF THE OBSERVED MASS-INDEPENDENT OXYGEN-ISOTOPE ENRICHMENT IN TROPOSPHERIC CO

Combined O-17 and O-18 measurements of tropospheric CO from two northe rn hemisphere (NH) sites reveal systematically enhanced O-17 levels, c onfirming the existence of significant mass independent oxygen isotope enrichment in this important trace gas. When CO levels increase in th e NH winter, the mass independent enrichment decreases proportionally. A possible source of this rare isotope effect in CO is transfer of th e mass independent enrichment from O-3 to the CO pool via ozonolysis o f unsaturated hydrocarbons such as isoprene and terpenes, Laboratory o zonolysis experiments indeed confirm that this process is a potentiall y important source of mass independently enriched CO. The extra O-17 e nrichment found in ozonolysis-derived CO is similar to the mass indepe ndent fractionation measured on ozone. If ozonolysis is the only sourc e of mass independent enrichment in CO, it is estimated that about 10% of all CO in middle to high latitude winter, at ground level, origina tes from the ozonolysis of unsaturated hydrocarbons, While the laborat ory 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 O-18 as well, this enrichment is also transfer red to the CO inventory. When 10% of the CO inventory has the strong O -18 enrichment, the measured low O-18 abundance of atmospheric CO requ ires for compensation the existence of a strongly O-18 depleted CO sou rce, 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 atmospheri c CO. Because the mass independent isotopic enrichment is by its natur e indestructible by common (i.e., mass dependent) fractionation proces ses, it is an ideal tracer.