Adjoint Sensitivity Analysis of Ozone Nonattainment over the Continental United States (original) (raw)

2006, Environmental Science & Technology

An application of the adjoint method in air quality management is demonstrated. In the adjoint applications (e.g., variational data assimilation), sensitivities of a receptor-based metric to a variety of parameters can be efficiently calculated. Here, we calculate the sensitivities of a nationwide U.S. ozone National Ambient Air Quality Standard (NAAQS) non-attainment metric to precursor emissions during the month of July 2004. We use the adjoint version of a continental scale chemical transport model (STEM) with 60 km horizontal grid resolution. The model shows low bias and error (3, and 21 percent, respectively), particularly for areas with high ozone concentrations that are subject of this study. The non-attainment metric is integrated over the month and entire domain, and accounts for both 1-hour and 8-hour ozone standards. The metric is quadratic, and therefore, higher concentrations are given larger overall weights in the calculations. The non-attainment metric for the month of July is heavily dominated by the 8-hour standard (98% of the combined metric). Using these spatial distributions of sensitivities we identify the regions and source categories with significant contribution/effect on the overall non-attainment metric. Largest values of sensitivities are found to be with respect to emissions in the southeast U.S. and in the Ohio River Valley. When non-attainment sensitivities are integrated over the entire U.S., NO x emissions account for the largest contribution (64% of the total), followed by the biogenic and anthropogenic VOCs (23% and 13%, respectively). For NO x emissions, point/area, mobile, and non-road mobile sources account for 51, 39, and 10 percent of the total sensitivities, respectively. Point/area sources also have the largest effect among the VOC source categories (62%). No significant difference is seen between weekday and weekend emission contribution patterns. We also provide a state-by-state comparison for the non-attainment magnitude, non-attainment sensitivity, and emission magnitudes to explore the influence of interstate transport of ozone and its precursors, and policy implications of the results. Our analysis of the nationwide ozone non-attainment metric suggests that simple cap-and-trade programs may prove inadequate in achieving the sought-after air quality objectives.