Dynamic evaluation of regional air quality models: Assessing changes in O3 stemming from changes in emissions and meteorology

Regional-scale air quality models are used to estimate the response of air pollutants to potential emission control strategies as part of the decision-making process. Traditionally, the model-predicted pollutant concentrations are evaluated for the “base case” to assess a modelʹs ability to reproduce past observations. Dynamic evaluation approaches, which evaluate a modelʹs ability to accurately simulate air quality changes from given changes in emissions, are critically important to regulatory applications. Here, we investigate approaches to evaluate the Community Multiscale Air Quality (CMAQ) modelʹs predicted ozone (O3) response to large NOx emission reductions associated with the NOx State Implementation Plan (SIP) Call and on-road mobile emissions. This case has the advantages that emission changes associated with the NOx SIP Call can be well characterized and substantial changes are observed in O3 levels. To consider the modeled response to emission changes in light of the strong meteorological influences on O3, two time periods after the NOx SIP Call are included with very different meteorological conditions. The sensitivity to chemical mechanisms is also considered by including simulations with the CB4, SAPRC, and CB05 chemical mechanisms. The evaluation results suggest that the air quality model predictions underestimate the O3 reductions observed after the NOx SIP Call was implemented. While the emission estimate uncertainties may also be a factor, the results suggest that the contribution of long-range transport of O3 and precursors is underpredicted, especially when using the CB4 chemical mechanism. Further investigation of the chemical mechanisms’ ability to characterize tropospheric chemistry aloft is recommended. Results based on the most recent CMAQ version 4.6 with CB05 and updated emission inventories show incremental improvements to the modeled O3 response to NOx emission reductions.