Air Pollution Control Program

Sulfur Dioxide

Primary Standard | Boundary Designation Process | State Implementation Plans | Links


Sulfur dioxide is a colorless gas with a strong, suffocating odor. Sources of sulfur dioxide emissions include fossil-fuel fired power plants, metal/ore processing industries, other industries that combust fossil fuels, and certain nonroad engines such as locomotives and marine vessels.  Exposure to elevated concentrations of sulfur dioxide can cause irritation of the throat and lungs, leading to difficulty breathing, increased asthma symptoms and other respiratory illnesses.   The department monitors sulfur dioxide levels in the air at certain locations across the state.

Sulfur dioxide is 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 that sources can emit into the atmosphere. Sulfur dioxide also contributes to the formation of fine particle pollution, another criteria pollutant, commonly known as particulate matter. Additional information about sulfur dioxide can be found on EPA’s sulfur dioxide webpage.

Sulfur Dioxide National Ambient Air Quality Standard (NAAQS)

In June 2010, EPA established a new 1-hour primary sulfur dioxide standard of 75 parts-per-billion. The previous sulfur dioxide primary standards set in 1971 included a 24-hour standard at 140 parts-per-billion and annual standard at 30 parts-per-billion.  There is also a secondary sulfur dioxide standard based on a 3-hour average set at 500 parts per billion.  When EPA revised the sulfur dioxide standard in 2010, they revoked the two previous primary standards, replacing them with the new 1-hour primary standard and they retained the existing secondary standard.

Based on ambient monitoring data from 2007 - 2009, areas in Kansas City and Herculaneum were in violation of the 1-hour sulfur dioxide standard. Based on the violations recorded at these monitors, these areas were designated nonattainment under the 2010 sulfur dioxide standard effective in October 2013.           

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Boundary Designation Process

When EPA revises a NAAQS, states are allowed to submit boundary designation recommendations to EPA to be considered when EPA establishes the final boundary designations.  For most criteria pollutants, states are given one year following a NAAQS revision to submit their recommendations and EPA finalizes boundary designations within two years of the NAAQS revision.

The EPA has chosen a different approach to establish boundary designations under 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. Additionally, EPA is splitting the boundary designation process into multiple rounds. The initial round for boundary designations was based on available ambient air quality monitoring data.  Future-round designations will be based on a hybrid approach involving emissions inventory analysis, an enhanced monitoring network and extensive use of refined air dispersion modeling. In May 2014, EPA proposed a data requirements rule discussing potential options for evaluating areas in the future-round sulfur dioxide boundary designations process.

A summary of the boundary designations timeline for the 1-hour sulfur dioxide standard based on EPA’s proposed data requirements rule is given in the table below:

Expected Date

Action

June 2011

States submit initial round boundary designation recommendations

February 2013

EPA notifies states of intended initial-round recommendations (120-day letters)

August 2013

EPA finalizes initial-round boundary designations

January 2016

States submit a list of SO2 sources in their state and indicate whether each source will be characterized by modeling or by newly placed air quality monitors

January 2016

Modeling protocols must be submitted for sources that are to be characterized with modeling

July 2016

Monitoring network plans that satisfy the data requirements rule must be submitted

January 2017

SO2 monitors that satisfy the data requirements rule must be operational

January 2017

States submit modeling analyses and recommendations for sources that are to be characterized with modeling

August 2017

EPA notifies states of intended designations for sources characterized by modeling (120-day letters)

December 2017

Final Designations for sources characterized by modeling

May 2020

Certified monitoring data submitted for 2017 – 2019 and states submit recommendations for sources characterized by monitoring

August 2020

EPA notifies states of intended designations for sources characterized by monitoring (120-day letters)

December 2020

Final Designations for sources characterized by monitoring

In June 2014, in an effort to resolve litigation surrounding the SO2 boundary designations process, EPA proposed a federal consent decree and solicited comments on the proposal.  The proposed federal consent decree would impose a strategy and timeline for future-round boundary designations that differs from EPA’s proposed data requirements rule.  The federal register notice for this proposed consent decree can be found here.

As the state develops new boundary designation recommendations, they will be made available for public review and comment online at the Air Program’s Public Notice webpage

Information regarding previous boundary designation processes for the sulfur dioxide NAAQS can be found at the Air Program’s NAAQS Boundary Designations webpage.

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State Implementation Plans (SIP) for Sulfur Dioxide

After a new NAAQS is finalized, states have three years to develop an infrastructure SIP that demonstrates the state’s ability to implement, maintain and enforce the NAAQS.  Missouri developed the infrastructure SIP for the 2010 sulfur dioxide NAAQS and submitted it to EPA for approval in July 2013.

Because areas in Jackson County and Jefferson County were designated SO2 nonattainment areas in the initial boundary designations process, additional nonattainment related SIP elements must be submitted to EPA.  As new SIP elements are released for public notice, they will be available for public review and comment online at the State Plans on Public Notice webpage.

If new areas in Missouri are designated nonattainment for the 2010 sulfur dioxide standard in the future rounds of designations then nonattainment related SIP elements will need to be developed for those areas as well.  The following table provides dates from EPA’s proposed data requirements rule regarding the timing for nonattainment related SIP submittals depending on when the area is designated nonattainment.

Expected Date

Action

April 2015

States submit nonattainment related SIPs for areas designated nonattainment in the initial-round boundary designations

August 2019

States submit nonattainment related SIPs for areas designated nonattainment based on modeling characterization

August 2022

States submit nonattainment related SIPs for areas designated nonattainment based on 2017 - 2019 monitoring data from new SO2 monitors

For information regarding Missouri’s past SIP submittals addressing sulfur dioxide, click the link below.

Sulfur Dioxide State Implementation Plan

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