Modeling Show Cars and Trucks Largest Human-Made Source of Ozone

Published on June 18, 2026

Colorado has been challenged with meeting the federal health standards for ground level ozone for decades. In late 2025, the local air planning agency responsible for developing control strategies to lower ozone levels conducted photochemical modeling to identify which sources of emissions contribute most to ozone production.

What does the modeling data tell us is contributing to ozone at these key monitors?

The Regional Air Quality Council (RAQC) modeling results show background ozone has a large contribution to ozone in the nonattainment area (up to 85%). Background ozone:

Natural sources, like wildfires, land biosphere interactions, ozone coming down from the ozone layer, and ozone produced by lightning

Ozone transported long distance, like across the ocean from Asia

Ozone transported regionally within our continent, like from Canada and Mexico

Focusing only on the human-made sources of emissions within the Colorado nonattainment area, the results show vehicles and commercial landscaping equipment are the primary contributor to ozone in the nonattainment area. The results show a difference between the southern and northern parts of the nonattainment area, where the northern monitors show a higher contribution to ozone production from oil and gas emissions.

How do we figure out which sources to target for ozone reductions?

Photochemical modeling incorporates emissions, weather, and chemistry to simulate changes in ozone amounts over time. When identifying which sources contributing to ozone we should focus on, we evaluate the results for four key monitors showing the highest ozone values:

Chatfield (CHAT) located at Chatfield State Park in Littleton

National Renewable Energy Lab (NREL) located northeast of South Table Mountain Park in Golden, and

Rocky Flats North (RFNO) located near the Rocky Flats National Wildlife Refuge north of Arvada/west of Superior

Fort Collins – West (FTCW) located at Christman Airfield in Fort Collins

It’s important to note that the amount of ozone from human-made sources within the nonattainment area in Colorado ranges from 4% of the total at the Fort Collins monitor to 15% of the total at the Golden monitor.

Why is ground level ozone a public health concern?

Ground level ozone is an odorless, colorless gas that is formed in the air when chemicals from combustion processes (NOx) interact with gaseous chemicals (VOCs) from natural and human-made sources in the presence of sunlight. Exposure to high amounts of ozone can cause eye irritation, difficulty breathing, and aggravation of asthma. High ozone amounts can also cause plant leaf injury to crops (snap peas, specific varieties of potatoes), wildflowers (milkweed), and trees (aspens, pines), reducing crop and forest yield.

How do I learn more and get involved in the conversation?

Check out:

RAQC’s Board meetings and Control Strategy Committee to learn more about the development of control strategies for reducing ozone

Air Pollution Control Division (APCD) public outreach page for information on upcoming regulatory concepts and much more

Consider watching the APCD’s Air Quality 101 library, especially the Ozone 101 and Air Monitoring 101 recordings

Visit our air.weld.gov events calendar for upcoming public participation opportunities

Resources:

RAQC Modeling Reports and Dashboards for 2026 SIP Planning Efforts - Provides modeling reports and data dashboards for the current ozone planning efforts and provided the data for the plots in this blog

2008 Severe Nonattainment - Provides background on the Colorado 2008 Serious Nonattainment designation

2015 Serious Nonattainment - Provides background on the Colorado 2015 Serious Nonattainment designation

APCD Air Quality 101 Library – Links to the recorded presentations and informational fact sheets intended to support the public’s understanding of topics frequently discussed in regulations and public meetings