The ozone-climate penalty: past, present, and future.

Climate change is expected to increase global mean temperatures leading to higher tropospheric ozone (O3) concentrations in already polluted regions, potentially eroding the benefits of expensive emission controls. The magnitude of the "O3-climate penalty" has generally decreased over the past three decades, which makes future predictions for climate impacts on air quality uncertain. Researchers attribute historical reductions in the O3-climate penalty to reductions in NOx emissions but have so far not extended this theory into a quantitative prediction for future effects. Here, we show that a three-dimensional air quality model can be used to map the behavior of the O3-climate penalty under varying NOx and VOC emissions in both NOx-limited and NOx-saturated conditions in Central and Southern California, respectively. Simulations suggest that the planned emissions control program for O3 precursors will not diminish the O3-climate penalty to zero as some observational studies might imply. The results further demonstrate that in a NOx-limited air basin, NOx control strategies alone are sufficient to both decrease the O3-climate penalty and mitigate O3 pollution, while in a NOx-saturated air basin, a modified emissions control plan that carefully chooses reductions in both NOx and VOC emissions may be necessary to eliminate the O3-climate penalty while simultaneously reducing base case O3 concentrations to desired levels. Additional modeling is needed to determine the behavior of the O3-climate penalty as NOx and VOC emissions evolve in other regions.