Retrospective Evaluation of Recent State Implementation Plans for Ozone

State implementation plans (SIPs) are a critical component of air quality management in the United States. SIPs set forth strategies for controlling air pollution and present evidence that the controls will be sufficient to attain ambient standards. However, retrospective analysis of the content and effectiveness of SIPs has rarely been conducted.

Here, we present a multi-pronged assessment of recent SIPs that aimed to attain the 84 part per billion 8-hour ozone standard. Regions designated as “moderate” non-attainment sought to attain this standard by 2009, enabling assessment of whether attainment was in fact achieved. Control plans are found to rely heavily on federal measures except in California, where state and local measures comprised a majority of reductions. Photochemical modeling conducted by states under US EPA's recommended methodology provided reasonably accurate predictions of whether attainment would be achieved by the 2009 target year, yielding more false alarms than false negatives. However, some states employed weight of evidence arguments to counteract modeling predictions of non-attainment, garnering approval for SIPs that later proved inadequate to achieve attainment. Ambient monitors observed substantial reductions in ground-level ozone across much of the country, with largest reductions at locations that were originally most polluted. Meanwhile, satellite observations of NO2 columns by the OMI instrument aboard Aura indicated substantial concurrent reductions in emissions of nitrogen oxides, a leading precursor of ozone. However, evidence was inconclusive regarding whether the ozone and NO2 reductions were accelerated by SIPs or reflected broad-based improvements from federal measures. Our work demonstrates that retrospective analysis of the SIP process can yield important insights as states seek to achieve more stringent new air quality standards for ozone and fine particulate matter.