1971-1997
In 1971 the US Environmental Protection Agency (EPA) established National Ambient Air Quality Standards for several air pollutants, including ground-level ozone pollution.
In 1978, Denver was designated as a nonattainment area under the EPA’s standard. The air quality in a nonattainment area does not meet one or more of the air quality standards set by the federal government.
In 1979, the EPA revised the national ozone standard of 120 parts per billion of ozone in ambient air, based on a 1-hour averaging time for the measurement.
In 1982, the Denver metropolitan area developed a plan to attain the standard and submitted it to the EPA. The EPA approved Colorado’s Ozone State Implementation Plan on December 12, 1983. The Denver metropolitan area has attained this standard and has not had a violation since 1987.
In 1997 the EPA disseminated a new standard of 80 parts per billion based on an 8-hour averaging time and 3 years of data. Implementation of the 1997 standard was delayed due to litigation and congressional action.
On September 11, 2001 the EPA changed the Denver metropolitan area 1-hour ozone designation to attainment and approved the region’s Ozone Maintenance Plan.
2004-2007
In April 2004, the EPA designated the Denver metro/north Front Range as a nonattainment area for the 1997 8-hour ozone standard. However, the EPA deferred the effective date of the classification in return for a commitment from the state of Colorado, the Regional Air Quality Council, and others to implement ozone pollution control measures sooner than required by the federal Clean Air Act. This commitment was contained in the Denver Early Action Compact.
That deferral expired on November 20, 2007, on which date the EPA designated the Denver metro/north Front Range as a "marginal" nonattainment area for the 1997 federal 8-hour ozone standard. The nonattainment classification was a result of a violation of the federal ozone pollution standard based on data from 2005-2007.
2008-2014
In April 2008, the EPA revoked the attained 1-hour standard for the Denver metropolitan area. A detailed Ozone Action Plan was developed by the Air Pollution Control Division, along with the Regional Air Quality Council and the North Front Range Metropolitan Planning Organization. The resulting attainment plan was approved by the Air Quality Control Commission in December 2008, and submitted by the governor to the EPA on June 18, 2009. The plan required greater reductions in ozone pollution levels than the 2004 Ozone Early Action Compact.
In March 2008, EPA decreased the ozone standard from 80 to 75 parts per billion averaged over an 8-hour period and 3 years of data. Colorado evaluated the impact of the new standard and determined that no areas outside the Denver metro/north Front Range violated the standard.
In November 2010, the motor vehicle inspection and maintenance program expanded from Denver into parts of Larimer and Weld Counties to include Fort Collins, Greeley, and nearby areas, as required by the 2008 Ozone Action Plan.
Colorado adopted a Regional Haze Plan in 2010 and 2011 that included new controls for nitrogen oxides at power and cement plants. These nitrogen oxides controls also assisted ozone pollution reduction efforts.
In 2011, the EPA partially approved the 1997 standard Attainment Demonstration State Implementation Plan. The Denver metro/north Front Range had not violated the 1997 ozone standard since 2008.
Effective July 20, 2012, the EPA classified the Denver metro/north Front Range as a "marginal" nonattainment area for the 2008 Ozone standard of 75 ppb based on 2008-2010 monitor data.
In early 2014 the Colorado Air Quality Control Commission adopted regulatory changes to reduce volatile organic compound emissions from the oil and gas sector. In addition, new federal vehicle emissions standards and Colorado's motor vehicle inspection and maintenance program also helped reduce pollutants that create ozone.
2015-2017
Progress towards 2008 ozone standard
While ozone pollution concentrations improved over time, the region did not attain the 2008 75 ppb standard by the July 20, 2015 attainment deadline. Thus, in July 2016 the Denver metro/north Front Range was reclassified from a “marginal” to a “moderate” nonattainment area. As a result, the division and Regional Air Quality Council developed a revised State Implementation Plan to reduce ozone pollution levels. The plan included detailed technical analysis of the formation of ozone pollution, future trends in ozone pollution levels, and strategies to reduce ozone pollution. The Air Quality Control Commission approved the moderate State Implementation Plan in November 2016, and approved changes in November 2017 that implemented additional reduction strategies.
Progress towards 2015 ozone standard
Effective October 1, 2015, the EPA revised the ozone standard to 70 parts per billion, averaged over an 8 hour period and based on 3 years of data. The state reviewed monitor data relative to the revised standard, and in September 2016 the Air Quality Control Commission adopted the recommendation that the Denver metro/north Front Range be designated as nonattainment with the new standard, based on monitoring data from 2013-2015.
In 2017, the division launched a voluntary emission reduction program with industry in the Denver metro/north Front Range area. Program participants receive emails two to three days before a forecasted high ozone pollution day and are encouraged to take emission reduction actions on those days. In addition, Colorado began to incentivize reductions of ozone-causing pollutants using resources available through the Volkswagen settlement to fund the replacement of diesel medium and heavy duty fleet vehicles and school and transit buses with newer, more efficient, and cleaner electric or renewable natural gas technologies.
2018-2019
Historically, when EPA issued a revised standard, the previous standard was revoked and states would focus on meeting the most current standard. As a result of a court decision in February 2018, revocation of the 2008 ozone standard was not included in the 2015 ozone standard rule, which was finalized in June 2018. Therefore the region continues to conduct planning efforts for both the 2008 and 2015 ozone standards. In June 2018, EPA classified the Denver metro/north Front Range as a “marginal” nonattainment area for the 2015 8-hour ozone standard, effective August 3, 2018 with an attainment date of August 3, 2024.
In 2018 the division developed guidance for implementing an existing rule under the Emission Reduction Credit program to incentivize sources to shut down or modify high emitting sources to permanently reduce emissions.
In 2017 Colorado had a “clean data” year with no exceedances of the 75 ppb standard, except for 2 days where there was a clear causal relationship between wildfires and monitored ozone pollution levels. A clean data year qualifies a nonattainment area to request a one-year extension to demonstrate attainment of the standard. In June 2018, Colorado submitted to EPA an exceptional events demonstration documenting the clear relationship between the wildfires and the monitored data for 2 days in September 2017, and a request to extend the 2008 attainment deadline for the nonattainment area. The EPA issued a letter of concurrence in July 2018. In March 2019, Governor Polis notified EPA to withdraw the extension request. In July 2018, November 2018, and December 2019 the commission approved changes to the State Implementation Plan to implement expanded and additional control measures.
To further reduce ground-level ozone pollution, improve air quality, and comply with federal requirements, the division, the Air Quality Control Commission, and the Regional Air Quality Council continued working with external stakeholders to identify further steps to reduce emissions from oil and natural gas sources beyond existing regulations. At the commission's directive, the division initiated and led the Statewide Hydrocarbon Emissions Reduction stakeholder process and the Pneumatic Controller Task Force over the 2018-2019 timeframe. The division also promulgated rules in 2018 and 2019 to promote the adoption of low emission and zero emission vehicles.
2020-2021
2008 ozone standard
In January 2020, the EPA designated the Denver metro/north Front Range area as a “serious” nonattainment area under the 2008 ozone standard with an attainment deadline of July 20, 2021, based on 2018-2020 ozone season data. The Air Pollution Control Division and Regional Air Quality Council developed a revised serious area State Implementation Plan that included additional strategies to reduce ozone pollution. In December 2020, the Air Quality Control Commission approved the revised State Implementation Plan. The commission also approved changes to the State Implementation Plan July 2021 and December 2021 to expand and implement additional reduction strategies.
2015 ozone standard
In June 2020, the Air Quality Control Commission approved the emissions statement, emissions inventory, and State Implementation Plan compliance certification to be submitted to the EPA, satisfying the marginal nonattainment area requirements for the 2015 standard. The attainment deadline for the 2015 standard was August 3, 2021, based on 2018-2020 ozone season data.
In 2021, the EPA revised the boundary of the nonattainment area under the 2015 standard. Under this standard, the nonattainment area boundary was expanded to include the entirety of Weld County, effective December 30, 2021. This boundary change only applied to the 2015 standard.
2022
2008 ozone standard
Because the area did not attain the 75 ppb standard by July 20, 2021, the Denver metro/north Front Range region was redesignated from a “serious” to a “severe” classification, effective November 7, 2022. During 2022, the Air Pollution Control Division and Regional Air Quality Council developed a detailed severe State Implementation Plan with assistance from the Denver Regional Council of Governments and the North Front Range Metropolitan Planning Organization.
2015 ozone standard
Because the area did not attain the 70 ppb standard by August 3, 2021, the Denver metro/north Front Range region was redesignated from a “marginal” to a “moderate” nonattainment area, effective November 7, 2022. During 2022 a detailed moderate State Implementation Plan was developed in parallel with the aforementioned severe State Implementation Plan. The full plan was presented to the Air Quality Control Commission. The plan was approved by the commission in December 2022 and the division and Regional Air Quality Council committed to continued work to identify and implement additional emissions reduction strategies.
2023
2008 ozone standard
During 2023, the Air Pollution Control Division and the Regional Air Quality Council revised the severe State Implementation Plan emissions inventory and associated modeling. In December 2023, the Air Quality Control Commission approved the severe State Implementation Plan to meet the 2008 federal standard. The State Implementation Plan will also help make progress towards meeting the 2015 standard. The commission approved additional regulations to reduce emissions of nitrogen oxides, an ozone precursor, from the oil and gas sector and to reduce emissions from lawn and garden equipment. For more information please read our press release.
2015 ozone standard
In June 2024, the Air Pollution Control Division requested the U.S. Environmental Protection Agency to update Colorado’s classification for ozone pollution. The EPA approved the request on July 24, 2024. This update primarily impacts northern Weld County. The update reclassifies the Denver metro/northern Front Range, including northern Weld County, from a “moderate” to a “serious'' nonattainment area. The update is based on air monitoring data collected between 2021 and 2023, which showed the area would not meet the EPA’s 2015 standard for ozone pollution.
This voluntary reclassification provides administrative relief for both state and federal agencies, allowing the division to continue focusing on identifying and implementing strategies to reduce ground-level ozone pollution.
The division will continue to explore more ways to reduce ozone pollution and will further update Colorado's state implementation plan in the future. This process will include continued opportunities for public feedback.