Classical smog (or London smog) is a reducing smog composed of smoke, fog, and SO2. It forms in cool, humid climates. Photochemical smog (or Los Angeles smog) is an oxidizing smog composed of ozone, NOx, and hydrocarbons. It forms in warm, sunny climates.
Carbon monoxide is poisonous because it has a very high affinity for hemoglobin, an iron-containing protein in red blood cells that transports oxygen. It binds to hemoglobin 200 times more effectively than oxygen, thereby severely reducing the blood's ability to transport oxygen to tissues and organs.
The greenhouse effect is the process by which certain gases in the Earth's atmosphere trap heat. These gases (like CO2 and CH4) allow sunlight to pass through to the Earth's surface but absorb the infrared radiation (heat) that the Earth radiates back. This trapped heat warms the planet.
The main sources of NOx are the high-temperature combustion of fossil fuels, particularly in automobile engines and thermal power plants. At high temperatures, atmospheric nitrogen and oxygen react to form these oxides.
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Atmospheric Pollutants and Reactions
1.0Introduction
The atmosphere is a delicate mixture of gases essential for life. Atmospheric pollution occurs when the concentration of certain substances in the air exceeds their natural abundance, causing harmful effects on living organisms and the environment. These substances, known as pollutants, can be gases, liquids, or solids and are primarily introduced into the atmosphere through human activities like the burning of fossil fuels and industrial processes.
2.0Classification of Pollutants
Atmospheric pollutants can be classified based on their origin.
Primary Pollutants: These are pollutants that are emitted directly into the atmosphere from a source. Examples include sulfur dioxide SO2, nitrogen oxides (NOandNO2)carbon monoxide (CO), and particulate matter.
Secondary Pollutants: These are pollutants that are not directly emitted but are formed in the atmosphere through chemical reactions between primary pollutants and other atmospheric components. Examples include sulfuric acid(H2SO4) nitric acid HNO3, ozone (O3)in the troposphere, and Peroxyacetyl Nitrate (PAN).
3.0Gaseous Air Pollutants and Their Effects
Oxides of Sulfur (SOx)
Sources: The burning of sulfur-containing fossil fuels like coal and petroleum in power plants and industries.
Pollutant: Sulfur dioxide (SO2) is the most common oxide of sulfur.
Effects: It is highly poisonous to living organisms. In humans, it can cause respiratory diseases like asthma and bronchitis. In plants, it can cause damage to leaves and inhibit growth.
Reaction: Sulfur dioxide can be oxidized to sulfur trioxide (SO3) in the presence of particulate matter and sunlight.
Oxides of Nitrogen (NOx)
Sources: Formed during high-temperature combustion processes, such as in automobile engines and power plants, where atmospheric nitrogen and oxygen react.
Pollutants: Nitric oxide (NO) and nitrogen dioxide (NO2)
Effects: High concentrations of can damage lung tissues and cause respiratory problems. It also contributes to the formation of photochemical smog.
Reaction: At high temperatures, nitrogen and oxygen react to form nitric oxide.
N2(g)+O2(g)heat2NO(g)
This NO is then rapidly oxidized to Nitrogen Dioxide⇒2NO(g)+O2(g)→2NO2(g)
Oxides of Carbon (COx)
Carbon Monoxide (CO): A colorless, odorless, and highly poisonous gas produced from the incomplete combustion of fuels. It is hazardous because it binds to haemoglobin in the blood about 200 times more strongly than oxygen, reducing the blood's oxygen-carrying capacity. This can lead to headaches, impaired vision, and death by asphyxiation.
Carbon Dioxide (CO2): A major greenhouse gas released from the burning of fossil fuels. While it is naturally present in the atmosphere, its increased concentration is the primary cause of global warming.
Hydrocarbons
Sources: Incomplete combustion of fossil fuels and natural emissions from plants.
Effects: Many hydrocarbons are carcinogenic (cancer-causing). They can also cause tissue damage and premature ageing in plants.
Reactions: Hydrocarbons and nitrogen oxides react in the presence of sunlight to form secondary pollutants that constitute photochemical smog.
4.0Major Atmospheric Chemical Reactions and Their Effects
Formation of Acid Rain
Acid rain is any form of precipitation (rain, snow, fog, etc.) that has a pH lower than 5.6. Normal rain is slightly acidic due to the dissolution of atmospheric CO2
Chemical Reactions: The primary culprits for acid rain are sulfur and nitrogen oxides.
Formation of Sulfuric Acid:2SO2(g)+O2(g)+2H2O(l)→2H2SO4(aq)
Formation of Nitric Acid:4NO2(g)+O2(g)+2H2O(l)→4HNO3(aq)
Effects: Acid rain damages historical monuments (especially marble), corrodes metal structures, harms aquatic life by lowering the pH of lakes and rivers, and damages forests and crops.
The Greenhouse Effect and Global Warming
The greenhouse effect is a natural phenomenon where certain gases in the atmosphere trap heat, keeping the Earth warm enough for life. However, an enhanced greenhouse effect due to increased concentrations of these gases leads to global warming.
Greenhouse Gases: The main greenhouse gases are carbon dioxide(CO2), methane (CH4), nitrous oxide (N2O) and chlorofluorocarbons (CFCs).
Mechanism: These gases absorb infrared radiation (heat) emitted from the Earth's surface and re-radiate it back, leading to a rise in the planet's average temperature.
Formation of Photochemical Smog
Photochemical smog is a type of air pollution that forms in warm, sunny, and dry climates. It is an oxidizing smog because it contains high concentrations of oxidants like ozone and PAN.
Key Components: A mixture of primary pollutants (nitrogen oxides and hydrocarbons) and secondary pollutants (ozone, aldehydes, and Peroxyacetyl Nitrate or PAN).
Chemical Reactions: The process is initiated by sunlight.
Nitrogen dioxide absorbs sunlight and breaks down into nitric oxide and an oxygen atom.
NO2(g)hνNO(g)+O(g)
The highly reactive oxygen atom combines with molecular oxygen to form ozone.
O(g)+O2(g)⇋O3(g)
Ozone reacts with the unburnt hydrocarbons to form products like acrolein, formaldehyde, and PAN.
Effects: Photochemical smog causes eye irritation, respiratory problems, and damage to plants and materials.
Ozone Layer Depletion
The ozone layer in the stratosphere protects life on Earth by absorbing harmful ultraviolet (UV) radiation from the sun. Depletion of this layer is a major concern.
Cause: Primarily caused by chlorofluorocarbons (CFCs), which are stable in the lower atmosphere but break down in the stratosphere under intense UV radiation.
Chemical Reactions:
A CFC molecule (e.g.,CF2Cl2) breaks down to release a chlorine free radical.
CF2Cl2(g)UVCl(g)+CF2Cl(g)
The highly reactive chlorine radical acts as a catalyst, destroying ozone.
Cl(g)+O3(g)→ClO(g)+O2(g)
The chlorine monoxide radical then reacts with an oxygen atom, regenerating the chlorine radical.
ClO.(g)+O(g)→Cl(g)+O2(g)
A single chlorine atom can catalytically destroy thousands of ozone molecules.
Table of Contents
1.0Introduction
2.0Classification of Pollutants
3.0Gaseous Air Pollutants and Their Effects
4.0Major Atmospheric Chemical Reactions and Their Effects