The U.S. Environmental Protection Agency (EPA) issued a memorandum April 16, 2013, providing direction to regions, and indirectly to states and tribes, on the appropriate way to identify nonattainment areas for the fine particulate matter (PM2.5) standard. As outlined in the memo, states should submit their preliminary designations of attainment, nonattainment and unclassifiable areas with identified boundaries by December 13. An area would be identified as “nonattainment,” if three years of certified air-monitoring data show that it either violates the PM2.5 limits or contributes to such a violation in a nearby area.

According to EPA, “violations of the annual PM2.5 NAAQS [national ambient air quality standards] are usually the result of emissions from a broad variety of sources that are typically located across a metropolitan area . . .” Setting the boundaries of a nonattainment area, EPA said, should begin with consideration of the entire metropolitan area. But EPA said it also will “evaluate any adjacent counties” to the metropolitan area that have “the potential to contribute.”

For locations where a high PM2.5 is affected by long-range or regional transport of pollutants, EPA said, “the CAA does not require that all contributing areas be designated nonattainment, only the contributing areas that are nearby.”