EPA Establishes New 1-hour Standard for Sulfur Dioxide

On June 2, 2010 the U.S. EPA established a new 1-hour primary National Ambient Air Quality Standard (NAAQS) for sulfur dioxide (SO2).  The new 1-hour standard was set at the level of 75 parts per billion (ppb) calculated as the three-year average of the 99th percentile of the annual distribution of daily maximum 1-hour average concentrations.  With the establishment of the new 1-hour SO2 standard, the EPA also revoked the two previous primary standards of 140 ppb (24-hour standard), and 30 ppb (annual standard) because they will not provide additional public health protection given a 1-hour standard at 75 ppb.  EPA’s evaluation of scientific information and the risks posed by exposure to SO2 indicate that the new standard will protect public health by reducing exposure to high short-term (5-minutes to 24-hour) concentrations of SO2, especially for children, the elderly, and people with asthma.  EPA did not revise the secondary NAAQS for SO2, which aims to protect public welfare, as part of this action and is assessing the need for changes to the secondary standard under a separate review.

Monitoring and Reporting Requirements

With the new 1-hour standard, EPA is also revising the ambient air quality monitoring requirements for SO2.  Under the new requirements, SO2 monitors must be placed in Core Based Statistical Areas (CBSAs) based on a population weighted emissions index for the area, as follows:

  •  3 monitors in CBSAs with index values of 1,000,000 or more 
  •  2 monitors in CBSAs with index values less than 1,000,000 but greater than 100,000 
  •  1 monitor in all CBSAs with index values greater than 5,000 

At least 163 SO2 monitoring sites nationwide are required by these criteria.  EPA estimates that 41 new monitoring sites will need to be established, nationwide, to meet the new monitoring requirements.  EPA regional administrators may also require additional monitoring sites for certain circumstances. 

Under the new regulations, EPA also finalized changes to SO2 data reporting requirements.  State and local agencies are now required to report two values for every monitored hour – (1) the 1-hour average SO2 concentration; and (2) the maximum 5-minute block average SO2 concentration for each hour.  States must make adjustments to monitoring networks to meet the new SO2 monitoring requirements by January 1, 2013. 

Based on currently available 2007-2009 air quality monitoring data from existing SO2 monitoring sites, 60 of the total 249 monitored counties violate the new 1-hour SO2 standard.  EPA expects to designate areas as attainment, nonattainment, or unclassifiable areas by June 2012 based on more recent air quality monitoring data, most likely 2008-2010. 

Dispersion Modeling Utilized

In an innovative approach, EPA also plans to utilize refined air dispersion modeling results, where available, as part of the attainment designation process.  EPA expects that, in areas without currently operating SO2 monitors but with sources that might have the potential to cause or contribute to violations of the NAAQS, the identification of NAAQS violations and compliance with the 1-hour SO2 NAAQS would primarily be accomplished through refined, source-oriented air quality dispersion modeling analyses, supplemented with the new, limited network of ambient air quality monitors.  EPA justifies this approach, per the rule’s preamble, saying that the revised approach would better address:

  1. The unique source-specific impacts of SO2 emissions
  2. The special challenges SO2 emissions present in terms of monitoring short-term SO2 levels for comparison with the NAAQS in many situations
  3. The superior utility that modeling offers for assessing SO2 concentrations 
  4. The most appropriate method for ensuring that areas attain and maintain the new 1-hour SO2 NAAQS in a manner that is as expeditious as practicable, taking into account the potential for substantial SO2 emissions reductions from forthcoming national and regional rules that are currently underway 

Of particular interest to specific sources of SO2 emissions is EPA’s recommendation, in the rule’s preamble, that states initially focus performance of attainment demonstration modeling on larger sources (e.g., those emitting > 100 tons per year (tpy) of SO2), and that states would also identify and eventually conduct refined modeling of any other sources that may be anticipated to cause or contribute to a violation to determine compliance with the new SO2 NAAQS.

This hybrid monitoring/modeling designation approach is planned as follows:

  • Any area that has monitoring data or refined modeling results showing a violation would be designated “nonattainment”
  • Any area that has both monitoring and refined modeling results showing no violations would be designated “attainment”
  • All other areas would initially be designated “unclassifiable”
  • The county boundary would be the presumptive nonattainment boundary unless state demonstrates otherwise in recommendations to EPA

States with counties classified as nonattainment areas must submit nonattainment State Implementation Plans (SIPs) by February 2014 and must demonstrate how those areas will be brought into attainment status by August 2017.  For all other areas classified as attainment or unclassifiable, maintenance SIPs, as required by the Clean Air Act, are due by June 2013.

The final rule will become effective August 23, 2010.  As a result of the new rule, any new or modified major sources will be required to demonstrate compliance with the new 1-hour SO2 NAAQS as part of federal permitting projects.  States may also require a similar compliance demonstration for state-level permitting projects.  The final rule does not include modeling significance levels or Class I / Class II Increment Standards associated with the SO2 1-hour primary standard, nor does it establish a new significant emission rate for purposes of new source review (although EPA holds this option available, per discussion in the final rule preamble).  EPA intends to issue separate guidance related to these implementation concerns of the new SO2 standard and allow for public comment before issuing it in final form.  For those areas designated as nonattainment under the new standard, additional nonattainment New Source Review permitting requirements will apply for new construction and source modification projects.  States may also implement emissions control based regulations (e.g., RACT, Cap and Trade) to address attainment issues related to the new 1-hour SO2 NAAQS.