U.S. EPA Proposes 1-hour National Ambient Air Quality Standard for Sulfur Dioxide

On November 17, 2009, U.S EPA announced that they were proposing to establish a new 1-hour sulfur dioxide (SO2) National Ambient Air Quality Standard (NAAQS) between 50 and 100 parts per billion by volume (ppbv) (i.e., approximately 130 to 260 mg/m3).  Currently, a Prevention of Significant Deterioration (PSD) permit application requires a demonstration that the existing annual, 24-hour, and 3-hour SO2 NAAQS will not be exceeded as a result of a project.  The establishment of a 1-hour SO2 NAAQS may make it more difficult for facilities to demonstrate compliance as part of an SO2 NAAQS analysis for two reasons.  First, the 1-hour standard would reflect worst-case dispersion conditions and elevated modeled concentrations due to a combination of either building aerodynamic downwash or adverse meteorological conditions.  Second, the high end of the proposed range of the 1-hour NAAQS is a relatively low standard in comparison to the current 3-hour NAAQS (1,300 mg/m3).  Note that the statistical criteria used to evaluate modeled concentrations against the proposed 1-hour standard and current 3-hour standard are different and do not allow a one-to-one comparison between the standards (they are not measured against peak modeled concentrations).  Regardless of the statistical considerations, an air quality modeling demonstration will need to address a relatively low numeric concentration standard (130 mg/m3 compared to 1,300 mg/m3) over a shorter averaging time (1-hour compared to 3-hour).  Therefore, it may be more difficult to demonstrate compliance with the proposed 1-hour NAAQS than with the existing 3-hour NAAQS.  In specific cases, the new 1-hour standard could be advantageous and result in less overlap of contributions from surrounding emission sources than previous air quality modeling studies.  With the proposed new SO2 NAAQS targeted for June 2, 2010, facilities may be well served to perform some exploratory air quality modeling analyses to evaluate compliance with the new NAAQS.