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Sulfur Dioxide (SO2)

Most Recent Action

On May 13, the Environmental Protection Agency (EPA) issued proposed options for states and local air agencies to use air quality monitoring or modeling to determine whether areas meet the 2010 air quality standards for sulfur dioxide (SO2). See here for a discussion of the proposed rule.


The Clean Air Act (CAA) requires the EPA to set National Ambient Air Quality Standards (NAAQS) for “criteria pollutants.” Currently, sulfur oxides, lead, ozone, nitrogen oxides, carbon monoxide, and particulate matter are listed as criteria pollutants. The law also requires the EPA to periodically review the standards to ensure that they adequately protect public health and welfare. Primary standards address public health concerns while secondary standards impact the public welfare.

The primary standard is a 1-hour SO2 standard set at 75 parts per billion (ppb). The EPA revoked the old 140 ppb evaluated over 24-hours, and 30 ppb evaluated over an entire year standards because, according to the EPA, those standards do “not provide additional health benefits.”

The secondary SO2 standards are being reviewed jointly with the secondary NO2 standards. The EPA estimates new standards will be adopted by 2012. This is the first time EPA has reviewed the environmental impacts separately from the health impacts of these pollutants.

Wisconsin, like all other states, must create, and submit to EPA for approval, a state implementation plan (SIP) that addresses each NAAQS. The state must also help the EPA designate attainment and nonattainment areas.

This rule is part of a group of rules known as the EPA’s Regulatory Train Wreck.



Two sections of the Clean Air Act govern the establishment and revision of the NAAQS.

Section 108 (42 U.S.C. 7408) directs the Administrator to identify and list each air pollutant that ‘‘in his [or her] judgment, cause or contribute to air pollution which may reasonably be anticipated to endanger public health and welfare’’ and whose ‘‘presence…in the ambient air results from numerous or diverse mobile or stationary sources’’ and to issue air quality criteria for those that are listed.

\Section 109 (42 U.S.C. 7409) directs the Administrator to propose and promulgate ‘‘primary’’ NAAQS for pollutants listed under section 108. Section 109(b)(1) defines a primary standard as one ‘‘the attainment and maintenance of which in the judgment of the Administrator, based on [air quality] criteria and allowing an adequate margin of safety, are requisite to protect the public health.’’



Primary Standard:

The primary standard is a 1-hour SO2 standard set at 75 parts per billion (ppb). The EPA revoked the old 140 ppb evaluated over 24-hours, and 30 ppb evaluated over an entire year standards because, according to the EPA, those standards do “not provide additional health benefits.”

The EPA also changed the monitoring requirements for SO2. The EPA set specific minimum requirements for where states must place the 163 SO2 monitoring sites nationwide. The monitoring regulations require monitors to be placed in Core Based Statistical Areas (CBSAs) based on a population weighted emissions index for the area. The final rule requires: 3 monitors in CBSAs with index values of 1,000,000 or more; 2 monitors in CBSAs with index values less than 1,000,000 but greater than 100,000; and 1 monitor in CBSAs with index values greater than 5,000. Wisconsin has three CBSAs which each require one monitor. Any new monitors required by the revised rule must begin operating no later than January 1, 2013.

The EPA expects to use refined dispersion modeling as well as monitoring to determine compliance with the new standard. Dispersion modeling simulates how air pollutants spread throughout the atmosphere and is used to estimate the concentration of air pollutants from sources such as industrial plants or highways.

State and local agencies are required to report to the EPA two data values for every hour of monitoring conducted: the 1-hour average SO2 concentration; and the maximum 5-minute block average SO2 concentration for each hour.

The EPA expects to identify or designate areas not meeting the new standard by June 2012 based on data from existing monitors and modeling. The EPA’s planned designation approach is:

  • Any area that has monitoring data (or refined modeling results) showing a violation would be designated “nonattainment.”
  • Any area that has monitoring and refined modeling results showing no violations would be designated “attainment.”
  • All other areas would initially be designated “unclassifiable.”

States with monitored or modeled air quality violations are required to submit “nonattainment” state implementation plans (SIPs) by February 2014 that demonstrate that the area will attain the standard by August 2017.

For all other areas, maintenance SIPs required by the CAA are due in June 2013. These plans must demonstrate, through refined air quality modeling, that all sources contributing to monitored and modeled violations of the new standard, or that have the potential to cause or contribute to a violation, will be sufficiently controlled to ensure timely attainment and maintenance of the new SO2 standard; and include enforceable emissions limitations, timetables for compliance, and appropriate testing and reporting to assure compliance.

The final rule also changes the Air Quality Index to reflect the revised SO2 standard.

The Wisconsin DNR is developing a program and plan to address expected SO2 modeled nonattainment areas.


Secondary Standard:

Secondary NAAQS are required to mitigate the environmental, social, and economic impacts of air pollution.

On March 20, 2012, the U.S. Environmental Protection Agency (EPA) took final action to retain the current secondary National Ambient Air Quality Standards (NAAQS) for oxides of nitrogen (NOx) and oxides of sulfur (SOx). This was a change from the proposed rule.

The existing secondary standards are:

  • For NO2: 0.053 ppm (parts per million) averaged over a year; and
  • For SO2: 0.5 ppm averaged over three hours, not to be exceeded more than once per year



The EPA is extending the deadline for area designations for the 2010 primary SO2 standard until June 3, 2013 due to insufficient information.

By taking the additional time, the EPA is now required under CAA section 107 to promulgate designations by June 3, 2013. The EPA expects to take additional time, as necessary, to appropriately assess designations. For some areas, EPA anticipates it will not be necessary to take the full additional year, and in those cases EPA will proceed sooner than June 2013. For example, the EPA intends to make its best effort to promulgate final designations for areas with monitored violations of the SO2 NAAQS by the end of calendar year 2012, subject to being able to resolve issues related to nonattainment boundary determinations and contributions from nearby areas, rather than take until June 2013 for those areas.


Additional Information

Primary Standard:

Secondary Standard: