History of ozone in Colorado

In April 2004, the US Environmental Protection Agency designated the Denver Metro/North Front Range (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.
Metropolitan Denver and the North Front Range became a "Marginal" nonattainment area for the 1997 federal ozone standard on Nov. 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, 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 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 went 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 Denver Metro/North Front Range 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.
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.

Metropolitan Denver and the North Front Range was classified as a "Marginal" nonattainment area by the EPA effective July 20, 2012. A nonattainment area is one in which air quality does not meet the ozone standards set by the federal government.
2013 was the fourth year that the Denver Metro/North Front Range 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). Through these changes, there will be 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 help reduce pollutants that create ozone.
2015 ozone standard
The EPA revised the ozone standard effective Oct. 1, 2015, to 70 parts per billion. Currently, the Denver Metro/North Front Range region is not meeting the ozone standard of 75 ppb. As a result, the division and the Regional Air Quality Council engaged in an extensive planning and implementation effort, using both voluntary and mandatory air pollution control measures to reduce ground-level ozone. The 2015 standard is currently under legal review by the EPA. 

2008 ozone standard
The Denver Metro/North Front Range region was bumped up from a “Marginal” to a “Moderate” ozone nonattainment area for the 2008 standard in early 2016. As a result, the division and the Regional Air Quality Council developed a revised State Implementation Plan 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. Colorado must meet the 2008 ozone standard by 2018, and the SIP includes the measures necessary to meet the deadline. The commission approved this SIP on Nov. 17, 2016. The rest of Colorado is meeting the 2008 ozone standard.
At this time, the EPA has delayed designating the nonattainment areas for the 2015 standard. Because of this delay, the 2008 standard is still in place and Colorado has until July 2018 to meet this standard. Given present data, Colorado might not meet this deadline. This means that Colorado could possibly be bumped-up again, to a “Serious” classification for our ozone nonattainment area. The division is preparing for the potential reclassification and working with external stakeholders to determine the best path forward to reduce emissions of ozone-causing pollutants.