Atmospheric Pollution, Pollutants, Sinks and Consequences

Our earth\’s ecosystem has four components. The atmosphere is one of them. The other three components are the biosphere, hydrosphere, and lithosphere. The atmosphere is the gaseous band that surrounds our earth\’s surface. It\’s the gravity of the earth that protects the atmosphere from being disappeared into outer space.

Availability of gases in the atmosphere

• Major gases

Nitrogen 78.08%

Oxygen  20.95%

Argon  0.93%

Carbon dioxide 0.04%

• Trace level gases (<0.002%)

Neon

Helium

Methane

Krypton

Nitrous oxide

Hydrogen

Xenon

Sulfur dioxide

Ozone

Nitrogen dioxide

Ammonia

Carbon monoxide

How important is our atmosphere?

While you are enjoying this blog in your subconscious mind you are taking air into your lungs and expelling it. So, No Atmosphere, No Human. You can argue that well scientist will be able to produce mass oxygen soon. As a chemistry professor I know how costly these chemicals are? You should thanks your Almighty Creator at least for the free oxygen. If you are not convinced yet, how importance is the atmosphere, I listed several key benefits of having air around us.

• The outer space is hostile. The atmosphere protects us from asteroids and other outer space objects. They burn into ashes in the atmosphere.
• The atmosphere is the source of carbon dioxide. We may be more aware of the global warming side of carbon dioxide. However, we often forget that plants cannot produce oxygen and sugar (carbohydrate) without carbon dioxide.
• It provides nitrogen to the nitrogen-fixing bacteria that produce chemically bound nitrogen for the survival of plant molecules.
• It absorbs the harmful outer space cosmic ray and electromagnetic radiation from the sun
• It also transports water from the ocean to land as rain.

Unfortunately, the atmosphere is getting polluted by industrialization, emissions from motor vehicles and development activities. Air pollution consists of three basic components

How many ways the air pollution can be classified?

Air pollution is studied in many discipline of science. Every discipline has their own way to classify it according to their scope and expertise. I have summarized them below.

According to source:

1. Natural air pollution

2. Anthropogenic air pollution

According to the manner in which they reach the atmosphere:

1. Primary pollutants (those emitted directly from the sources)

2. Secondary pollutants (those formed in the atmosphere by chemical interactions among primary pollutants and normal atmospheric conditions)

According to physical state:

1. Gaseous

2. Liquid (aqueous)

3. Solid.

According to chemical composition:

1. Sulfur-containing compounds

2. Nitrogen-containing compounds

3. Carbon-containing compounds

4. Halogen-containing compounds

5. Toxic substances

6. Radiative compounds

Criteria air pollutants (six major pollutants defined by EPA) :

1. Ozone

2. Carbon monoxide

3. Sulfur dioxide

4. Nitrogen oxides

5. Lead

6. Particulates, PM₁₀

I am going to discuss the major air pollutants in light of inorganic and organic air pollutants.

What are the major inorganic air pollutants?

• Oxides of carbon

• Sulfur dioxide

• Oxides of nitrogen

• Aerosols (particulates consist of finely divided solids or liquids and often exist as colloidal states)

What are the major organic air pollutants?

•Hydrocarbons

•Oxygen-Containing organic compounds (aldehydes and ketones) 

•Organohalide compounds

•Chlorofluoro carbons  

•CFC substitutes   

•Organo nitrogen compounds

•Organic particles in the atmosphere  

Sources of carbon monoxide (CO) air pollution

Carbon monoxide is a colourless, odourless tasteless gas and one of the criteria pollutants.

Natural sources of carbon monoxide air pollution

Volcanic action

Natural gas emission

Electrical discharge during storms

Seed germination

Marsh-gas production

Forest fires

Agricultural burning

Anthropogenic sources of carbon monoxide air pollution

• Incomplete combustion of carbon containing compounds in the industrial processes

• Reactions between carbon-containing materials at high temperatures as in blast furnaces

• Dissociation of CO₂ at high temperature

• Vehicle exhaust

Major sink for carbon monoxide (CO)

The sink is a reservoir that retains the pollutants for a longer period of time or brings it to the natural cycle in a different form. The ocean is well-known sink for many pollutants. However, in the case of carbon monoxide (CO) the soil, and the atmosphere act as a major sink.

1. Soil is considered as a major natural sink for canon monoxide. The activity of soil microorganism is the reason to act as a sink for carbon monoxide.
2. The atmosphere converts carbon monoxide (CO) to carbon dioxide (CO₂). The average residence time of carbon monoxide in the atmosphere is around 4 months before it converts to carbon dioxide by reaction with hydroxyl radical, (HO•)

The atomic hydrogen then reacts with oxygen and produces hydroperoxyl radical.

M is an energy absorbing third body, usually a molecule of O₂or N₂

HO• is regenarated from HOO• by the following reactions

The hydrogen peroxide formed undergoes photochemical dissociation to regenerate HO•

Detrimental effect of Carbon monoxide (CO) pollution

Due to the greater affinity of carbon monoxide (CO) for hemoglobin, it forms irreversible carboxy hemoglobin (COHb) that reduces the ability of blood to supply the proper amount of oxygen throughout the body.

This oxygen deficit affect brain functionality, increase heart rate and in severe exposure may causes death.

However, the level of CO pollution that occur in an urban area has no detrimental effects on materials or plants.

How can we control carbon monoxide pollution?

• Carbon monoxide emissions may be lowered by can be lowered by using a relatively low air-fuel mixture in automobiles. If the air-fuel ratios exceeding approximately 16 : 1 (w/w), an internal combustion engine emits virtually no carbon monoxide.
• Strict enforcement of using catalytic converter in automobile exhaust will reduce the emission of carbon monoxide. A catalytic converter oxidizes CO to CO₂.

Carbon dioxide

In atmosphere carbon dioxide is a relatively non – pollutant species (sounds dubious). There is no doubt that its increasing concentration in the atmosphere is of serious environmental concern. This green house gas are the main culprit of global warming and sea level rising. So, what makes a greenhouse gas from other gases? Gases that are radiatively active and absorb wavelengths longer than 4 mm are called green house gases. The major greenhouse gases of the atmosphere are methane, chloro fluoro carbons, ozone, nitrous oxide, water vapour and carbon dioxide. These gases capture portion of the outgoing thermal radiation attempting to leave the earth\’s surface. Therefore, the earth\’s temperature raises beyond beyond the equivalent temperature. The major greenhouse gases and there share in global warming is tabulated below:

The anthropogenic activity has increased the amount of carbon dioxide in the atmosphere and this trend is continuing. Ideally, In the atmosphere the CO₂ should balance with photosynthesis. There is a trend- every year the CO₂ declines to a minimum concentration in summer when photosynthesis in forests of northern hemisphere converts CO₂ to sugar and it rises to a maximum in winter when the dead vegetation days, releasing its stored carbon as CO₂.

Major Sinks of Carbon Dioxide(CO₂)

The major important sink for CO₂ is ocean. Sea water is alkaline and CO₂ is acidic, therefore the oceans are vast reservoir of CO₂. However the capacity is limited and only the top 75 meters of the ocean is in equilibrium with the atmosphere. Eventually, the absorbed CO₂ of surface layer exchanges with deep oceans but it takes hundreds of years. The remaining carbon dioxide is absorbed by the vegetation.

Although natural sink sources of CO₂ is vast but, it\’s level is continuously increasing resulting in global warming. The consequences of global warming in human health, quality of life, civilization, geography, flora and fauna are widely accepted.