An Expanded Definition of the Odd Oxygen Family for Tropospheric Ozone Budgets: Implications for Ozone Lifetime and Stratospheric Influence

Models of tropospheric ozone commonly define an "odd oxygen" family (Ox), comprising ozone and species with which it rapidly cycles, in order to compute tropospheric ozone budgets and lifetimes. A major O-x loss is the O(D-1) + H2O -> 2OH reaction, but this may not be an actual loss because the resulting hydrogen oxide (HOx) radicals regenerate ozone in the presence of nitrogen oxides. Here we introduce an expanded odd oxygen family, O-y = O-x + O-z, to include both O-x and an additional subfamily, O-z, consisting of HOx and its reservoirs. We incorporate this new accounting into the GEOS-Chem model, revealing a longer global mean ozone lifetime (73 days vs. 24 days) and greater stratospheric contribution (26% vs. 9%) under present-day conditions than derived from the standard O-x budget. Tracking the O-y budget may provide better understanding of the discrepancies between global models in their computations of ozone sources and sinks. Plain Language Summary Ozone in the lower atmosphere (troposphere) is a greenhouse gas, a strong oxidant, and a surface air pollutant. It is produced chemically in the atmosphere from gaseous precursors that have both natural and anthropogenic sources. While the amount of ozone in the troposphere is easily measured, the processes by which it is produced and lost are not, so we need global models to estimate these processes and their contributions to the ozone budget. This paper describes a new way of accounting for the budget of ozone in models, including the cycling with radicals, thus relating the production and loss of ozone to molecular oxygen (the ultimate source and sink). By implementing our method in a global model, we show that ozone has a much longer effective global mean lifetime than previously thought, extending the global influence of sources. We also find that downwelling of natural ozone from the stratosphere is more important for the tropospheric ozone budget than previously thought.