Isotopic ordering in atmospheric O2 as a tracer of ozone photochemistry and the tropical atmosphere

The distribution of isotopes within O-2 molecules can be rapidly altered when they react with atomic oxygen. This mechanism is globally important: while other contributions to the global budget of O-2 impart isotopic signatures, the O(P-3)+O-2 reaction resets all such signatures in the atmosphere on subdecadal timescales. Consequently, the isotopic distribution within O-2 is determined by O-3 photochemistry and the circulation patterns that control where that photochemistry occurs. The variability of isotopic ordering in O-2 has not been established, however. We present new measurements of (OO)-O-18-O-18 in air (reported as (36) values) from the surface to 33km altitude. They confirm the basic features of the clumped-isotope budget of O-2: Stratospheric air has higher (36) values than tropospheric air (i.e., more (OO)-O-18-O-18), reflecting colder temperatures and fast photochemical cycling of O-3. Lower (36) values in the troposphere arise from photochemistry at warmer temperatures balanced by the influx of high-(36) air from the stratosphere. These observations agree with predictions derived from the GEOS-Chem chemical transport model, which provides additional insight. We find a link between tropical circulation patterns and regions where (36) values are reset in the troposphere. The dynamics of these regions influences lapse rates, vertical and horizontal patterns of O-2 reordering, and thus the isotopic distribution toward which O-2 is driven in the troposphere. Temporal variations in (36) values at the surface should therefore reflect changes in tropospheric temperatures, photochemistry, and circulation. Our results suggest that the tropospheric O-3 burden has remained within a 10% range since 1978.