Smog

Smog is mixture of smoke and fog containing particulates. Two types of smog exist, classical or original smog and the more recent photochemical smog.

Classical smog

The word smog was used initially to describe 'smoke fog' prevalent in England, in early and middle of the last century. Coal was then widely used to heat homes and as an industrial fuel. London is famous for its smoke fog of 1952 when 4000 people died from its effects over a four-day period.

Now days, the mixture of particulates with gaseous oxides of sulphur present in atmosphere is called classical smog. The principal source of classical smog is the combustion of industrial and household fuels (coal and petroleum). Because of the presence of SO₂ and carbon (soot) particles, classical smog has a reducing character. It occurs in winter months particularly in early morning hours. It causes severe lung and throat irritation.

Photochemical smog

Photochemical smog occurs due to the action of sunlight on air pollutants, generating photochemical reactions. The principal constituents of the photochemical smog are gaseous hydrocarbons (leading to ozone formation) and oxides of nitrogen (NO_x) that are a part of automobile exhaust.)

The reaction of sunlight with hydrocarbons and NO₂ results in a variety of chemical products. One of these is ozone, made up of 3 atoms of oxygen (O₃), which is an irritating noxious gas. Other undesirable chemicals such as aldehydes also result from this photochemical reaction. This type of smog is oxidizing in character because of the presence of O₃, NO₂ and some photochemical oxidants.

Note: While Ozone formed during photochemical reaction in the lowest region of the atmosphere is an air pollutant, in the upper atmosphere, the natural existence of the ozone layer helps protect living organisms from harmful U.V. rays from the Sun.

Because sunlight is essential to this type of smog, the concentration of ozone and other measurable chemicals it is maximum around mid day of summer months and falls of considerably at night.

Photochemical smog causes eyes irritation and coughs due to the presence of ozone and can lead to respiratory problems and reduced physical (athletic) ability.

Mechanism of photochemical smog

Morning and evening office hours lead to an increase in vehicular traffic. The emissions of CO, hydrocarbons and oxides of nitrogen increase during these times. These chemicals are also emitted by industries and power plants. NO₂ a good absorber of U.V. ray and visible radiation from the sun, starts a series of reactions as given below.

NO₂ + U.V. radiations NO + O

Ozone formed reacts with NO to form NO₂ and O₂

NO + O₃ NO₂ + O₂

Oxygen emitted by photolysis of NO reacts with hydrocarbons to produce free radicals that bring about a variety of chemical reactions as shown below.

O + Hydrocarbons RCO RCO₃

Free radicals Peroxyacyl free radicals (PAN)

The peroxy free radicals react with hydrocarbons, O₂ and NO₂ present in the atmosphere.

The role played by these radicals is that they react with NO to give back NO₂.

RC₃ + NO RC₂ + NO₂

Due to elimination of NO, the consumption of O₃ and NO gets reduced. As a result the level of O₃ and NO₂ in atmosphere increases. The presence of excessive O₃, together with aldehydes, ketones, PAN etc. constitute photochemical smog. From the above it can be concluded that,

• CO does not play any role in photochemical smog.

• As these reactions are catalyzed by light, photochemical smog is possible only in daytime.

Harmful effects of smog

• Smog causes respiratory problems like asthma attacks, bronchitis, heart related disorders.

• Being noxious, it causes irritation to eyes, throat and nose.

• It reduces visibility and affects road as well as air traffic.

• It damages plants; Ozone is very toxic to leafy vegetation such as tomatoes, tobacco and grape plantation. Ozone also damages old growth forests closer to sea level. Small animals incur respiratory and other health stress disorders.

• Other materials like electronic and electrical equipments get affected.

• Extreme and unexpected conditions due to smog have caused death. (1952 London smog.)

The Asian Brown Haze

A report by scientists at the United Nations Environment Program (UNEP) said the Asian Brown haze is a three kilometer deep blanket of pollution in the sky, which stretches across South Asia, and has been caused by the spectacular economic development experienced by this part of the world.

The haze is a result of a dramatic increase in the burning of fossil fuels in vehicles, industry and power stations; vast forest fires in South Asia, rural practices of burning of agricultural waste and emissions from millions of inefficient wood and cow dung stoves.

Research has shown that sunlight levels are reducing yearly; falling ash on plant leaves from the haze can further aggravate this impact. Acid in the haze has a further potential of damaging trees and crops.

Research by UNEP has shown that the haze could be leading to several hundreds of thousands of premature deaths as a result of higher levels of respiratory diseases.

V. Ramanathan, at the Scripps Institute of Oceanography (USA) and Paul J. Crutzen, Noble laureate, at the Max Planck Institute for Chemistry in Germany have indicated that the effect of the Asian Brown Haze, will intensify over the next thirty years and is a cause for concern to European nations also. Smog that extends its width to 3 km in the sky can travel half the way across the world within a week, depending upon the wind conditions. The Asian Brown Haze, which has startled environmental scientists the world over, is a major focus of concern at the Earth Summit 2002, at Johannesburg, South Africa

Problem

Give some favourable effects of Ozone gas?

Solution

When Ozone is present in the higher reaches of the atmosphere it absorbs harmful U.V. radiation from the Sun. It keeps the Earth warm due to heat absorbed by it.