Following major controls of NO_x, ozone has come down more quickly than anticipated. This is good news. But it also poses a challenge because states rely upon models to predict whether they’ll attain ozone standards in the future. If the models have key uncertainties that affect their responsiveness, that can affect the states’ control strategies.

—Daniel Cohan, co-author

Ozone is not emitted directly but instead forms near the ground from precursor emissions of NO_x and hydrocarbons. Modeling of this complex chemistry is important to help states comply with federal standards for ozone, which now stand at 75 parts per billion (ppb) and may be tightened by the Obama administration. A recent Rice study showed a positive correlation between high ozone levels and cardiac arrest. (Earlier post.)

In 2002, the EPA implemented a cap-and-trade program known as the NO_x SIP Call to curtail emissions of ozone-forming NO_x from industries in Eastern states. The significant reduction in emissions over the subsequent four years provided a real-world experiment for the researchers to test how well computer models predict improvements in air quality.

When a model fails to replicate an observed change, a key challenge is to discern whether the discrepancy is caused by errors in meteorological simulations, errors in emission magnitudes and changes, or inaccurate responses of simulated pollutant concentrations to emission changes. In this study, the Community Multiscale Air Quality (CMAQ) model is applied to simulate the ozone (O₃) change after the NO_x SIP Call and mobile emission controls substantially reduced nitrogen oxides (NO_x) emissions in the eastern U.S. from 2002 to 2006.

For both modeled and observed O₃, changes in episode average daily maximal 8-h O₃ were highly correlated (R² = 0.89) with changes in the 95_th percentile, although the magnitudes of reductions increased nonlinearly at high percentile O₃ concentrations.

Observed downward changes in mean NO_x (−11.6 to −2.5 ppb) and 8-h O₃ (−10.4 to −4.7 ppb) concentrations in metropolitan areas in the NO_x SIP Call region were under-predicted by 31%–64% and 26%–66%, respectively.

The under-predicted O₃ improvements in the NO_x SIP Call region could not be explained by adjusting for temperature biases in the meteorological input, or by considering uncertainties in the chemical reaction rate constants. However, the under-prediction in O₃ improvements could be alleviated by 5%–31% by constraining NO_x emissions in each year based on observed NO_x concentrations. This demonstrates the crucial need to accurately characterize changes in precursor emissions when dynamically evaluating a model’s ability to simulate O₃ responses to those changes.

—Zhou et al.

Faster-than-expected reductions in NO_x emissions may explain some but not all of the gap. The remaining gap may result from inaccuracies in how the model represents the chemistry and transport of air pollutants, Cohan said.

How ozone responds to changes in NO_x and hydrocarbons is a nonlinear chemistry. So it’s certainly possible that even the best models could be slightly inaccurate in defining those relationships. It tells us that, as modelers, we need to revisit the formulations, especially the chemistry.

—Daniel Cohan

The observed (dash line) and modeled (solid line) daily maximal 8-h O3 as a function of percentile n, averaged over all monitors inside the NOx SIP Call region. Zhou et al. Click to enlarge.

While it may be preferable for models to be a bit conservative rather than too aggressive in predicting ozone improvements, Cohan said, the models are intended to represent air pollution as accurately as possible. A study by Cohan’s research group last year showed that regulatory modeling by states tended to slightly under-predict the ozone reductions that were actually achieved.

The goal of everyone in the process is to reach attainment in the most cost-effective manner possible, and we need accurate models to inform those decisions.

—Daniel Cohan

Resources

• Wei Zhou, Daniel S. Cohan, Sergey L. Napelenok (2013) Reconciling NO_x emissions reductions and ozone trends in the US, 2002–2006, Atmospheric Environment, Volume 70 Pages 236-244 doi: 10.1016/j.atmosenv.2012.12.038