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Photochemical SmogSmog is a kind of air pollution. The word "smog" is a combination of smoke and fog. Photochemical smog is one of the most common urban air pollution problems Classic smog results from large amounts of coal burning in an area caused by a mixture of smoke and sulfur dioxide. In the 1950s a new type of smog, known as photochemical smog, was first described. It does not usually come from coal but from vehicular emissions from internal combustion engines and industrial fumes that react in the atmosphere with sunlight to form secondary pollutants that also combine with the primary emissions to form.

Photochemical smog forms when sunlight hits various pollutants in the air and forms a mix of inimical chemicals that can be very dangerous. A photochemical smog is the chemical reaction of sunlight, nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the atmosphere, which leaves airborne particles (called particulate matter) and ground-level ozone.

Nitrogen oxides are released by nitrogen and oxygen in the air reacting together under high temperature such as in the exhaust of fossil fuel-burning engines in cars, trucks, coal power plants, and industrial manufacturing factories. VOCs are released from man-made sources such as gasoline (petrol), paints, solvents, pesticides, and biogenic sources, such as pine and citrus tree emissions.

This noxious mixture of air pollutants can include the following:

Tropospheric ozone Nitrogen oxides, such as nitrogen dioxide Volatile organic compounds (VOCs) Peroxyacyl nitrates (PAN) Aldehydes (RCHO)

All of these chemicals are usually highly reactive and oxidizing.

Good and Bad Ozone Ozone has the same chemical structure whether it occurs miles above the earth or at ground level "Good" ozone occurs naturally in the stratosphere approximately 10 to 30 miles above the earth's surface

and forms a layer that protects life on earth from the sun's harmful rays. In the earth's lower atmosphere, ground-level ozone is considered "bad". It causes breathing difficulties,

headaches, fatigue and can aggrevate respiratory problems.

The summertime Peak ozone levels typically occur during hot, dry, stagnant summertime conditions. The length of the ozone season varies from one area to another.

Not Just a Local Problem Ozone and the pollutants that form it can cause air quality problems hundreds of miles away.

Natural causes An erupting volcano can also emit high levels of sulphur dioxide along with a large quantity of particulate

matter; two key components to the creation of smog. However, the smog created as a result of a volcanic eruption is often known as "vog" to distinguish it as a natural occurrence.

The radiocarbon content of some plant life has been linked to the distribution of smog in some areas. For example; presence of Creosote bush in the Los Angeles area has been shown to have an effect on smog distribution that is more than fossil fuel combustion alone.

Formation of Photochemical Smog Motor vehicles produce exhaust gases containing oxides of nitrogen such as nitrogen dioxide (NO2) and nitric oxide

(NO). At the high temperatures of the car's combustion chamber (cylinder)or during lightning, nitrogen and oxygen from

the air react to form nitric oxide(free radical) (NO)N2(g) + O2(g) 2NO(g)

Hydrocarbons and carbon monoxide (CO) will also be emitted by cars. Hydrocarbons are volatile organic compounds that may include acetaldehyde, formaldehyde, ethylene, and many other compounds.

In the air, nitric oxide combines with molecular oxygen to form nitrogen dioxide (NO2) within a few hours:

Page 2 of 22NO(g) + O2(g) 2NO2(g)

The mixture of nitric oxide (NO) and nitrogen dioxide (NO2) is sometimes referred to as NOx.

When the nitrogen dioxide (NO2) concentration is well above clean air levels and there is plenty of sunlight, then an oxygen atom splits off from the nitrogen dioxide molecule:

NO2(g)sunlight NO(g) + O(g)

This oxygen atom (O) can react with oxygen molecules (O2) in the air to form ozone (O3):O + O2 O3

Nitric oxide can remove ozone by reacting with it to form nitrogen dioxide (NO2) and oxygen (O2):NO(g) + O3(g)< > NO2(g) + O2(g)

When the ratio of NO2 to NO is greater than 3, the formation of ozone is the dominant reaction. If the ratio is less than 0.3, then the nitric oxide reaction destroys the ozone at about the same rate as it is formed, keeping the ozone concentration below harmful levels.

When hydrocarbons are present, nitric oxide reacts with them instead of the ozone. This reaction produces a variety of toxic products, such as a volatile compound known as PAN (peroxyacetyl nitrate).

NO + hydrocarbons PAN and various other compounds.   Also,NO2 + hydrocarbons PAN and various other compounds

So, there are two results from the reaction of nitrogen oxides with hydrocarbons.  One is that a lot of volatile, reactive organic compounds are generated directly.  The other is that when the nitric oxide (NO) is busy reacting with hydrocarbons, it is not reacting with ozone to break it back down to molecular oxygen.  So the amount of ozone in the air increases.  With nitric oxide reacting with hydrocarbons, ozone may accumulate to damaging levels.

The result, then, is an accumulation of ozone and volatile organic compounds such as PAN. These are referred to as secondary pollutants, because they are formed by the reaction of primary pollutants, nitrogen oxides and hydrocarbons, emitted by burning fossil fuels.

Health effectsSmog is a serious problem in many cities and continues to harm human health.

Ground-level ozone, sulfur dioxide, nitrogen dioxide and carbon monoxide are especially harmful for:

Senior citizens, children, and people with heart and lung conditions such as emphysema, bronchitis, and asthma. It can inflame breathing passages, decrease the lung's working capacity, and cause shortness of breath, pain

when inhaling deeply, wheezing, and coughing. It can cause eye and nose irritation and it dries out the protective membranes of the nose and throat and

interferes with the body's ability to fight infection, increasing susceptibility to illness. Hospital admissions and respiratory deaths often increase during periods when ozone levels are high. PAN damage younger plant leaves. Ozone exposure causes formation of yellow spots on leaves of plants, a condition called chlorotic stippling. Visibility-reducing atmospheric aerosol particles are the most common manifestation of the harm done to

atmospheric quality by smog.

Areas affectedSmog can form in almost any climate where industries or cities release large amounts of air pollution, such as smoke or gases. It often stays for an extended period of time over densely populated cities or urban areas, such as London, Atlanta, Houston, Phoenix, Las Vegas, New Delhi, New York, Cairo, Los Angeles, Sacramento, São Paulo, Mexico City, Santiago of Chile, Toronto, Milan, Athens, Beijing, Shanghai, Manila, Hong Kong, Seoul, the Randstad or Ruhr Area and can build up to dangerous levels.