Carbon monoxide concentrations have declined in the past decade. The major source of CO in the Tucson area comes from motor vehicles. This graph illustrates the downward trend in CO concentrations. This has been attributed mostly to the use of cleaner burning oxygenated fuels, fuel efficient computer controlled vehicles, locally adopted Clean Air and Travel Reduction Programs and various local traffic control measures. The levels of CO remain around 20% of the standard but with population growth and increased number of cars on the roads, higher CO levels may be measured in the future.

Ground level ozone concentrations have remained relatively steady, approaching but not exceeding the NAAQS. These concentrations are currently at 90% of the pre-March 2008 NAAQS of .080ppm. In March of 2008, the EPA set a new standard for ozone of .075ppm. Compliance with this new standard will probably be determined in 2009 and be based on the 2006-2008 three year average of 4th maximum averages.

Ground level ozone concentrations are the highest in the summer months due to the intense sunlight and heat. Oxides of nitrogen (NOx) and volatile organic compounds (VOCs) are the “precursor” pollutants that react in the presence of sunlight to form ozone. In the Tucson area, ozone levels generally decline after sunset as the photochemical reactions cease. The highest ozone levels generally are not found near major intersections. Instead they are found when precursor pollutants are released and travel, due to wind or simple dispersion, away from the area of concentration before reacting with sunlight to form ozone. The increased pollutant emissions from the growing number of vehicles contribute to the formation of ozone. The Saguaro National Park East site generally records the highest ozone levels.

PM10 is particulate matter with an aerodynamic diameter of 10 microns or less. Particulate matter is a health concern because when inhaled, the particles are able to pass through the body’s protective filtration system and enter the lungs. Pima County violated the PM10 standard in 1999 with four recorded exceedances at the Orange Grove location. High winds and unusually long periods without rain are considered factors contributing to the high particulate readings for that year. A Natural Events Action Plan ordinance was adopted December 3, 2002. This policy includes measures to minimize contributing controllable sources using the best available control measures, increased enforcement and education to help protect public health and welfare on days with high levels of PM10. There were three exceedances of the PM10 NAAQS in 2002, one at the Orange Grove location and two at the South Tucson location. There was one exceedance in 2003 at the Orange Grove location, this exceedance was considered a Natural Event due to forest fires in the nearby Catalina Mountain Range.

PM2.5 is particulate matter with an aerodynamic diameter of 2.5 microns or less.PM2.5 travels deeper into the lungs and can be more harmful than PM10. It can also be composed of toxic substances such as metals and organic compounds. Pima County’s concentrations are currently at 40% of the standard.

Nitrogen dioxide levels remain well within federal standards. Pima County has monitored NO2 since 1973, measuring typical neighborhood NO2 concentrations. Much of the data has been used in studies measuring the effects of NO2 as a precursor to ozone formation.

Ambient concentrations of sulfur dioxide (SO2) in Tucson have historically remained well below all federal standards, and in recent years have been extremely low (approaching the noise level of the instrumentation). SO2 is not considered a problem in Pima County.