Air Pollution Control Program

Sulfur Dioxide

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On Feb. 7, 2013, EPA sent a letter to all states with regard to the 1-hour sulfur dioxide, or SO2, National Ambient Air Quality Standard, or NAAQS.  This so-called “120-day”  letter sent to Missouri is available here.  Per Missouri’s letter, EPA intends to designate parts of Jackson and Jefferson Counties as nonattainment, the two parts of the state with monitors in violation of the 1-hour SO2 standard.  The Air Program plans to formally revise our boundary recommendation by removing the Springfield area as a recommended SO2 nonattainment area based on 2010-2012 monitoring data.  Based on these plans and ongoing communication, EPA does not intend to designate the Springfield, Greene County area as nonattainment.  EPA expects to finalize this round of designations based on monitored violations of the SO2 NAAQS by June 2013.

A draft technical support document supporting the 120-day letter provides additional information.  Within this document, EPA agrees with the recommended nonattainment area boundaries in Jackson and Jefferson Counties.

In other news related to SO2 NAAQS implementation, EPA released a DRAFT implementation document outlining an updated strategy for addressing SO2 areas of concern that are not near existing monitors in violation of the 1-hour SO2 standard.  EPA notes this strategy recognizes existing SO2 monitors may not adequately characterize air quality around many of the largest SO2 sources across the country.  Accordingly, the strategy presents a process and timetable by which states would characterize air quality in these areas, either through expanded air quality monitoring or air quality modeling techniques.  The strategy also mentions plans for early reduction incentives associated with permanent and enforceable SO2 emission reductions.  For more information, see the "Area Designations and Implementation Strategy" heading at the top of EPA's SO2 implementation webpage:

Sulfur Dioxide

Sulfur dioxide is a colorless gas with a strong, suffocating odor. It can cause irritation of the throat and lungs, leading to difficulty breathing, increased asthma symptoms and more respiratory illnesses.

Sulfur dioxide is sometimes referred to as one of the Environmental Protection Agency's criteria air pollutants. Criteria pollutants are commonly found air pollutants that, at high enough levels, can harm human health and the environment. Because of the potential to harm people and the environment, EPA and the Department of Natural Resources limit the amount of sulfur dioxide sources can emit into the atmosphere. The department monitors sulfur dioxide levels in the air at certain locations across the state. Sulfur dioxide also contributes to the formation of fine particle pollution, another criteria pollutant, also known as particulate matter.

Sulfur Dioxide - National Ambient Air Quality Standard Revision

In June 2010, EPA established a new 1-hour primary sulfur dioxide standard of 75 parts-per-billion. States must submit boundary recommendations for area designations using this new standard to EPA by summer 2011, and EPA expects to issue final designations by June 2013. EPA anticipates initially designating areas based on 2008 to 2010 monitoring data or refined dispersion modeling results if provided by the state. Areas violating the standard based on monitoring or modeling data would be designated as nonattainment and all other areas would be designated either attainment or unclassifiable.

EPA plans to issue guidance on the use of air quality models to support designations, state implementation plan development and other implementation issues related to the new sulfur dioxide standard. EPA expects to propose this guidance in summer 2013. As the sulfur dioxide modeling and implementation guidance is finalized, the department intends to conduct refined dispersion modeling of sulfur dioxide sources in accordance with EPA’s guidelines and provide additional technical justification as needed for the recommended nonattainment area boundaries.

Based on current data, monitors located in Kansas City and Herculaneum are in violation of the 1-hour sulfur dioxide standard. Based on violations recorded at these monitors, the Air Program is required to recommend a nonattainment area boundary for each of these areas.

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Boundary Designation Process

EPA requires each state to determine which areas in the state will not meet the 2010 National Ambient Air Quality Standard for sulfur dioxide, known as nonattainment areas. Since the 2010 sulfur dioxide standard does not require modeling for initial nonattainment determinations and EPA’s modeling guidance was not available in time to accommodate our public hearing deadlines, the department is basing boundary recommendations on factors other than modeling, such as monitoring data, emissions data, and meteorology. The department presented draft nonattainment recommendations at public information sessions held in areas with violating monitors during early April 2011 and at the formal public hearing on April 28, 2011. The department submitted the nonattainment boundary designation to EPA on July 19, 2011. EPA has the authority to make the final determination of nonattainment area boundaries.

The EPA has chosen a different approach to determine attainment status for the 2010 sulfur dioxide standard. Unlike other criteria pollutants, sulfur dioxide is almost exclusively a point source-emitted pollutant. Additionally, transport of sulfur dioxide emissions is typically more localized and is less likely dependent on a regional scale. A monitoring network large enough to adequately cover all large sources would be prohibitively expensive and an affordable network would leave large gaps in coverage. Therefore, EPA has decided to use a hybrid monitoring-modeling approach for sulfur dioxide. This hybrid approach involves a modest monitoring network and extensive use of refined air dispersion modeling. A combination of monitoring and modeling will also be used to assess compliance with the 2010 1-hour sulfur dioxide standard.

A summary of the implementation timeline for the 1-hour sulfur dioxide standard is as follows:

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To help communities that may be designated as nonattainment, the department has and will be holding public meetings with local city, county and business leaders. Documents and presentations from these meetings will be posted here.


Kansas City


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