The 2006 United States standard for particulate matter smaller than 2.5 microns (PM2.5) under the Clean Air Act is 35 micrograms per cubic meter of air (μg m-3) taken over a 24 hour average. This standard was a revision of a previous standard of 65 μg m-3. Many locations around the country were in compliance with the earlier 65 μg m-3 limit, but not with the new value of 35 μg m-3. The rationale for the standard is for the protection of human health, especially sensitive individuals such as the elderly, infants, and those with cardiovascular and respiratory disease.
Two PM2.5 monitors in the state of Iowa have 2005-2007 values that are above the 35 μg m-3 attainment threshold (Scott Country/Davenport Wellman St., and Muscatine’s Garfield School). Furthermore, many other Eastern Iowa monitors are just below the standard. While it is widely accepted that pollution levels at individual sites are the combination of regional episodic processes, urban scale pollution, and local source impacts, an in depth investigation of these elements for Iowa sites was desired as part of overall efforts to deal with PM2.5 levels at or near the 35 μg m-3 threshold.
Final attainment or nonattainment status will be heavily influenced by 2006-2008 monitoring data (which has the two sites in question below the attainment threshold). However, the objectives and results of this work are applicable regardless of attainment status. Furthermore, as the report shows, unless ongoing decreases are achieved in the contributing impact categories identified herein, future years of non-attainment are likely