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Air Pollutants


Carbon Monoxide

Carbon monoxide (CO) is a colorless, odorless gas emitted by motor vehicles and other sources. CO is formed when fuels such as gasoline, wood or coal, are not completely burned. 

Emissions primarily occur from automobiles, buses and trucks and some industrial processes. Carbon monoxide is found in high concentrations along the roadside, especially where there is heavy traffic. Parking garages and poorly ventilated tunnels also can have high concentrations.

CO can affect the cardiovascular and nervous systems and cause death in high concentrations. Lower concentrations of CO have been shown to affect people with heart disease and can cause dizziness, headaches and fatigue.

The U.S. Environmental Protection Agency (EPA) sets National Ambient Air Quality Standards.

Current EPA CO Standards

  • 1-hour average: 35 parts per million
  • 8-hour average: 9 parts per million

Carbon Monoxide short-term trends

Carbon monoxide (CO) is a localized pollutant that disperses rapidly. In Pima County, on-road motor vehicle exhaust produces about 26 percent of the carbon monoxide. Non-road vehicles produce about 25 percent  of the county's CO emissions.

CO concentrations tend to be highest in winter, when temperatures are cool, wind speeds are low and temperature inversions are present. This occurs when a stable atmospheric layer restricts the mixing of pollutants.

Carbon monoxide levels tend to rise following traffic peaks, with the highest levels occurring soon after rush hour.


Since the region violated the EPA health standard for carbon monoxide frequently in the 1970s, the Tucson Air Planning Area was classified as a nonattainment area, and a state implementation plan was required to comply with the Clean Air Act.

In 2000, Arizona requested the EPA reclassify the Tucson region in attainment with the national health-based standard for carbon monoxide and, in the same year, the EPA approved a maintenance plan to ensure ongoing compliance. The CO Maintenance Plan was updated and approved by the EPA in 2009, with an effective date of January 2010.

The 2008 Revision to the CO Limited Maintenance Plan maintains existing controls and contingency provisions through 2020.


Carbon Monoxide long-term trends

Since monitoring began in Pima County in 1973, carbon monoxide levels have decreased. The primary reasons for this are technological advances and the implementation of the following:   

  • Federal Motor Vehicle Control Program (tailpipe emission standards for new cars)
  • State Inspection and Maintenance Programs
  • Use of oxygenated fuels each winter from September 30 to March 31, since 1990 in Pima County



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Ozone is an invisible gas that occurs naturally in the upper atmosphere (about nine to 13 miles above the earth’s surface), and protects life on earth by filtering out harmful ultraviolet radiation. Ozone at ground level, however, is a harmful pollutant. It is a major component of smog and is the result of complex chemical reactions involving precursor chemicals and sunlight. Oxides of nitrogen (NOx) and volatile organics (VOC) (known as precursors to ozone formation) come from many different sources. Typical urban area sources are emissions from on-road and off-road vehicles (such as bulldozers, planes and trains), and power plants and factories. 

Ozone is a severe irritant to the respiratory system and can cause shortness of breath, coughing, wheezing and stinging eyes. It can damage lung tissue and make people more susceptible to respiratory infections. Ozone is especially harmful to children, the elderly, and those with impaired health.

In 2015, the EPA revised the primary and secondary ozone standards to 0.070 ppm. The previous standard was 0.075 ppm. Currently, Pima County is in attainment for the 2015 ozone standards. Current ozone monitoring levels are reaching 99 percent of the EPA's health standard.

For more information, please view the FAQ on the New Ozone Standards from PAG's Air Quality Program.

2018 Ozone Status Report

Current EPA Ozone Standards

  • 0.070 ppm (Annual fourth-highest daily maximum 8-hour concentraton averages over 3 years)

Ozone Trends in Pima County

In Pima County, ozone can be produced at any time of year, due to the high percentage of days with little or no cloud cover. Ozone concentrations are usually the highest during summer afternoons, when there is intense sunlight, moderate heat, and stable air conditions.

Levels are also dependent on the topography and air flow within the Tucson valley. High ozone concentrations tend to occur in more suburban and rural sites where there is less NOx to react with and break down the ozone (scavenging).


Although ozone levels in Pima County have been relatively constant over the past 15 years, Pima County is at 99% of reaching the 2015 standard. If ozone concentrations exceed this standard, stricter regulations limiting emissions from vehicles and industry could be enacted. This translates to increased costs for the public and local businesses and industries. With small efforts from everyone in the community, ozone levels can continue to remain at healthy levels.



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Particulate Matter

Particulate matter is made up of small solid particles or liquid droplets from smoke, dust, fly ash and condensing vapors. It can be suspended in the air for long periods of time. It is generated from travel on paved and unpaved roads, woodsmoke, burning fuels, earth moving, mining, construction, vacant lots and agricultural activities.

These microscopic particles can affect breathing and respiration, cause lung damage and possibly cause premature death. Children, the elderly and those suffering from heart or lung disease are especially at risk.

There are two types of particulate matter that are regulated by the EPA: Coarse Particulate Matter (PM10) and Fine Particulate Matter (PM2.5). The number indicates the size of the particle in microns. Research shows that the smaller particles pose a greater threat than the larger particles because they are able to travel deeper into lung tissue. 

Current Particulate Matter Standards

  • 24-hour average: 150 µg/m3
  • 24-hour average:  35 µg/m3
  • Annual average:   12 µg/m3

Particulate Matter Trends in Pima County

In Pima County, dust from the natural desert background comprises about one half of the typical particulate matter concentrations. Particulate concentrations tend to be higher in winter and increase during dry periods.

This graph shows the monthly average trends of PM10 and PM2.5 concentrations at the Orange Grove monitoring site for 2016.

For dust control, Title 17 of the Pima County Code outlines activity permits and performance standards required for construction activities.



Pima County generally meets the EPA standard for PM10 concentrations but dry and windy conditions can cause concentrations to exceed the standard.


Pima County's annual PM2.5 concentrations are well below the EPA standards.

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Greenhouse Gases

Regional GHG Inventories and Trends

Many chemicals in the atmosphere act as greenhouse gases (GHG) because they absorb infrared radiation and trap heat. They are both naturally reoccurring and human generated. Naturally occurring GHG include water vapor, carbon dioxide, methane and nitrous oxide. In addition, some human activities, such as electricity and natural gas consumption, vehicle travel, manufacturing and agricultural practices also produce GHG including carbon dioxide, methane, nitrous oxide and fluorinated gases.

When accessing the amount of human- generated GHG, carbon dioxide equivalents (CO2e), a weighted value, is usually used. This allows the various gases to be compared based upon the relative global warming potential and their persistence in the atmosphere.  

PAG conducts GHG inventories for various communities and government operations in eastern Pima County. Community inventories include GHG emissions from electricity and natural gas use, vehicle use, waste disposal, wastewater reclamation, and locomotive, aircraft and industrial activities. Results from the various community inventories show a similar trend: energy use and transportation are responsible for most regional GHG emissions.

Government operations’ inventories include emissions produced by energy used in government buildings, water delivery and wastewater treatment and streetlights; government fleet travel and employee commuting, and from waste disposal. The results from the government inventories indicate that the energy used in buildings and in water handling are the major sources of governments’ GHG emissions.




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