Modeled and observed fine particulate matter reductions from state attainment demonstrations.

States rely upon photochemical models to predict the impacts of air quality attainment strategies, but the performance of those predictions is rarely evaluated retrospectively. State implementation plans (SIPs) developed to attain the 1997 U.S. standard for fine particulate matter (PM2.5; denoting particles smaller than 2.5 microns in diameter) by 2009 provide the first opportunity to assess modeled predictions of PM2.5 reductions at the state level. The SIPs were the first to rely upon a speciated modeled attainment test methodology recommended by the U.S. Environmental Protection Agency to predict PM2.5 concentrations and attainment status. Of the 23 eastern U.S. regions considered here, all but one achieved the 15 microg/m3 standard by 2009, and the other achieved it the following year with downward trends sustained in subsequent years. The attainment tests predicted 2009 PM2.5 design values at individual monitors with a mean bias of 0.38 microg/m3 and mean error of 0.68 microg/m3, and were 95% accurate in predicting whether a monitor would achieve the standard. All of the errors were false alarms, in which the monitor observed attainment after a modeled prediction of an exceedance; in these cases, the states used weight-of-evidence determinations to argue that attainment was likely. Overall, PM2.5 concentrations at monitors in the SIP regions declined by 2.6 microg/m3 from 2000-2004 to 2007-2009, compared with 1.6 microg/m3 in eastern U.S. regions originally designated as attainment. Air quality improvements tended to be largest at monitors that were initially the most polluted. Implications: As states prepare to develop plans for attaining a more stringent standard for fine particulate matter, this retrospective analysis documents substantial and sustained air quality improvements achieved under the previous standard. Significantly larger air quality improvements in regions initially designated nonattainment of the 1997 standard indicate that this status prompted heightened control efforts: The speciated modeled attainment test is found to be accurate and slightly conservative in predicting particulate concentrations for the cases considered here, providing confidence for its use in upcoming attainment plans.