An Expanded Definition of Odd Oxygen for Improved Tropospheric Ozone Accounting

Tropospheric ozone, an important atmospheric oxidant and greenhouse gas, can be produced within the troposphere by photochemical oxidation of volatile organic compounds (VOCs) and CO in the presence of nitrogen oxide radicals (NO_x = NO + NO₂) or transported from the stratosphere, where it is produced by O₂ photolysis. Global 3-D chemical transport models (CTMs) resolving the coupling between chemistry and transport have become standard tools for developing an understanding of the factors controlling tropospheric ozone, but the chemical cycling between ozone and various chemical species presents a difficulty in tracking ozone and quantifying ozone budgets in the models. Models define for this purpose an "odd oxygen" (O_x) chemical family, including ozone and the species with which it cycles, as the relevant quantity for computing the ozone budget. However, different models use various definitions of the O_x family, leading to ambiguities in model intercomparisons, and the definitions commonly used have prominent theoretical deficiencies. We present here an expanded definition of the O_x family to enable rigorous accounting of tropospheric ozone budgets, and a comprehensive treatment of O_x burden, lifetime, production, and loss by various definitions in GEOS-Chem, a widely used CTM. We pay particular attention to the changes in O₃ burden caused by perturbations to stratosphere-troposphere exchange and to tropospheric NO_x emissions, and the ease with which the various O_x definitions can explain these changes. Our new, expanded definition of O_x provides a more intuitive understanding of the influence of stratospheric-troposphere exchange on tropospheric ozone burdens, and enables improved accounting of O_x tagging by source type.