U.S. Environmental Protection Agency


Date of this Version



Atmospheric Environment 115 (2015), pp. 389-403, http://dx.doi.org/10.1016/j.atmosenv.2015.02.043.


U.S. government work.


Several participants in Phase 2 of the Air Quality Model Evaluation International Initiative (AQMEII-2) who are applying coupled models to the North American domain are comparing model results for two years, 2006 and 2010, with the goal of performing dynamic model evaluation. From a modeling perspective, the differences of interest are the large reductions in domain total emissions of NOx (21%) and SO2 (37%) from 2006 to 2010 and significant differences in meteorological conditions between these two years. The emission reductions occurred mostly in the eastern U.S, with some reduction in emissions from western wildfires in 2010. Differences in meteorological conditions both confound the impact of emission reductions on ambient air quality and provide an opportunity to examine how models respond to changing meteorology. This study is aimed at documenting changes in emissions, modeled large-scale background concentrations used as boundary conditions for the regional models, and observed meteorology and air quality to provide a context for the dynamic model evaluation studies performed within AQMEII-2. In addition to warmer summer temperatures, conditions in the eastern U.S. summer of 2010 were characterized by less precipitation than in 2006, while western portions of the U.S. and Canada were much cooler in 2010 due to a strengthening of the thermal trough over the Southwest and associated onshore flow. Summer ozone levels in many portions of the Northeast and Midwest were largely unchanged in 2010 despite reductions in precursor emissions. Normalization of the ozone trend, to account for differences in meteorological conditions, including warmer summer temperatures in 2010, shows that the emission reductions would have resulted in lower ozone levels at these locations if not for the countervailing influence of meteorological conditions. Winter mean surface temperatures were generally above average in 2006 whereas below average temperatures were noted in the Southeast and northern plains in 2010, consistent with a greater frequency of cold arctic air outbreaks. In general, changes in observed air quality as measured at U.S. monitoring sites appear to be consistent with differences in emissions and meteorological conditions between 2006 and 2010. Two potential inconsistencies were noted which warrant further investigation: 1) an increase in particulate nitrate during the winter in the Midwest despite lower emissions of NOx and 2) lower than expected SO2 reductions in the Southeast during the winter.