History of ozone in Colorado

Early Standards

In 1971 the US Environmental Protection Agency (EPA) established National Ambient Air Quality Standards (NAAQS) for several air pollutants, including photochemical oxidants. Under the standard, the Denver Metropolitan Area (Adams, Arapahoe, Boulder, Denver, Douglas, and Jefferson counties) was designated as nonattainment in 1978. An ozone nonattainment area is one in which air quality does not meet the ozone standards set by the federal government. In 1979, EPA revised the photochemical oxidant standard to a national ozone standard of 120 parts per billion of ozone in ambient air, based on a 1-hour averaging time for the measurement. EPA approved Colorado’s 1-hour Ozone State Implementation Plan (SIP) on December 12, 1983. The region has attained this standard and has not had a violation since 1987. On September 11, 2001 the EPA approved the Denver Metropolitan Area 1-hour ozone designation to attainment and maintenance plan. 

In 1997 the EPA promulgated an 8-hour ozone standard of 80 parts per billion. Implementation of the 1997 standard was delayed due to litigation and congressional action.  


In April 2004, the US Environmental Protection Agency designated the Denver Metro/North Front Range (DM/NFR) (Adams, Arapahoe, Boulder, Broomfield, Denver, Douglas, Jefferson and parts of Larimer and Weld counties) as nonattainment for the 1997 8-hour ozone standard, but 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 control measures sooner than required by the federal Clean Air Act.
This commitment was contained in the Denver Early Action Compact. In return for this early action and for meeting certain milestones, the EPA agreed to defer the effective date of the nonattainment classification. That deferral expired on Nov. 20, 2007.

In April 2008, EPA revoked the 1-hour standard for the Denver Metropolitan Area.

DM/NFR became a "marginal" nonattainment area for the 1997 federal ozone standard on November 20, 2007, when the EPA deferral expired. The nonattainment classification was a result of a violation of the federal ozone standard based on data from 2005-2007. 
A detailed Ozone Action Plan was developed by the Air Pollution Control Division (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 (Commission) in December 2008, and submitted by the governor to the EPA on June 18, 2009.

The plan requires greater reductions in ozone levels than in the 2004 Ozone Early Action Compact

Colorado evaluated the impact of a new ozone standard that the EPA issued in March 2008 that was lowered from 80 parts per billion to 75 parts per billion of ozone averaged over an 8-hour period. The state determined that no areas outside the DM/NFR violated the standard.

The motor vehicle inspection and maintenance program expanded from metropolitan Denver into parts of Larimer and Weld Counties to include Fort Collins, Greeley and nearby areas. The expansion was implemented in November 2010, and was required by the 2008 Ozone Action Plan.

The DM/NFR Area had not violated the 1997 ozone standard since 2008, and in 2011, EPA partially approved the 1997 standard Attainment Demonstration SIP. 

Colorado adopted a Regional Haze Plan in 2010 and 2011 that includes new controls for oxides of nitrogen (NOx) at power and cement plants. These NOx controls will also benefit ozone reduction efforts.

EPA classified the DM/NFR as a "marginal" nonattainment area for the 2008 Ozone standard (75 ppb), effective July 20, 2012. 

2013 was the fourth year that the DM/NFR was above the standard for ozone. In early 2014 the Commission adopted regulatory changes to reduce VOC emissions from the oil and gas sector. The regulatory revisions will reduce VOC emissions by 93,000 tons per year in Colorado. In addition, the Commission approved a regional haze plan in 2011 that includes emission reductions for oxides of nitrogen (NOx). These changes resulted in more than 35,000 tons per year of NOx reductions throughout Colorado by 2018. In addition, new federal vehicle emissions standards and Colorado's motor vehicle inspection and maintenance program also helped reduce pollutants that create ozone.

2008 ozone standard

While ozone concentrations have improved over time, the region did not attain the 75 ppb standard by the July 20, 2015 attainment deadline, and in July 2016 the DM/NFR region was bumped up from a “marginal” to a “moderate” ozone nonattainment area. As a result, the Division and the Regional Air Quality Council developed a revised SIP to reduce ozone levels. The plan includes detailed technical analysis of the formation of ozone, future trends in ozone levels, and strategies to reduce ozone. The Commission approved changes to the SIP in November 2016, November 2017, July 2018, and November 2018.  


2015 ozone standard

The EPA revised the ozone standard effective October 1, 2015, to 70 parts per billion. In September 2016, the Commission adopted the recommendation that the DM/NFR region 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 DM/NFR area. Program participants receive emails two to three days before a forecasted high ozone 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 vehicles with newer more efficient and cleaner technologies.

As a of result of a court decision in February 2018, revocation of the 2008 ozone standard was not included in the final 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 DM/NFR as a “marginal” nonattainment area for the 2015 8-hour ozone standard, effective August 3, 2018. 

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 June 2018, Colorado submitted to EPA an exceptional events demonstration and request to extend the 2008 ozone attainment deadline for the nonattainment area. In March 2019, Governor Polis notified EPA to withdraw that request. 

To further reduce ground-level ozone, improve air quality, and comply with federal requirements, the Division, the 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 (SHER) stakeholder process and the Pneumatic Controller Task Force (PCTF) over the 2018-2019 timeframe.   

In January 2020, EPA designated the DM/NFR area as a “serious” nonattainment area under the 2008 ozone standard. The serious area attainment deadline is July 20, 2021, based on 2018-2020 ozone season data. 

In June 2020, the Commission approved the emissions statement, emissions inventory, and SIP 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 is August 3, 2021, based on 2018-2020 ozone season data. 

Due to high ozone levels recorded during July and August 2020, the area may be reclassified as “severe” under the 2008 standard and “moderate” under the 2015 standard in late 2021 or early 2022.