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Ozone is an invisible gas which is a form of molecular oxygen (three oxygen atoms linked together). It 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 from the sun. 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 vehicle exhaust and sunlight.

Ozone is not emitted directly from mobile or stationary sources but is formed when volatile organic compounds and oxides of nitrogen react in the presence of sunlight. The rate at which the reactions proceed is related to both temperature and intensity of the sunlight. 

Elevated ozone levels typically occur on hot summer afternoons. Oxides of nitrogen and volatile organics (known as precursors to ozone formation) originate from many different sources. Typical urban area sources are emissions from cars, buses and trucks, and off-road mobile sources, such as construction vehicles, planes and trains, 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. This includes people with respiratory problems such as asthma, emphysema, chronic bronchitis and cardiovascular patients. It also inhibits plant growth and can cause damage to crops and forests. 

The EPA strengthened the ozone standards in 2008 and is currently reviewing data to possibly lower this 2008 standard. The EPA is expected to issue a new ozone standard in 2014.

Current Ozone Standard

0.075 ppm (8-hour average)
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.



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