Ozone Molecule

named from the Greek "ozein" for smell. Pronunciation: 'O-"zOn 

Function: noun 

Etymology: German Ozon, from Greek ozOn, present participle of ozein to smell


Ozone is a molecule that contains three atoms of oxygen and thus has the formula O3.

Ozone was first discovered in 1839 by German scientist Christian Friedrich Schonbein. 

NASA Graphic


Ozone is a molecule that contains three atoms of oxygen and thus has the formula O3.The name ozone is derived from a Greek word meaning "to smell". 

Ozone is a relatively simple molecule, consisting of three oxygen atoms bound together. Yet it has dramatically different effects depending on where ozone resides, it can protect or harm life on Earth. High in the atmosphere about 15 miles (24 km) up ozone acts as a shield to protect Earth's surface from the sun's harmful ultraviolet radiation. Without this shield, we would be more susceptible to skin cancer, cataracts, and impaired immune systems. 


Closer to Earth, in the air we breathe, ozone is a harmful pollutant that causes damage to lung tissue and plants. Near Earth's surface, where ozone comes into direct contact with life forms, it primarily displays a destructive side. At ground level, ozone is a health hazard it is a harmful pollutant that causes damage to lung tissue and plants- it is a major constituent of  smog.

  • Good Ozone. Ozone occurs naturally in the Earth's upper atmosphere - 10 to 30 miles above the Earth's surface - where it forms a protective layer that shields us from the sun's harmful ultraviolet rays. This beneficial ozone is gradually being destroyed by manmade chemicals. An area where ozone has been significantly depleted - for example, over the North or South pole - is sometimes called a "hole in the ozone."

  • Bad Ozone. In the Earth's lower atmosphere, near ground level, ozone is formed when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources react chemically in the presence of sunlight. Ozone at ground level is a harmful pollutant. Ozone pollution is a concern during the summer months, when the weather conditions needed to form it - lots of sun, hot temperatures - normally occur.

Earth's atmosphere


 Because it reacts strongly with other molecules, large concentrations of ozone near the ground prove toxic to living things. Motor vehicle exhaust and industrial emissions, gasoline vapors, and chemical solvents are some of the major sources of NOX and VOC, also known as ozone precursors. 

Environmental Protection Agency  graphic


Strong sunlight and hot weather cause ground-level ozone to form in harmful concentrations in the air. Many urban areas tend to have high levels of "bad" ozone, but other areas are also subject to high ozone levels as winds carry NOx emissions hundreds of miles away from their original sources.


Smog kills more people than Car accidents

 At higher altitudes, where 90 percent of our planet's ozone resides, it does a remarkable job of absorbing ultraviolet radiation.


 In the absence of this gaseous shield in the stratosphere, the harmful radiation has a perfect portal through which to strike Earth. 

The amounts of "good" and "bad" ozone in the atmosphere depend on a balance between processes that create ozone and those that destroy it. An upset in the ozone balance can have serious consequences for life on Earth. Scientists are finding evidence that changes are occurring in ozone levels - the "bad" ozone is increasing in the air we breathe, and the "good" ozone is decreasing in our protective ozone shield.


How Ozone is Created

Graphic: Emily Shuckburgh Cambridge University

Ozone is produced naturally in the stratosphere when highly energetic solar radiation strikes molecules of oxygen, O2, and cause the two oxygen atoms to split apart in a process called photolysis.


 If a freed atom collides with another O2, it joins up, forming ozone O3. Most of the ozone in the stratosphere is formed over the equatorial belt, where the level of solar radiation is greatest. The circulation in the atmosphere then transports it towards the pole . So, the amount of stratospheric ozone above a location on the Earth varies naturally with latitude, season, and from day-to-day.


Under normal circumstances highest ozone values are found over places such as Canada and Siberia, whilst the lowest values are found around the equator. The ozone layer varies naturally with season. Over Canada is normally about 25% thicker in winter than summer. Weather conditions can also cause considerable daily variations.

Ozone is also naturally broken down in the stratosphere. In an unpolluted atmosphere there is a balance between the amount of ozone being produced and destroyed and so the total concentration remains relatively constant. At different temperatures and pressures (i.e. varying altitudes), there are different production and destruction reaction rates leading to a variation in concentration. The highest ozone concentrations are in the lower stratosphere, between about 18 and 26 km. 

Ozone also occurs in very small amounts in the troposphere. It is produced at ground level through a reaction between sunlight and, e.g., gases emitted from cars. As a pollutant it should not be confused with the separate problem of stratospheric ozone depletion.

Ozone Measurement

1 Dobson Unit (DU) is defined to be 0.01 mm thickness at STP (standard temperature and pressure). Ozone layer thickness is expressed in terms of Dobson units, which measure what its physical thickness would be if compressed in the Earth's atmosphere. In those terms, it's very thin indeed. A normal range is 300 to 500 Dobson units, which translates to an eighth of an inch-basically two stacked pennies. In space, it's best not to envision the ozone layer as a distinct, measurable band. Instead, think of it in terms of parts per million concentrations in the stratosphere (the layer six to 30 miles above the Earth's surface).


The unit is named after G.M.B. Dobson, one of the first scientists to investigate atmospheric ozone . In 1923 he produced the first Dobson Ozone Spectro-meter, to be succeeded in 1931 by his Spectrophotometer - a device which is still in use worldwide, with a network of over 150 instruments making daily observations. The Dobson spectrometer measures the intensity of solar UV radiation. A single measurement uses two wavelengths of UV, but for normal operation pairs of readings are taken at two different wavelength settings for a total of four wavelengths, two of which are absorbed by ozone and two of which are not.

Environmental Protection Agency  graphic

Chlorofluorocarbon (CFC): a compound consisting of chlorine(CI), fluorine, and carbon

The very thing that makes Ozone good for filtering UV radiation makes it easily destroyed: it is very unstable.

Environmental Protection Agency  graphic

Natural events such as Volcanic Eruptions can strongly influence the amount of Ozone in the atmosphere.

However, man-made chemicals such as CFCs or chlorofluorocarbons are now known to have a very dramatic influence on Ozone levels too. CFCs a were once widely used in aerosol propellants, refrigerants, foams, and industrial processes.

Ozone creation and depletion process text- Emily Shuckburgh Cambridge University


EPA Air Quality Index

The AQI is an index for reporting daily air quality. It tells you how clean or polluted your air is, and what associated health concerns you should be aware of. The AQI focuses on health effects that can happen within a few hours or days after breathing polluted air. EPA uses the AQI for five major air pollutants regulated by the Clean Air Act: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. For each of these pollutants, EPA has established national air quality standards to protect against harmful health effects.


Air Quality Index (AQI): Ozone


of Health

Cautionary Statements
0-50 Good None
51-100* Moderate Unusually sensitive people should consider limiting prolonged outdoor exertion.
101-150 Unhealthy for Sensitive Groups Active children and adults, and people with respiratory disease, such as asthma, should limit prolonged outdoor exertion.
151-200 Unhealthy Active children and adults, and people with respiratory disease, such as asthma, should avoid prolonged outdoor exertion; everyone else, especially children, should limit prolonged outdoor exertion.
201 - 300 Very Unhealthy Active children and adults, and people with respiratory disease, such as asthma, should avoid all outdoor exertion; everyone else, especially children, should limit outdoor exertion.
301 - 500


Everyone should avoid all outdoor exertion.

Each category corresponds to a different level of health concern. For example, when the AQI for a pollutant is between 51 and 100, the health concern is "Moderate." Here are the six levels of health concern and what they mean:

  • "Good" The AQI value for your community is between 0 and 50. Air quality is considered satisfactory and air pollution poses little or no risk.

  • "Moderate" The AQI for your community is between 51 and 100. Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of individuals. For example, people who are unusually sensitive to ozone may experience respiratory symptoms.

  • "Unhealthy for Sensitive Groups" Certain groups of people are particularly sensitive to the harmful effects of certain air pollutants. This means they are likely to be affected at lower levels than the general public. For example, children and adults who are active outdoors and people with respiratory disease are at greater risk from exposure to ozone, while people with heart disease are at greater risk from carbon monoxide. Some people may be sensitive to more than one pollutant. When AQI values are between 101 and 150, members of sensitive groups may experience health effects. The general public is not likely to be affected when the AQI is in this range.

  • "Unhealthy" AQI values are between 151 and 200. Everyone may begin to experience health effects. Members of sensitive groups may experience more serious health effects.

  • "Very Unhealthy" AQI values between 201 and 300 trigger a health alert, meaning everyone may experience more serious health effects.

  • "Hazardous" AQI values over 300 trigger health warnings of emergency conditions. The entire population is more likely to be affected.



  • Ozone, Nitrogen Change the Way Rising CO2 Affects Earth's Water-Click Here

source EPA


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