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EPA Issues Draft White Paper on Implementation of the 1-Hour SO2 NAAQS


In May 2012, EPA issued a draft White Paper for discussion with stakeholders regarding options for implementing the 2010 primary 1-hour SO2 NAAQS.  The 1-hour SO2 NAAQS was promulgated on June 2, 2010 to address short-term exposure to elevated SO2.  The 1-hour SO2 standard is set at 75 parts per billion (3-year average of the 99th percentile daily maximum 1-hour concentration).

EPA had previously issued draft guidance in September 2011 for states for implementation of the new 1-hour SO­2 standard and for preparing related SIP submissions.  The September 2011 guidance proposed a hybrid modeling and monitoring approach for states to use to demonstrate continued or ultimate compliance with the 1-hour SO2 NAAQS.  The modeling component of that guidance was very controversial as attainment designations have historically been based upon ambient air monitoring.  As noted in prior issues of the Air Quality Letter, Kentucky and Indiana submitted proposed attainment and nonattainment designations to EPA in June 2011 (and Indiana submitted additional technical addendums to its preliminary recommendation on January 6, 2012 and April 26, 2012) based upon data from existing SO2 ambient air monitors.  Only Jefferson County, Kentucky and the following townships in Indiana: New Albany Township in Floyd County; Harrison Township in Vigo County; Wayne Township in Wayne County; Center, Perry and Wayne Townships in Marion County; Clay and Washington Townships in Morgan County; Veale Township in Daviess County; and Washington Township in Pike County were proposed by the states as nonattainment areas and all other areas of the states were proposed as unclassifiable.

By letter dated April 12, 2012, EPA advised states of the status of its development of final implementation guidance for SIPs for the 1-hour SO2 standard.  Those SIPs are to be submitted by June 2013.  EPA noted in its letter that states and other stakeholders had identified a number of concerns in response to the September 2011 draft implementation guidance.  To address those concerns, EPA indicated it would take several actions.  First, EPA will move forward with area designations as soon as possible based upon the recommendations submitted by the states in June 2011 that relied upon data from existing ambient air monitors.  Second, EPA will initiate focused stakeholder outreach to help define the approach for states to use to determine whether air quality in a given unclassifiable area is meeting the SO2 NAAQS.  Finally, EPA noted that it no longer expected states’ June 2013 SIP submittals to contain modeling demonstrations to assess attainment of the standard in unclassifiable areas as was proposed in the September 2011 implementation guidance.

The two main topics to be addressed in the stakeholder outreach meetings are:  (1) how best to assess compliance with the SO2 NAAQS through modified monitoring networks and/or through appropriate modeling; and (2) how to implement the new approach, including SIP deadlines for areas in which violations are identified by the monitoring and/or modeling.  In EPA’s May 2012 draft White Paper, EPA focuses on options for evaluating NAAQS compliance in “future-designated unclassifiable areas” that currently lack adequate compliance information/data by developing approaches to modeling and/or monitoring that would enable EPA to affirmatively determine whether areas of concern are attaining the 1-hour SO2 standard.  EPA noted that stakeholder concerns with the prior September 2011 implementation guidance included: the difficulty in meeting the suggested modeling approach for attainment demonstrations, especially in multi-source areas; the time and resource burden associated with such attainment demonstration modeling; and legal and policy objections to basing attainment demonstrations on modeling, especially since the modeling may over-estimate actual SO2 ambient concentrations.

In response to the concerns, EPA has proposed approaches that focus on how to determine current attainment status, rather than future attainment.  With respect to ambient air monitoring, EPA anticipates there will need to be expansion of current monitoring networks and reallocation of monitors in existing networks to focus on either high population concentrations or major sources of SO2 emissions.  For any ambient air monitoring alternative, EPA expects that modeling or other similar analyses would likely need to be completed in order to site monitors at the areas of highest concentration.  EPA expects that such modeling would be less resource intensive than attainment demonstration modeling.  With respect to modeling options, EPA notes that in recognition of the limitations of monitoring to identify high concentrations in the vicinity of SO2 sources and the resource burden associated with expanding monitoring networks, modeling may also need to be used to supplement monitoring or replace monitoring where appropriate.  EPA states that modeling of emissions from major sources should be conducted using AERMOD, and EPA is soliciting input on whether it would be based on actual emissions, allowable emissions, or potential emissions.  Finally, EPA notes that with respect to implementation of the standard, it may issue final guidance or a rulemaking that adopts minimum monitoring/modeling/ hybrid requirements for determining the attainment status of unclassifiable areas.  EPA’s modeling proposals will remain controversial due to the conservative nature of modeling as well as its use to designate areas as nonattainment in the absence of confirming monitoring data.

To view a complete PDF of the Second Quarter 2012 issue of the Air Quality Letter, click HERE.

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