Accurate estimates for North American background (NAB) ozone (O 3 ) in surface air over the United States are needed for setting and implementing an attainable national O 3 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). where it correlates with observed O 3 . At these sites, a 27-year GFDL AM3 simulation simulates observed O 3 events above 60 ppb and indicates a role for year-to-year variations in NAB O 3 in driving their frequency (contributing 50-60 ppb or more during some events). 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 NO x , 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 O 3 and related species from multiple platforms, and thereby reduce the error on NAB estimates needed for air quality planning.

We evaluate nitrogen dioxide (NO2) simulations from a widely used air quality model, the Environmental Protection Agency (EPA) Community Multiscale Air Quality (CMAQ) model, using ground‐ and satellite‐based observations. In addition to direct comparison of modeled and measured variables, we compare the response of NO2 to meteorological conditions and the ability of the model to capture these sensitivities over the continental U.S. during winter and summer periods of 2007. This is the first study to evaluate relationships between NO2 and meteorological variables using satellite data, the first to apply these relationships for model validation, and the first to characterize variability in sensitivities over a wide geographic and temporal scope. We find boundary layer height, wind speed, temperature, and relative humidity to be the most important variables in determining near‐surface NO2 variability. Consistent with earlier studies on NO2‐meteorology relationships, we find that, in general, NO2 responds negatively to planetary boundary height, negatively to wind speed, and negatively to insolation. Unlike previous studies, we find a slight positive association between precipitation and NO2, and we find a consistently positive average association between temperature and NO2. CMAQ agreed with relationships observed in ground‐based data from the EPA Air Quality System and the Ozone Monitoring Instrument over most regions. However, we find that the southwest U.S. is a problem area for CMAQ, where modeled NO2 responses to insolation, boundary layer height, and other variables are at odds with the observations.

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