OZONE: Ozone is a colourless gas made up of three oxygen atoms. Ozone is not emitted directly into the air but is formed through chemical reactions between natural and man-made emissions of nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. These gaseous compounds mix in the ambient, or outdoor air, and when they interact with sunlight, ozone is formed. Ozone can be split into two major types: Stratospheric Ozone and Ground-Level Ozone.

An ozone molecule

Stratospheric Ozone: Stratospheric ozone, also known as the “ozone layer” forms high in the atmosphere, 6-30 miles above the earth’s surface, when intense sunlight causes oxygen molecules (O2) to break up and re-form as ozone molecules (O3). These ozone molecules from the ozone layer and are commonly referred to as “good ozone.” At concentrations as high as 12,000 ppb (much higher than 70 ppb- safe limit), this ozone protects and shields people, trees, crops, property, and microorganisms from the harmful effects of the sun’s ultraviolet light.

Ground-Level Ozone: Ground-level ozone (GLO) forms just above the earth’s surface (up to about 2 miles above ground) and impacts human, animal, and plant respiration. Although ground-level ozone is less concentrated than stratospheric ozone, its impacts on human health and welfare make ground-level ozone “bad ozone.” GLO  is an irritant and can negatively affect human health and welfare.  Ground-level ozone concentrations are the highest on hot sunny days with low humidity when wind speed is low or stagnant.



The ground level ozone is formed from nitrogen oxides (NOx) and volatile organic compounds (VOCs). They are produced primarily from the pollutants already present in the air. For example, when fossil fuels like gasoline, oil are burnt in automobiles or coal are burned or when some chemicals, like solvents, evaporate.

NOx is emitted from power plants, motor vehicles and other sources of high-heat combustion.

VOCs are emitted from motor vehicles, chemical plants, refineries, factories, gas stations, paints and solvents.

If these ingredients are present under the right conditions, they react to form ozone in the presence of sunlight. This pollution is mainly outdoor pollution but can be carried away far from where it was formed, even internationally across borders and even the oceans.

How GLO forms

Harmful effects of Ozone Pollution:

Premature death: Breathing ozone can shorten our life. Strong evidence exists of the deadly impact of ozone from large studies conducted in cities across the U.S., in Europe and in Asia. Researchers repeatedly found that the risk of premature death increased with higher levels of ozone and even low levels of ozone may be deadly. Researchers found that ozone at those lower levels was associated with deaths from cardiovascular disease, strokes, and respiratory causes.

Immediate breathing problems. Immediate problems may include:

  • shortness of breath, wheezing and coughing;
  • asthma attacks;
  • increased risk of respiratory infections;
  • increased susceptibility to pulmonary inflammation; and
  • Increased need for people with lung diseases, like asthma or chronic obstructive pulmonary disease (COPD), to receive medical treatment and to go to the hospital.

Cardiovascular effects. Inhaling ozone may affect the heart as well as the lungs. Exposure to high ozone levels for as little as one hour may cause cardiac arrhythmia that itself increases the risk of premature death and stroke.  Several studies around the world have found an increased risk of hospital admissions for cardiovascular disease due to ozone contamination.

Ozone causes harm to vegetation and ecosystems including forests, parks, wildlife refuges etc.

Breathing other pollutants in the air may make our lungs more responsive to ozone—and breathing ozone may increase our body’s response to other pollutants. For example, research warns that breathing sulfur dioxide and nitrogen oxide—two pollutants common in the air can make the lungs react more strongly than to just breathing ozone alone. Breathing ozone may also increase the response to allergens in people with allergies. A large study published in 2009 found that children were more likely to suffer from hay fever and respiratory allergies when ozone and PM2.5 levels were high.

The Chemistry of GLO formation:

Hydrocarbon Oxidation

Automobiles use hydrocarbons, gasoline or diesel, as fuel. The hydrocarbons are burning in the internal combustion engines. Complete combustion of a hydrocarbon should produce only carbon dioxide and water. For example:

C7H16 + O2  7 CO2 + 8 H2O

However, the fuels are not pure hydrocarbons. Nitrogen impurities burn with the fuel to form NO and NO2.
A significant amount of volatile hydrocarbons are released without combustion or with only partial combustion. These can be oxidized by hydroxyl radical and oxygen to form alkyl peroxy radicals. For example:

Nitric Oxide and Alkylperoxy Radicals

Nitric oxide from the gasoline impurities can react with peroxy radicals to make nitrogen dioxide and another radical.



The next step is the reaction of the new radical with more hydrocarbon. This generates a more stable carbon molecule called alcohol.


Ozone Formation

The brown gas NO2 is photolyzed by visible light to make NO and oxygen atom. The oxygen atom can combine with O2to make ozone.

The Catalytic Cycle

NO is converted to NO2, then reconverted to NO as ozone forms. The nitrogen oxide is not used up. We can combine the reactions to make a catalytic cycle.

One component of Photochemical smog is ozone.

Photochemical smog is a type of smog produced when UV light from the Sun reacts with nitrogen oxides in the atmosphere. It is visible as a brown haze, and is most prominent during the morning and afternoon, especially in densely populated, warm cities. Cities that experience this smog daily include Los Angeles, Sydney, Mexico City, Beijing, and many more.

Sources, effects and Chemistry of Ozone formation is well understood and the preventive measures are also well discussed in my earlier articles.

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