Denver, Colorado
An overview of air quality contributors in the Denver Metro/North Front Range Area
Introduction
Why do we care about ground-level ozone?
Tropospheric ozone or ground-level ozone (O 3 ) is formed through reactions involving volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of sunlight.
Ozone levels about the NAAQS standard (currently 70 ppb as of 2015) can contribute to eye and nose irritation, decreased lung function, cardiovascular disease (COPD) (Crooks et al., 2021; U.S. EPA 2020 ISA).
According to the American Lung Association's 2023 State of the Air Report , Denver is the 6th most ozone-polluted city in the United States.
Ozone in the Denver Metro/North Front Range Area
Current Air Quality Conditions
A History of Ozone Nonattainment
Since the 1980s, ozone pollution has improved significantly in the United States, but the the Denver Metro North Front Range area still struggles with attainment of the NAAQS standard (70 ppb as of 2015).
Factors Influencing Ozone Production in the Denver Metro/North Front Range Area
Regional Factors
1) Biomass Burning: agricultural practices, prescribed burns, and wildfires supply numerous amounts of precursor emissions into the area.
2) Interstate Transport: studies have shown impacts of regional transport contributing to enhanced ozone in the Front Range area originating from southern California (Huang et al. 2013).
3) Springtime Stratospheric Intrusion Episodes: most frequent in the western U.S. after strong La Nina winters.
Meteorological Influence
Enhanced surface ozone is typical in the summertime associated with prolonged periods of high temperatures, sunlight, low wind, and high pressure systems. When westerly winds are absent, precursor emissions (i.e., VOCs, NO x ) from the east get trapped in the foothills region of the Rocky Mountains contributing to enhanced ozone production. For example, a 2017 report summarizing findings from the FRAPPÉ and DISCOVER-AQ 2014 field campaigns in the Denver region noted strong components of upslope flow observed on high ozone days in the afternoon and westerly and northeasterly wind components on days with lower observed ozone.
Emissions
Local emission sources in Denver enhance local-level ozone. Within Denver, the pollution from vehicles along the I-25 highway, which cuts through the center of Denver, have a large contribution. Another significant source is from oil and gas refinery activity at the heart of Denver.
The oil refineries play a big role in local Denver ozone because of the volatile organic compounds (VOCs) that they emit. A study done by the NOAA Chemical Sciences Laboratory found that the VOCs contributed to around 5 - 10 ppb of ozone.
Mackenzie Solomon, NOAA Chemical Sciences Laboratory Fact Sheets, https://csl.noaa.gov/factsheets/csdDenverOzone.pdf
Local Emission Sources
Mobile sector and the oil and gas are the main sources of local emissions in the Denver non nonattainment area.
Source: McDuffie, E. E., Edwards, P. M., Gilman, J. B., Lerner, B. M., Dubé, W. P., Trainer, M., ... & Brown, S. S. (2016). Influence of oil and gas emissions on summertime ozone in the Colorado Northern Front Range. Journal of Geophysical Research: Atmospheres, 121(14), 8712-8729.
The National Emission Inventory (NEI) by the EPA highlights the impact of oil and gas industries in the northern part of the nonattainment area, contributing significantly to the production of VOCs and NOx. In contrast, the southern part of the area is predominantly affected by emissions from mobile sources and power plants. Specifically, PM2.5 emissions in the southern nonattainment area are largely attributable to mobile sources.
Satellite Observation
The TROPOspheric Monitoring Instrument (TROPOMI) is a state-of-the-art satellite sensor onboard the European Space Agency's (ESA) Sentinel-5 Precursor (S5P) satellite. The spatial and temporal resolutions of TROPOMI data is approximately 5.5 × 3.5 km2 at nadir and The overpass time of TROPOMI over Denver occurs in the early afternoon.
Colocation of high tropospheric NO2 columns with nonattainment areas.
The weekend effect
The weekend effect occurs when O 3 is higher on weekends (i.e., Saturday-Sunday in the U.S.), even though NO x emissions decrease with reduced vehicular traffic.
Daily TROPOMI NO 2 tropospheric columns for 2019-2022, masked to the Denver nonattainment area, shown separately for weekdays and weekends.
Ozone Production Chemical regime
Formaldehyde-to-nitrogen dioxide ratio or FNR is an indicator that can be used to investigate ozone production chemical regime. The effectiveness of FNR in indicating local ozone production is limited by the variability in the HCHO and NO 2 levels. There are different thresholds for classification of NOx/VOC limited and transition regime.
weekdays and weekends FNR based on TROPOMI NO 2 and HCHO for 2019-2022 ( (<3.2 for VOC-limited and >4.1 for NO x -limited )
Temporal Change in NO 2 and HCHO tropospheric columns
TROPOMI shows the seasonal change in NO 2 and HCHO values and also the change in ozone production chemical regime inside the nonattainmnet area
Daily TROPOMI NO 2 and HCHO tropospheric columns for 2019-2022, masked to the Denver nonattainment area
Air quality Impact
Ozone, a Polluant with Two Faces_Blog_ISGLOBAL, https://www.isglobal.org/en/healthisglobal/-/custom-blog-portlet/el-ozono-un-contaminante-del-aire-con-dos-caras
Ozone pollution affects our lungs, making it difficult to breathe. UCAR https://scied.ucar.edu/learning-zone/air-quality/effects-air-pollution
1) Health: Ozone exposure reduces lung function and causes respiratory symptoms, such as coughing and shortness of breath. It can cause chest pain, throat, eye and nose irritation. Ozone exposure also aggravates asthma and lung diseases such as chronic obstructive pulmonary disease (COPD) and emphysema leading to increased medication use, hospital admissions, and emergency department visits. In additional, both short- and long-term exposures to ozone at concentrations below the current regulatory standards are associated with increased deaths from both respiratory and cardiovascular disease. https://www.nature.com/articles/s41370-021-00375-9 https://education.nationalgeographic.org/resource/air-pollution/
2) Vegetation: Ozone reduce biodiversity and decrease plant uptake of CO2. Sulfur dioxide and nitrogen oxide particles in the air, can create acid rain when they mix with water and oxygen in the atmosphere. When acid rain falls to Earth, it damages plants by changing soil composition; degrades water quality in rivers, lakes and streams; damages crops and reduce their yield.
Downtown Denver is shrouded in haze as seen from the Ken Caryl Sledding Hill on Wednesday, July 21, 2021. https://www.denverpost.com/2021/07/24/ozone-air-pollution-colorado-front-range/
3) Smog: Photochemical smog, often referred to as "summer smog", is the chemical reaction of sunlight, nitrogen oxides (NO & NO2) and volatile organic compounds (VOC) in the atmosphere, which leaves airborne particules and ground-level ozone Photochemical smog is a type of air pollution derived from vehicular emission from internal combustion engines and industrial fumes. These pollutants react in the atmosphere with sunlight to form secondary pollutants that also combine with the primary emissions to form photochemical smog. https://fr.wikipedia.org/wiki/Smog
Environmental Justice
While ozone is a problem for the whole of Denver, even within Denver, there are populations that are disproportionately impacted by ozone pollution. These populations, which tend to be low-income and lower socioeconomic status, tend to be clustered around local emission sources. This also leads to disproportionate impacts on health, such as asthma.
Future Drivers of Pollution
Mitigation Efforts
What is being done to mitigate ozone nonattainment?
Denver was classified from severe to a serious ozone nonattainment area in December 2019 based on the National Ambient Air Quality Standards (NAAQS) established under the Clean Air Act, which can increase regulatory compliance requirements. The state of Colorado was given the deadline of July 20, 2027 to reach new attainment status.
State Implementation Plan (SIP) revisions include:
- Implementation of Reasonably Available Control Technology (RACT).
- Require major sources to fees to the state until nonattainment reclassification is reached.
- Implementation of the reformulated gasoline (RFG) program.
- Oil and gas emission reductions, which aim to reduce nitrogen oxide (NO x ) via operators (i.e., drilling, pre-production operators, stationary engines). The goal is to reduce emissions from the oil and gas sector by 30% in 2025 and 50% by 2050.
- Reduce emissions from running lawn and gardening equipment.
Modeling efforts have shown that with the State Implementation Plan, the state of Colorado will be able to reach the 2008 federal ozone pollution standard of 75 ppb by 2026.
Denver's 2019 Greenhouse Gas Emissions (Source: https://denvergov.org/files/assets/public/v/2/climate-action/cpf_fiveyearplan_final.pdf )
Top greenhouse gas (GHG) emissions come from buildings, homes, and transportation.
Addressing these GHG emissions include:
- Net Zero Building Emissions
- Building code will require new constructions to achieve net zero energy.
- Renewable Energy
- Electrification
- Electric vehicle adoption. Currently, 21 electric vehicles for every 1,000 people.
- Enhance public transit, biking, and walking options by making streets cleaner and safer.
What can we do to better understand ozone and provide helpful solutions as scientists?
- Modeling studies
- Use and interpret observations
- Conduct field campaigns
2014 The Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ)
The NSF/NCAR FRAPPÉ campaign, carried out jointly with the NASA DISCOVER-AQ Colorado campaign, aimed to quantify the contributions of various emission sectors on ozone production in the Northern Front Range Metropolitan Area (NFRMA), which includes Denver. It was found that the transportation and oil and gas sectors were the largest contributors of photochemical ozone formation, with meteorological conditions playing a significant role on high-ozone days.
Flocke et al., JGR Atmospheres, Volume: 125, Issue: 2, First published: 22 December 2019, DOI: (10.1029/2019JD031197)
Flocke et al. (2019) found that thermally-driven upslope flows were prominent on high-ozone days under weak synoptic conditions. This led to high ozone concentrations from the Front Range to be transported to the mountains.
Conclusion
Air pollution, especially ozone, is a serious problem in Denver. Unfortunately, as with many other places in the US, ozone exposure and the resultant negative health impacts are disproportionately borne by the most-vulnerable communities. While some air pollution sources in Denver are transported from other regions in the US, there are local emission sources that can be addressed, such as traffic along the I-25 and also emissions from oil refineries. By doing so, not only will the most-vulnerable communities who are currently most-affected by air pollution will stand to benefit the most, but everyone in Denver will also benefit from better air quality and better health.
Meet the Team
From left to right: Julie Christopoulos, Marie Yapo, Amin Mirrezaei, Glen Chua, and Noribeth Mariscal