The Ozone-Climate Penalty: Past, Present, and Future

Climate change is expected to increase global mean temperatures leading to higher tropospheric ozone (O-3) concentrations in already polluted regions, potentially eroding the benefits of expensive emission controls. The magnitude of the "O-3-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 O-3-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 O-3-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 O-3 precursors will not diminish the O-3-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 O-3-climate penalty and mitigate O-3 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 O-3-climate penalty while simultaneously reducing base case O-3 concentrations to desired levels. Additional modeling is needed to determine the behavior of the O-3-climate penalty as NOx and VOC emissions evolve in other regions.