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May 20, 2014
Proposal would allow SO2 monitoring or modeling

As part of its strategy for the 2010 National Ambient Air Quality Standards (NAAQS) for sulfur dioxide (SO2), the EPA has proposed requirements applicable to states that must characterize air quality in priority source areas through either monitoring or modeling and submit such data to the Agency.  According to the EPA, the two approaches would provide more assurance that the next round of designations accurately reflects actual air quality conditions that states will address with their state implementation plans (SIPs). 


In the 2010 NAAQS, the EPA set a new 1-hour daily maximum primary SO2 standard at 75 parts per billion.  Following establishment of the NAAQS, a number of issues came up, including the concern that the existing SO2 air monitoring network is insufficient to develop the data needed to accurately designate areas that are in nonattainment with the NAAQS. 

Traditionally, air agencies have relied on monitoring to develop data on the concentrations of SO2 in the ambient air.  This approach reached its peak in 1980 when, nationwide, there were approximately 1,500 monitors measuring SO2.  But because of improved air quality and reduced agency resources, that number has declined to about 450.  Perhaps more significantly, only about one-third of these monitors are located in areas with the highest SO2 concentrations.  Thus, many existing monitors are effectively underreporting peak 1-hour concentrations. 

Unlike other NAAQS wherein regional measurement is critical (e.g., for ozone), the SO2 NAAQS is more source-oriented in accordance with the short-term health issues of sensitive individuals, which are addressed by the 1-hour form.  Based on the 2011 National Emissions Inventory, there are about 1,500 sources exceeding 100 tons of annual SO2 emissions.  The largest sources of SO2 include coal-fired electric utilities, industrial boilers, refineries, pulp and paper-related industries, and chemical manufacturing plants.

Cost of monitoring

The intent of the proposal is to provide state agencies with effective means to characterize 1-hour SO2 concentrations in priority locations across the country such that these data could inform future area designations for the SO2 NAAQS, while taking into consideration limited EPA and state resources.   The capital cost of siting a new monitor can be on the order of $50,000 to $100,000.  Routine operations and maintenance costs would be in addition to those up-front capital costs.  Thus, the EPA recognizes that a monitoring-only approach may not be practicable. 

Proposed requirements

Accordingly, the proposal would provide air agencies with the flexibility to choose either monitoring or modeling to characterize air quality around or in proximity to identified sources.  Specific proposed steps include the following:

  • By January 15, 2016, the air agency must submit a list of applicable sources located in its jurisdiction.  This list may be revised by the EPA regional administrator after review based on available SO2 emissions data.
  • For each area containing an applicable source, the air agency will state by January 15, 2016, whether it will characterize air quality through ambient air quality monitoring or through air quality modeling techniques.
  • For any area for which air quality will be characterized through ambient monitoring, the monitors must be sited and operated in a manner that comports with federal reference methods or federal equivalent methods.  The resulting data should be reported to the Air Quality System (AQS) and would be subject to annual data reporting and certification requirements.  The data will also be available to the public.
  • For each area for which air quality will be characterized through modeling, the air agency must submit by January 15, 2016, a technical protocol for conducting such modeling to the EPA regional administrator for review.  The air agency must consult with the appropriate  EPA regional office in developing these modeling protocols.
  • The air agency must conduct the modeling analysis for any applicable source pursuant to Section 40 CFR 51.1203(a) and for its associated area and any nearby area, as applicable, and submit the modeling analysis to the EPA regional office by January 13, 2017.

While the modeling approach is less resource intensive, it is generally not supported by industry because modeling techniques inherently overpredict SO2 concentrations by assuming a constant rate of peak emissions and worst-case meteorological conditions.  Predictably, environmental groups prefer the use of modeling, which, they say, can be done more quickly, with less expense, and for more locations (including locations where physically siting a monitor would be very difficult) than monitoring.

Also, the EPA has issued separate nonbinding draft technical assistance documents on how air agencies can conduct monitoring or modeling.

The proposal was published in the May 13, 2014, FR.

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