Module IV - Smog - photochemical smog

Smog
Smog is a kind of air pollution; the word "smog" is a portmanteau [A portmanteau word is a combination of two or more words and their definitions, into one new word.] of smoke and fog. Classic smog results from large amounts of coal burning in an area and is caused by a mixture of smoke and sulphur dioxide.
The term "smog" was first coined by Dr. Henry Antoine Des Voeux in his 1905 paper, “Fog and Smoke,” for a meeting of the Public Health Congress. The 26 July 1905 edition of the London newspaper Daily Graphic quoted Des Voeux, “It required no science to see that there was something produced in great cities which was not found in the country, and that was smoky fog, or what was known as ‘smog.’” The following day the newspaper stated that “Dr. Des Voeux did a public service in coining a new word for the London fog.”
Smog is a problem in a number of cities and continues to harm human health. Ground-level ozone, Sulfur dioxide, Nitrogen dioxide 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, decreasing the lungs' working capacity, and causing 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.
In the 1950s a new type of smog, known as photochemical smog, was first described. This is a noxious mixture of air pollutants including the following:
nitrogen oxides, such as nitrogen dioxide, tropospheric ozone, volatile organic compounds (VOCs), peroxyacyl nitrates (PAN), aldehydes (R'O)
All of these chemicals are usually highly reactive and oxidizing. Due to this fact, photochemical smog is considered to be a problem of modern industrialization.
Photochemical smog is a concern in most major urban centres but, because it travels with the wind, it can affect sparsely populated areas as well. Smog is caused by a reaction between sunlight and emissions mainly from human activity. Photochemical smog is the chemical reaction of sunlight, nitrogen oxides (NOx) and volatile organic compounds (VOC's) in the atmosphere, which leaves airborne particles (called particulate matter) and ground-level ozone. Nitrogen oxides are released in the exhaust of fossil fuel-burning engines in cars, trucks, coal power plants, and industrial manufacturing factories. VOC's are vapors released from anthropogenic (man-made) sources such as gasoline, paints, solvents, pesticides, and biogenic sources, such as pine and citrus tree emissions.

Photochemical Smog
Photochemical smog is a type of air pollution produced when sunlight acts upon motor vehicle exhaust gases to form harmful substances such as ozone (O3), aldehydes and peroxyacetylnitrate (PAN) and mixed with particulate matters. Ozone causes breathing difficulties, headaches, fatigue and can aggravate respiratory problems. The peroxyacetylnitrate (CH3CO-OO-NO2) in photochemical smog can irritate the eyes, causing them to water and sting.

Ozone Production
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), nitrogen and oxygen from the air react to form nitric oxide (NO):
N2(g) + O2(g) -----> 2NO(g)
Some of the nitric oxide (NO) reacts with oxygen to form nitrogen dioxide (NO2):
2NO(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.
The reaction of hydrocarbons (unburnt petrol) with nitric oxide and oxygen produce nitrogen dioxide also in the presence of sunlight, increasing the ratio of nitrogen dioxide to nitric oxide.

Peroxyacetylnitrate (PAN) Production
Nitrogen dioxide (NO2), oxygen (O2) and hydrocarbons (unburnt petrol) react in the presence of sunlight to produce peroxyacetylnitrate (CH3CO-OO-NO2):
NO2(g) + O2(g) + hydrocarbons + sunlight----------> CH3CO-OO-NO2(g)

1. It can cause headaches, eye, nose and throat irritations. It may cause the lung function impaired, coughing and wheezing. 2. It can cause rubbers and fabrics to deteriorate. 3. It can damage plants, leading to the loss of crops.

Catalytic Converters
Catalytic converters on motor vehicle exhausts are a way of trying to reduce the carbon monoxide and nitrogen oxide emissions.
The catalyst used is either platinum or a combination of platinum and rhodium.
The platinum catalyses the reaction of unburnt hydrocarbon (such as pentane) and oxygen (O2) to produce carbon dioxide (CO2) and water vapour (H2O):
C5H12(pentane)+8O2  (platinum catalyst)------------------->5CO2+    6H2O
The rhodium catalyses the reaction of carbon monoxide (CO) and nitric oxide (NO) to form carbon dioxide (CO2) and nitrogen gas (N2):
2CO + 2NO    rhodium catalyst------------------->2CO2 + N2
The reduction of nitric oxide (NO) to nitrogen gas (N2) must proceed more quickly than the oxidation of carbon monoxide (CO) to carbon dioxide (CO2) or else all the carbon monoxide will be oxidised to carbon dioxide before it can be used to reduce the nitric oxide.
Motor vehicles can only use catalytic converters if they use unleaded petrol since the lead in petrol renders the catalyst inactive.
In urban areas, air pollutants contribute to the formation of smog.  There are two types of smog.  The first to be identified, with reports dating back to the 17th century, was sulphurous smog.  This smog is characterized by high concentrations of sulphur compounds (i.e., SO2 and sulphates), resulting from the combustion of coal and high-sulphur content fuels.