Atmospheric Pollutants And The Reactions - Practice Questions & MCQ

It is a protective blanket of gases which surrounds the earth. The atmosphere extends to thousands of kilometre above the earth's surface. It has no well defined upper limits and gradually merges with the outer space. The atmosphere is held to the earth by the force of gravity of the total mass of atmosphere, aobut 99% wi within a height of 30km from the earth's surface. The total mass of the atmosphere is about 5 x 10¹⁵ metric tonnes.
The atmosphere is divided into four major regions depending on the height discussed below:

Tropospheric pollution
The tropospheric pollution is caused by two types of particles, viz,

1. Gaseous pollutants: These are those kinds of pollutants which exist in gaseous form. Common examples are oxides of sulphur, nitrogen, carbon, hydrogen sulphide, etc.
2. Particulate pollutants: These are those kinds of pollutants which exist as particles. Some examples include dust, mist, fumes, smoke, smog, etc.

Air pollutants are classified into two types:

1. Primary pollutants
2. Secondary pollutants

Primary pollutants: These are harmful chemical substances which enter directly into the atmosphere as a result of natural and human activities. These include inorganic gases such as sulphur dioxide, carbon monoxide, hydrogen sulphide, nitric oxide, carbon dioxide, ammonia, hydrogen fluoride, etc, hydrocarbons and radioactive substances, particulate matter such as ash, smoke, fumes, aerosols, etc.

Secondary pollutants: These are harmful chemical substances which are formed by chemical interactions among primary pollutants and normal atmospheric constituents. For example, sulphur dioxide is a primary pollutant. It reacts with oxygen in the atmosphere to form sulphur trioxide which further reacts with water vapours to form sulphuric acid.

Thus, sulphur trioxide and sulphur acid are secondary pollutants.

Major air pollutants of Tropospheric pollution
The major air pollutants are:

• Carbon monoxide
• Oxides of sulphur
• Oxides of nitrogen
• Hydrocarbons
• Particulates

Global Warming
Whereas the total amount of water vapour in our atmosphere has not altered over the years, the carbon dioxide concentrations have risen by about 25% in the past century. This increase is as a result of the extra burning of fossil fuels in order to meet the energy needs. Several measurements indicate that during the past nearly 120 years, the average temperature of the planet has increased by somewhere between 0.4 and 0.8^oC. Current estimates are that doubling the CO₂ concentration will result in a temperature increase of between 1.0 to 3.5^oC. Recent estimates suggest that about half of global warming may be attributable to compounds other than carbon dioxide. The amount of warming which a greenhouse gas imparts to a square metre of the earth is called radiative forcing. The radiative forcing exerted by each greenhouse gas depends on its concentration and its ability to absorb long-wavelength IR-radiation. The effectiveness is quantified by the greenhouse factor, a number that represents the relative contribution of a molecule of the gas to global warming. 

Consequences of global warming
Global warming has the following effects:

• Rise in sea level
• More evaporation of water from aquatic systems
• Changing patterns of rainfall
• Effect on plants
• Effect on human health and wildlife

Acid Rain
The term acid rain is used to describe all precipitations - rain, snow, fog, dew - which are more acidic than normal. The normal rain is slightly acidic having a pH of about 5.6 as carbon dioxide gas reacts with it to form a weak carbonic acid. It results from the presence of two strong acids in polluted air, i.e, H₂SO₄ and HNO₃.

As carbon dioxide is present in unpolluted air in traces the acidity is extremely mild and may be regarded as neutral. Acid rain, thus, refers to any precipitation which has pH less than 5.6. The pH of the acid rain can range between 5.6 and 3.5 and in some cases even lower upto 2.

Consequences of Acid rain
Acid rain has the following effects:

• Damage to animals
• Damage to plants
• Material damage
• Effect on human beings

The word smog was coined to describe the combination of smoke and fog.

The name was proposed to describe the phenomenon which occured due to condensation of smoke kind of fog on the carbon particles present in the smoke produced due to combustion of fuels like coal and petroleum.

Smoke is of two types:

1. Classical smog
2. Photochemical smog

Classical smog
It is also called London smog or sulphurous acid smog. This was observed in London in 1952 and killed about 4000 to 5000 people. This type of smog is produced from gases like SO₂, particulates and high humidity in the atmosphere. The particulates catalyse the oxidation of SO₂ to SO₃ which gets mixed with moisture in the tiny droplets of fog producing sulphuric acid drops which condense on particulates of smoke or dust to give colloidal dispersion called smog. This type of fog is usually formed during the winter season in the early morning hours. However, after sunrise, its intensity increases due to photochemical oxidation of SO₂ to SO₃.

It occurs in the lower part of the atmosphere. It decreases visibility and is responsible for breathing troubles like bronchitis and asthma. It produces irritation to eyes, nose and throat. The London smog is also called reducing smog as it contains SO₂ and carbon.

Photochemical smog
It is also called Los Angeles smog as it was observed for the first time in Los Angeles in 1950. This type of smog is formed by the combination of particulates(smoke, dust, fog) with oxides of nitrogen and hydrocarbons in the presence of sunlight. As photochemical reactions are involved in the formation of smog, it is called photochemical smog. It occurs in those areas which have warm, sunny and dey climate along with the atmosphere has a large concentration of automobile exhausts. It usually occurs in the months of summer during the day time. Photochemical smog is characterized by brown haze of irritating nature. It is oxidising in character as it has high concentration off oxidants.

The upper stratosphere consists of considerable amount of ozone (O₃), which protects us from the harmful ultraviolet (UV) radiations (λ 255 nm) coming from the sun. These radiations cause skin cancer (melanoma) in humans. Therefore, it is important to maintain the ozone shield.
Ozone in the stratosphere is a product of UV radiations acting on dioxygen (O₂) molecules. The UV radiations split apart molecular oxygen into free oxygen (O) atoms. These oxygen atoms combine with the molecular oxygen to form ozone.

Ozone is thermodynamically unstable and decomposes to molecular oxygen. Thus, a dynamic equilibrium exists between the production and decomposition of ozone molecules. In recent years, there have been reports of the depletion of this protective ozone layer because of the presence of certain chemicals in the stratosphere. The main reason of ozone layer depletion is believed to be the release of chlorofluorocarbon compounds (CFCs), also known as freons. These compounds are nonreactive, non-flammable, non-toxic organic molecules and therefore used in refrigerators, air conditioners, in the production of plastic foam and by the electronic industry for cleaning computer parts etc. Once CFCs are released in the atmosphere, they mix with the normal atmospheric gases and eventually reach the stratosphere. In stratosphere, they get broken down by powerful UV radiations, releasing chlorine free radical.

The chlorine radical then react with stratospheric ozone to form chlorine monoxide radicals and molecular oxygen.

Reaction of chlorine monoxide radical with atomic oxygen produces more chlorine radicals.

The chlorine radicals are continuously regenerated and cause the breakdown of ozone. Thus, CFCs are transporting agents for continuously generating chlorine radicals into the stratosphere and damaging the ozone layer.

The Ozone Hole
In 1980s atmospheric scientists working in Antarctica reported about the depletion of the ozone layer commonly known as the ozone hole over the South Pole. It was found that a unique set of conditions was responsible for the ozone hole.

In summer season: In summer season, NO₂ and CH₄ reacts with  and chlorine radicals respectively forming chlorine sinks and thus prevent the ozone depletion. The reaction occurs as follows:

In winter season: In this season, special type of clouds called Polar Stratospheric Clouds are formed over Antarctica. The reaction occurs as follows:

In spring season: Sunlight returns and the clouds are broken and HOCl and Cl₂ are photolysed and Chlorine radical thus formed initiates the ozone depletion process

Effects of Depletion of the Ozone Layer
With the depletion of the ozone layer, more UV radiation filters into the troposphere. UV radiations lead to ageing of skin, cataract, sunburn, skin cancer, killing of many phytoplanktons, damage to fish productivity etc. It has also been reported that plant proteins get easily affected by UV radiations which leads to the harmful mutation of cells. It also increases the evaporation of surface water through the stomata of the leaves and decreases the moisture content of the soil. Increase in UV radiations damage paints and fibres, causing them to fade faster.