Lifetimes and Timescales of Tropospheric Ozone (Invited Presentation)

Ozone is a secondary pollutant, produced in the atmosphere from other pollutants (Haagen-Smit, 1952). Because of this lack of direct emissions, our understanding and modeling of O₃ is quite different from that of other chemical pollutants and greenhouse gases. For example, we do not create atmospheric budgets tied to emissions, natural and anthropogenic, and thus do not derive lifetimes as we do for other gases like methane from the atmospheric burden divided by the emissions. In this work, we take a new approach to the study of tropospheric O₃ by hypothesizing a direct emission source of O₃ and then following its perturbation on the background atmosphere to derive that source's lifetime for O₃ and the atmospheric time scale to remove the excess O₃.

We study the perturbation to O₃ from hypothesized primary emissions associated with surface industrial pollution, aviation, and stratosphere-troposphere exchange using the UC Irvine chemistry-transport model with stratospheric-plus-tropospheric chemistry. A constant 3D emission pattern of 100 Tg-O₃/yr is run, identifying the pattern and burden of excess O₃ over four years. As expected for most any trace gas, the O₃ lifetime varies with season and location of emissions. When emissions are suddenly cut, we can observe and calculate the timescale of the O₃ perturbation, which is expected to differ from the lifetime due to the coupled O₃-HOx-NOx chemistry.

We find a number of not unexpected, but still surprising facts about tropospheric O₃. For example, (1) summertime NH urban O₃ pollution tends to have minimal hemispheric impact per mole produced; (2) the STE influx of O₃ is responsible for <10% of the tropospheric O₃ burden, and even less at the surface; (3) comparing aviation direct O₃ emissions vs. NOx emissions, we calculate an ozone production efficiency of about 30 moles of O₃ per mole of NOx; (4) O₃ perturbations decay with a timescale of 10-20 days in summer and 30-40 days in winter.