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Sulfur Dioxide

Sulfur Dioxide Implementation - Programs and Requirements for Reducing Sulfur Dioxide

The Clean Air Act requires EPA to set National Ambient Air Quality Standards (NAAQS) for six criteria pollutants; sulfur dioxide (SO2) is one of these.  EPA works with partners at state, local, and tribal air quality agencies to meet these standards.

General Information on Designation and Implementation Requirements

State Implementation Plans designed to reduce emissions in areas designated as nonattainment will be due within 18 months of the effective date of designation, and the attainment date for nonattainment areas will be within 5 years of designation.

Under the Clean Air Act (CAA), as amended in 1990, each state must develop a plan describing how it will attain and maintain the NAAQS.  In other words, how it plans to clean up polluted areas and keep them clean.  This plan is called the State Implementation Plan (SIP) and is required under Section 110 of the CAA (40 CFR Part 51, Subparts F & G).  In general, the SIP is a collection of programs, including:

  • a monitoring program, which is a collection of monitoring devices throughout the country which provide actual measurements of the concentrations in the air, to identify whether an area is meeting the air quality standards, and if not, how much reductions are needed to meet those standards;
  • air quality calculations and computer modeling, which are used to predict future trends and the effects of emissions reduction strategies;
  • emissions inventories, which describe the sources and categories of emissions to the air for a given pollutant, and how much is emitted by each source or source category;
  • control strategy studies whose goal is finding the best way to reduce emissions in order to meet air quality standards;
  • formal adoption of measures (enforceable by EPA, States and citizens) which ensure that we will achieve the reductions deemed necessary in the planning process;
  • periodic review to evaluate whether those needed reductions were achieved in reality, and whether they had the predicted result.

The air quality agency responsible for the State Implementation Plan, (usually a state agency) must provide the public an opportunity to review the plan before sending it to EPA for approval.

In cases where the EPA fails to approve a state implementation plan, the Agency can issue and enforce a Federal Implementation Plan (FIP) to ensure attainment and maintenance of the NAAQS.   In addition, the Clean Air Act contains penalties, referred to as "sanctions" which EPA can impose in areas not satisfying the State Implementation Plan requirements. 


Guidance for Sulfur Dioxide Nonattainment Areas

April 23, 2014 - EPA is providing guidance to assist air agencies with implementing the 2010 1-hour National Ambient Air Quality Standard for sulfur dioxide (SO2) in areas designated as nonattainment for the standard.  Children, the elderly and people with asthma are susceptible to health problems associated with breathing SO2, including narrowing of the airways, which can cause difficulty breathing, and increased asthma symptoms.

EPA Proposes Options to Assess Sulfur Dioxide Air Quality

April 17, 2014 - After carefully considering input from a broad range of stakeholders, EPA is proposing options that would allow state and local air agencies to use air quality monitoring or modeling to determine whether areas across the United States meet the 2010 air quality standards for sulfur dioxide. 

EPA Updates Monitoring and Modeling Technical Assistance Documents for Implementing the 2010 Sulfur Dioxide Standard

January 7, 2014 - EPA is making available two updated draft documents that provide technical assistance for states implementing the 2010 health-based, sulfur dioxide (SO2) standard.  These documents provide technical advice on the use of modeling and monitoring to determine if an area meets the 2010 SO2 air quality standard.  

EPA appreciates the comments it received on the May 2013 drafts of these documents.  Many of those comments have been addressed, others referred to parts of EPA’s forthcoming proposed Data Requirements Rule for Implementing the 1-Hour SO2 standard. To allow for further comments once that rule is proposed, EPA plans to make an updated version of these documents available around the time the Data Requirements Rule is finalized.

Area Designations and Implementation Strategy

February 7, 2013 - EPA issued the final primary NAAQS for SO2 on June 2, 2010.  In early June 2013, after the consideration of recommendations from air agencies, EPA intends to designate as "nonattainment" those areas with air quality monitors that violate the SO2 standard. EPA has also developed an updated strategy for identifying and addressing unhealthy levels of SO2 in the rest of the country. This strategy recognizes that 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.

Stakeholder Outreach

April 12, 2012 - EPA is seeking additional input from states, tribes, and other interested parties to refine the agency's approach for implementing the SO2 standard. We have set up an outreach process that allows for focused discussion of monitoring, modeling and implementation issues. This effort will ensure that the agency has the information it needs to protect public health from unhealthy levels of SO2 in the outdoor air.

EPA provided letters to states and tribes to notify them of these plans. Samples of the state and tribal letters are included below. These letters do not represent final agency action.

EPA is convening three small group meetings with stakeholders to discuss issues associated with implementation of the SO2 standard.

Schedule for Stakeholder Discussions

  • Session 1: Environmental and Public Health Organization Representatives
    May 30, 2012, Washington, DC
  • Session 2: State and Tribal Representatives
    May 31, 2012, Research Triangle Park, NC
  • Session 3: Industry Representatives
    June 1, 2012, Research Triangle Park, NC

For more information about these meetings, please contact Carolyn Childers at (919) 541-5604.

White Paper - To stimulate discussion at the small group meetings and to promote progress toward a workable approach for implementing the 1-hour SO2 standard, EPA developed a white paper that identifies important questions about how we could determine whether an area is meeting the SO2 standard and how we might use monitoring, modeling, or a combination of monitoring and modeling. The current version of the white paper is included below, along with summary information for the stakeholder discussions. This white paper is not a final agency action.

Summary of Key Comments from Stakeholder Meetings on SO2 Implementation

June 18, 2012 - High level summaries of key comments from the stakeholder meetings are provided below.

September 22, 2011 - Draft Implementation Guidance for the Primary National Ambient Air Quality Standard for Sulfur Dioxide


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