U.S. EPA Proposes a Short-Term NO2 NAAQS

In the more than 200 air quality modeling analyses that ALL4 meteorologists have conducted, demonstrating compliance with the annual nitrogen dioxide (NO2) National Ambient Air Quality Standard (NAAQS) of 53 parts per billion (ppbv) has rarely been very difficult.  The ability to easily demonstrate compliance with the annual NO2 NAAQS is due to two factors.  First, the NAAQS for NO2 is on an annual basis and thus any adverse dispersion that occurs based on worst-case meteorological conditions is averaged over the course of a year, minimizing its affect.  Second, U.S. EPA recommends the use of scaling factors to adjust the modeled concentrations of oxides of nitrogen (NOx) to reflect the fact that NOx is converted to NO2 after being emitted from the source.  This adjustment results in a reduction of the modeled annual concentrations.  

On June 29, 2009, U.S. EPA announced that they were proposing to establish a new short-term NO2 NAAQS between 80 and 100 ppbv.  The establishment of a short-term NO2 NAAQS may make it more difficult for NO2 air quality modeling analyses to demonstrate compliance with a NAAQS compliance demonstration under New Source Review (NSR).  Specifically, a short-term standard would reflect worst-case dispersion conditions and elevated modeled concentrations due to a combination of either building aerodynamic downwash or adverse meteorological conditions.  Based on ALL4 modeling experience, SO2 air quality modeling analyses often hinge on compliance with the existing 3-hour SO2 standard.  The ratio between the SO2 3-hour NAAQS (500 ppbv) and the annual SO2 NAAQS (30 ppbv) is 16.7, while the ratio between a 1-hour NO2 NAAQS (100 ppbv) and the annual NO2 NAAQS (53 ppbv) is 1.9.  The lower ratio for NO2 indicates that, relative to SO2, it will be more difficult to demonstrate compliance with a new 1-hour NO2 NAAQS than with the existing 3-hour SO2 NAAQS.  With the proposed new NO2 NAAQS targeted for January 22, 2010, facilities may be well served to perform some exploratory air quality modeling analyses to evaluate compliance with a new NAAQS.