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Ozone Trends in Pima County

(click on graphs below to view the pdf version of the graph)

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, but levels are the highest during summer. 

Short-term Trends:

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).



Local data indicate that the Tucson region experiences ozone levels close to the 8-hour standard set by the EPA. Concentrations tend to be highest in summer when temperatures are warm and wind speeds are low. 


Long-term Trends: 

Since ozone monitoring began in Pima County in 1973, levels have remained fairly uniform throughout the Tucson metropolitan area. Higher levels tend to be recorded further from the central urban area. NOx scavenging in the urban core prevents the build up of ozone. In areas such as Saguaro National Park East, the relatively low NOx levels combined with the effects of topography, result in higher overall ozone levels. One of the most effective national controls has been the Federal Motor Vehicle Control Program. This has reduced NOx and hydrocarbon emissions by placing stringent emissions regulations on auto manufacturers. These regulations require vehicles to have systems capable of capturing excess gasoline vapors and cleansing tailpipe emissions. However, these systems don’t always perform as designed and can deteriorate. Poorly tuned vehicles and tampered vehicles all increase emissions.

Other controls in the Tucson area that help reduce ozone levels by promoting the use of alternate modes include:

Alternate modes such as carpooling, taking the bus, walking, and bicycling, and the use of telecommuting all serve to reduce the number of motor vehicles on the road.

Although ozone levels in Pima County have been relatively constant over the past 15 years, a possible lowering of the ozone stander in 2013, may place the County closer to violating the health standard.  If ozone concentrations exceed this level, 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. For information on actions you can take to reduce ozone levels visit Clean Air Starts With Me


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