Evaluation of Present and Future Surface Ozone as Simulated by Chemistry-Climate Models

In evaluating a future scenario for air quality, one can identify four major factors driving change: (1) global emissions that alter atmospheric composition and thence baseline levels of surface ozone; (2) global changes in climate that also alter these baselines (via temperature, water vapor, convection); (3) climate-driven changes in the meteorological regimes of polluted regions that lead to air quality extreme (AQX) episodes; and (4) changes in the efficacy of local emissions to produce pollution within a governance region. While these factors are all part of a coupled system, a model that combines all would be difficult to verify. Thus an assessment approach needs evaluate each factor separately using observations and an ensemble of models. In this study, we focus on factor (3), evaluating the ability of the models in the Atmospheric Chemistry & Climate Model Intercomparison Project to reproduce the observed present-day climatology (e.g. diurnal/seasonal cycles, AQX episode size) of surface ozone  in North America (NA) and Europe (EU). We can characterize future climate (2090s RCP8.5 climate vs. 2000s) changes within NA and EU, as well as for south Asia (SA) where we lack suitable observations to validate models.  The chemistry-climate models simulated the 2090s decade for RCP8.5 climate using both current and 2100 emissions for air pollutants.  We find that most models simulate the observed climatology well, albeit biased high over the most of the probability distribution, ranging from baseline (30^th percentile) to AQX threshold (100 worst days in a decade).  For RCP8.5, the model ensemble mean shows an increase of ~10% in the mean annual maximum daily 8-h average ozone (MDA8) over all domains, with the largest changes in winter months. For RCP8.5 holding 2000s pollutant emissions, the modeled NA shows a small increase (+1%) in annual mean MDA8 while EU and SA show small decreases (-2% and -3%, respectively). Also for RCP8.5, most models show decreases in the mean size (S) and mean duration (D) of AQX episodes in EU (S = -28%, D = -17%) and increases in SA (+54%, +15%). The ensemble mean shows decreases in D (-7%) and increases in S (+21%) in NA, but the sign of the change in S is split between the models. For RCP8.5 with 2000s pollutants, we find similar but smaller changes in EU (-10%, -11%) and SA (+16%, +5%), and small decreases in NA (-3%, -8%). Thus we conclude future changes in AQX episode size to be driven more so by changes in precursor emissions rather than climate. This also applies to changes in duration for SA but is reversed for NA and EU.