Estimating North American background ozone in U.S. surface air with two independent global models: Variability, uncertainties, and recommendations
Accurate estimates for North American background (NAB) ozone (O3) in surface air over the United States are needed for setting and implementing an attainable national O3 standard. These estimates rely on simulations with atmospheric chemistry-transport models that set North American anthropogenic emissions to zero, and to date have relied heavily on one global model. We examine, for the first time, NAB estimates for spring and summer 2006 with two independent global models (GEOS-Chem and GFDL AM3). Evaluation of the standard simulations, which include North American anthropogenic emissions, with mid-tropospheric O3 retrieved from space and ground-level O3 measurements shows that the models often bracket the observed values, implying value in developing a multi-model approach to estimate NAB O3. Consistent with earlier studies, the models robustly simulate the largest nation-wide NAB levels at high-altitude western U.S. sites (average values of ~40-50 ppb in spring and ~25-40 ppb in summer) where it correlates with observed O3. At these sites, a 27-year GFDL AM3 simulation simulates observed O3 events above 60 ppb and indicates a role for year-to-year variations in NAB O3 in driving their frequency. During summer over the eastern United States (EUS), when photochemical production from regional anthropogenic emissions peaks, NAB is largely uncorrelated with observed values and it is lower than at high-altitude sites (average values of ~20-30 ppb). We identify four processes that contribute substantially to model differences in specific regions and seasons: lightning NOx, biogenic isoprene emissions and chemistry, wildfires, and stratosphere-to-troposphere transport. Differences in model representation of these processes contribute more to uncertainty in NAB estimates than the choice of horizontal resolution within a single model. We propose that future efforts seek to constrain these processes with targeted analysis of multi-model simulations evaluated with observations of O3 and related species from multiple platforms, and thereby reduce the error on NAB estimates and needed for air quality planning.