On June 2, 2010, the Environmental Protection Agency (“EPA”) finalized a new primary National Ambient Air Quality Standard (“NAAQS”) for sulfur dioxide (“SO2”), after finding that the existing standards are inadequate to protect public health. 

For the first time, EPA will implement a one-hour standard for SO2 NAAQS, at a level of 75 parts per billion (ppb).  EPA, in proposing revisions to the SO2 NAAQS, proposed a standard between 50 and 100 ppb, and requested comments on a standard of up to 150 ppb, before finalizing the standard at 75 ppb.  This standard replaces the two existing primary standards of 140 ppb evaluated over 24 hours, and 30 ppb evaluated over one year.  The new standard is significantly more stringent than the newly-revoked standards, as EPA’s justification for revoking those standards is that the one-hour NAAQS will require sources to maintain SO2 emissions concentrations well below the levels of the 24-hour and yearly standards.

The new standard is especially significant for the largest sources of SO2 emissions, such as electric generating utilities (“EGUs”).  The implementation of an hourly standard poses potential problems even for scrubbed EGUs, as short, isolated “burps” of emissions during normal operation or at boiler startup or shutdown could cause plant emissions to exceed the one-hour NAAQS.

EPA is also revising the ambient air monitoring requirements for SO2, proposing to rely on air quality modeling and monitoring to determine compliance with the new NAAQS.  This approach will be targeted primarily at areas surrounding medium and large emissions sources such as EGUs.  EPA plans to review both monitoring data from 2008-2010 and the results of refined dispersion modeling studies to designate nonattainment areas; areas that violate the standard under either approach would be designated as non-attainment.  Both attainment and non-attainment areas must meet the new standard no later than August 2017.

EPA cites increased health benefits, particularly for at-risk populations, in adopting the one-hour SO2 NAAQS.  According to EPA’s explanation of the new standard, sulfur oxides, of which SO2 is one, can react with other compounds in the atmosphere to form small particles that enter the lungs and can cause or worsen respiratory disease and aggravate existing heart disease.  It is noteworthy, however, that EPA is developing and will soon promulgate a new particulate matter NAAQS for these small particles (“PM2.5”), as well as a replacement for the overturned Clean Air Interstate Rule that will also aim to significantly lessen the atmospheric concentrations of PM2.5.