SMOG

By burning the fossil fuels like coal or petrol in industries or in homes or in automobiles poisonous gases like SO₂ or NO₂ are produced. These poisonous gases are mixed with the droplets of fog suspended in the air near the surface of earth. A colloidal dispersion is formed, when the droplets of fog which contains poisonous gases like SO₂ and NO₂ gets condensed on the solid particles of smoke or dust present in air. This colloidal dispersion is called smog. During the winter season smog usually present in the lower atmosphere is taken by men and animals during breathing and damages their health.

Photochemical Smog or Los Angeles smog

The poisonous gases like oxides of nitrogen (mainly NO₂), SO₂, CO, CO₂, or unburnt hydrocarbons (mainly R-H) released by homes, industries, or by automobiles sector. In the poisonous gases NO₂ is present, which is cleavage by photochemically by ultraviolet (UV) radiations present in the sunlight, and produced atomic oxygen (O).

NO₂ + U.V light ------> NO + O

The produced atomic oxygen (O) reacts with hydrocarbons (R-H), and forms a variety of free hydrocarbon radicals, ozone (O₃) and PAN (Peroxy Acetyl Nitrate).

Formation of ozone and hydrocarbon radicals--

O₂ + O + M -----> O₃ + M (M = Catalyst = N₂)

R-H + O -----> R• (hydrocarbon radical) + •OH

R• + O₂ -----> RO₂• (peroxy radical)

RO₂• + R-H ---> RO₂H (organic peroxide) + R•

RO₂• + O₂ ------> RO• (alkoxy radical) + O₃

Formation of PAN (Peroxy Acetyl Nitrate) ---

From ketone---

R―CO―R + hv -----> R• + RCO• (acetyl radical)

RCO• + O₂ -->R―CO―O―O• (peroxy acetyl radical)

R―CO―O―O•+NO₂-->R―CO―O―O―NO₂ (PAN)

From aldehyde---

R―CO―H + •OH -----> R―CO• + H₂O

R―CO• + O₂ -----> R―CO―O―O•

R―CO―O―O•+NO₂-->R―CO―O―O―NO₂ (PAN)

If PAN i.e., Peroxy Acetyl Nitrate is present, in the concentration of 0.1 ppm (parts per million) in atmosphere, it is harmful to human or animals. When PAN is mixed with fog and gets condensed on dust particles or smoke present in the atmosphere, a colloidal dispersion is formed, which is called photochemical smog.

PAN mixed with fog + smoke or dust particle-> photochemical smog.

Due to presence of heavy automobile traffic in Los Angeles, a city of United States of America a huge number of poisonous gases (mainly NO₂) released in the atmosphere, which produced a huge amount of photochemical smog. So photochemical smog is also called Los Angeles smog.

Sulphurus Smog

This smog is basically a mixture of liquid particles of H₂SO₄, SO₂ and other particles. If these particles and gases in the air are accompanied by stagnant air, humidity, cloudy skies, temperature contrast and dense fog, then sulphurus smog is formed. The smell of sulphurus smog is different from photochemical smog.

Particulate Smog

Particulate smog is the sum of small dust particles, the diameter of the particles is 0.1 µm. Combustion of fossil fuels in automobiles, dust emitted from thermal power plants create this type of smog. The difference between photochemical smog and particulate smog is that, particulate smog does not contain ozone gas.

Harmful Effects of Smog

Various types of harmful effects which are creates by smog are---

Smog produces asthma, bronchitis, and heart problem for all the human beings who breath in it.

Irritation in eyes, nose, throat, and skin is creates by smog.

Natural growth and development of human and animals is affected by smog.

In the presence of photochemical smog, the leaves of the plants develop a metallic sheen, which is harmful for plant.

In the presence of photochemical smog rubber loses its elasticity, and becomes inflexible and brittle.

The visibility is reduced by smog, so hindrance is produced in the air and in road traffic.