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Nitrogen Dioxide (NO2)

Most Recent Action

The EPA has designated the entire nation as “unclassifiable/attainment” for the 2010 primary NO2 standard.


The Clean Air Act 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 health and welfare.

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.

The EPA first set standards for NO2 in 1971, setting both a primary standard, to protect health, and a secondary standard, to protect the public welfare, at 0.053 parts per billion (53 ppb), averaged annually.  The primary standard was changed in 2010, and the secondary standard is currently under review.



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

Section 108 (42 U.S.C. 7408) directs the EPA 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 Standards:

In 2010 the EPA set a new 1-hour NO2 standard at the level of 100 ppb. This level defines the maximum allowable concentration anywhere in an area. The EPA is retaining, with no change, the current annual average NO2 standard of 53 ppb.

In addition to establishing an averaging time and level, EPA also is setting a new “form” for the standard. The form for the 1-hour NO2 standard is the 3-year average of the 98th percentile of the annual distribution of daily maximum 1-hour average concentrations.

To ensure compliance with the revised NO2 standard, the EPA also made changes to the NO2 air quality monitoring network requirements.

  • At least one monitor will be located near a major road in any urban area with a population greater than or equal to 500,000 people. A second monitor will be required near a major road in areas with either a population greater than or equal to 2.5 million people, or one or more road segments with an annual average daily traffic count greater than or equal to 250,000 vehicles.
  • A minimum of one monitor will be placed in any urban area with a population greater than or equal to 1 million people to assess community-wide concentrations
  • Working with the states, the EPA Regional Administrators will site at least 40 additional NO2 monitors to help protect communities that are susceptible and vulnerable to NO2 -related health effects.

The EPA estimates the revised NO2 monitoring network will require 126 NO2 monitoring sites near major roads in 102 urban areas; 53 additional monitoring sites to assess community-wide levels across wider urban areas; and 40 monitors in low income or minority at-risk communities. All new NO2 monitors must begin operating no later than January 1, 2013.

The EPA’s Regional Administrators have the authority to require additional monitoring in certain circumstances, such as in areas impacted by major industrial point sources or a combination of sources where there is an indication that the standards may be exceeded. The Regional

Administrators also have the authority to require additional near-road monitoring in urban areas where multiple peak concentration areas may be caused by a variety of mobile source factors including fleet mix, traffic congestion patterns, or terrain.

Implementation Schedule
January 2011 States send designation recommendations to the EPA based on existing network data.
January 2012 Areas with monitors recording violations of the new standards will be designated “nonattainment.” The EPA will designate all or most areas as “unclassifiable” because near road monitors are not yet in place.
January 2013 All monitors in the new NO2 monitoring network are operational.
January 2015 Next NO2 NAAQS review completed.
January 2016/2017 Depending on the date that sites become operational, the EPA will issue nonattainment re-designations.
January 2021/2022 Attainment date (5 years after date of nonattainment designations).



There are currently five NO2 monitors operating in Wisconsin. Three of these monitors, Ashland, Cassville and Potawatomi, have only been measuring NO2 concentrations during 2010. The Ashland and Cassville monitors are part of a special study by the WDNR and are expected to cease operations on or around June 30, 2011. The Manitowoc monitor is part of the national Photochemical Assessment Monitoring Stations (PAMS) network, as required by the U.S. EPA to better understand ozone and its precursors. As such, this monitor only operates from June 1 – August 31. The Milwaukee monitor has measured NO2 concentrations for several years.

The DNR plans to install NO2 near-roadway monitors in Madison and Milwaukee. Final locations of the monitors are still being determined. These monitors must be operational by January 1, 2013 to meet the NO2 monitoring requirements. Section 107(d)(1)(A) of the CAA requires states to provide area attainment / nonattainment designation recommendations based on the 2010 NO2 NAAQS to the U.S. EPA no later than January 22, 2011.

Based on the available data, the Governor recommended all 72 Wisconsin counties be designated as attainment.


Secondary Standards:

EPA plans to retain the current nitrogen dioxide (NO2) and sulfur dioxide (SO2) secondary standards to provide protection for the direct effects on vegetation from exposure to gaseous oxides of nitrogen and sulfur in the ambient air. The EPA is proposing to add secondary standards identical to the NO2 and SO2 primary 1 – hour standards and not set a new multi-pollutant secondary standard in this review. The proposed 1-hour secondary standard would be set at a level of 100 ppb and the proposed secondary SO2 standard would be set at 75 ppb.


Additional Information

The EPA’s NO2 regulatory action webpage