Assessing multi-year changes in modeled and observed urban NOX concentrations from a dynamic model evaluation perspective

An investigation of the concentrations of nitrogen oxides (NOX) from an air quality model and observations at monitoring sites was performed to assess the changes in NOX levels attributable to changes in mobile emissions. This evaluation effort focused on weekday morning rush hours since urban NOX concentrations are strongly influenced by the significant loading of emissions associated with heavy commuter traffic. On-road vehicle NOX emissions generated by the MOBILE6 model revealed a steady decline with an overall decrease of 25% for 2002–2006. In this study, a dynamic model evaluation was undertaken that entails an assessment of the predicted concentration response of the Community Multiscale Air Quality (CMAQ) model due to changes in NOX emissions as well as to meteorological variability spanning 3-month summer periods over five consecutive years (2002–2006) against observed concentration changes at NOX monitoring sites located primarily in urban areas of the eastern United States. Both modeled and observed hourly NOX concentrations exhibited maximum values that coincided with the morning peak NOX emissions. The notable results, based on 3-h average (6–9 AM local time) NOX concentrations, derived between the 50th and 95th percentiles of cumulative concentration distributions, revealed that modeled changes at these elevated NOX levels generally tracked the year-to-year variations in the observed concentration changes. When summer 2002 values were used as a reference, both modeled and observed results also showed definitive decreases in weekday morning urban NOX concentrations over this multi-year period, which can be primarily attributed to the reductions in mobile source emissions. Whereas observed NOX concentrations have declined by about 25% over this period consistent with the decline in the modeled mobile emission sector, modeled NOX concentration changes were close to the decreases exhibited in all (mobile + other sectors) surface NOX emissions whose overall decline was about 15% over this multi-year period.