Primary & Secondary Air Pollutants |

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Air Pollutants

Air pollution refers to the release of pollutants into the air—pollutants which are detrimental to human health and the planet as a whole. Thus, Air Pollutants can be solid particles, liquid droplets, or gases. They can be natural or manmade. The pollutants have been classified into primary and secondary categories.

Primary Air Pollutants

The primary pollutants are “directly” emitted from the processes such as fossil fuel consumption, Volcanic eruption and factories.
The major primary pollutants are Oxides of Sulphur, Oxides of Nitrogen, Oxides of Carbon, Particulate Matter, Methane, Ammonia, Chlorofluorocarbons, Toxic metals etc.

Five major materials released directly into the atmosphere in unmodified forms.
-Carbon monoxide
-Sulfur dioxide
-Nitrogen oxides
-Hydrocarbons
-Particulate matter

Oxides of Sulphur

Sulphur Oxides are generally a product of the Volcanoes, Industrial processes, Coal and petroleum, because most of them have Sulphur as a component. The Sulphur Dioxide in presence of a catalyst such as NO2 causes Acid Rain, because of the formation of Sulphuric Acid. The Indian Coal is though low in sulphur content but still coal consumption is a major danger of acid rain because of the coal based power plants.

About 99% of the sulfur dioxide in air comes from human sources.
The highest concentrations of sulfur dioxide in the air are found around petrol refineries, chemical manufacturing industries, mineral ore processing plants, and power stations.
Sulphur Dioxide can lead to lung diseases.

It is a major contributor to smog and acid rain.

Oxides of Nitrogen

Most of the Nitrogen Oxides are produced due to high temperature combustion. In the cities the brown haze dome above the cities is mostly because of the Nitrogen Oxides. The most important toxic gas is Nitrogen dioxide which is brown, with sharp odour.

Nitrogen dioxide is part of a group of gaseous air pollutants produced as a result of road traffic and other fossil fuel combustion processes.
Its presence in the air contributes to the formation and modification of other air pollutants, such as ozone and particulate matter, and to acid rain.
High levels of NOx can have a negative effect on vegetation, including leaf damage and reduced growth.
It can make vegetation more susceptible to disease and frost damage.

Oxides of Carbon

Carbon dioxide

Carbon dioxide, a greenhouse gas, is the main pollutant that leads to global warming.

The Carbon Dioxide is associated with the Ocean Acidification and is emitted from combustion, factories and respiration of living organisms.
Though living things emit carbon dioxide when they breathe, carbon dioxide is widely considered to be a pollutant when associated with cars, planes, power plants, and other human activities that involve the burning of fossil fuels such as gasoline and natural gas.
In the past 150 years, such activities have pumped enough carbon dioxide into the atmosphere to raise its levels higher than they have been for hundreds of thousands of years.

Carbon Monoxide

Carbon Monoxide, which is colourless, odourless and non irritating but very poisonous gas is the product of incomplete combustion of the natural gas, coal or wood. The vehicle exhaust is the major source of CO.

Produced by burning of organic material (coal, gas, wood, trash, etc.)
Automobiles biggest source (80%)
Cigarette smoke another major source
Toxic because binds to hemoglobin, reduces oxygen in blood
Not a persistent pollutant, combines with oxygen to form CO2
Most communities now meet EPA standards, but rush hour traffic can produce high CO levels

Hydrocarbons

Hydrocarbons – organic compounds with hydrogen, carbon

From incomplete burning or evaporated from fuel supplies
Major source is automobiles, but some from industry
Contribute to smog
Improvements in engine design have helped reduce

Particulate Matter

The particulate matters are the fine particles which may be either solid or liquid, suspended in a gas. They are different from the Aerosols. Aerosols are particle and gas referred together.

Particulate matter is the sum of all solid and liquid particles suspended in air many of which are hazardous.

This complex mixture includes both organic and inorganic particles, such as dust, pollen, soot, smoke, and liquid droplets.
Aerosols are minute particles suspended in the atmosphere. When these particles are sufficiently large, we notice their presence as they scatter.
Particulates – small pieces of solid materials and liquid droplets (2.5 mm and 10 mm)
PM_2.5 (also known as fine fraction particles) is generally defined as those particles with an aerodynamic diameter of 2.5 microns or less. Sources of fine particles include all types of combustion activities (motor vehicles, power plants, wood burning, etc.) and certain industrial processes.
PM_10-2.5(also known as coarse fraction particles) is generally defined as those particles with an aerodynamic diameter greater than 2.5 microns, but equal to or less than a nominal 10 microns. Sources of coarse particles include crushing or grinding operations, and dust from paved or unpaved roads.

Other PM may be formed in the air from the chemical change of gases. They are indirectly formed when gases from burning fuels react with sunlight and water vapor. The sources can be from fuel combustion in motor vehicles, at power plants, and in other industrial processes.
Examples: ash from fires, asbestos from brakes and insulation, dust
It aggravates asthma
It decreases lung function
Premature death in people with heart or lung disease
Increased respiratory symptoms, such as irritation of the airways, coughing or difficulty breathing.
Some particulates are known carcinogens
Those working in dusty conditions at highest risk (e.g., miners)

Other Primary Pollutants

Then, we have primary pollutants such as Volatile Organic Compounds or VOCs which are methane (CH4) and non-methane (NMVOCs). Methane is a GHG which contributes to Global Warming. Methane comes from swamps and gas emitted by livestock. The NMVOCs include the aromatic compounds such as Benzene, Toluene, Xylene which are proved or suspected carcinogens. Another dangerous compound is the 1,3-butadiene, often associated with industrial uses.

Another category of the primary air pollutants is toxic metals such as Cadmium, Lead and Copper, which are products of the Industrial processes. The Chlorofluorocarbons (CFCs) are proved to be harmful to the ozone layer emitted from products currently banned from use. In agriculture process, Ammonia is emitted which has characteristic pungent odor. It is a precursor to foodstuffs and fertilizers. Ammonia is also a building block for the synthesis of many pharmaceuticals.

Secondary Air Pollutants

The secondary pollutants are not emitted directly. The secondary pollutants form when the primary pollutants react with themselves or other components of the atmosphere. Most important secondary level Air Pollutants are Ground Level Ozone, Smog and POPs (Persistent Organic Pollutants).

Primary pollutants interact with one another, sunlight, or natural gases to produce new, harmful compounds.
Secondary pollutants are harder to control because they have different ways of synthesizing and the formation are not well understood. They form naturally in the environment and cause problems like photochemical smog.

Examples : Ozone, PAN (peroxy acetyl nitrate), Photochemical smog, Aerosols and mists (H2SO4)

Ground level Ozone

The most important secondary pollutant is the Ground Level Ozone or Tropospheric Ozone. Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapours, and chemical solvents are some of the major sources of Nox and VOC. Ground Level Ozone forms due to reactions of the NOx, Carbon Monoxide and VOCs in presence of sunlight.

Ozone (O3) is a highly reactive gas composed of three oxygen atoms.
It is both a natural and a man-made product that occurs in the Earth’s upper atmosphere (the stratosphere) and lower atmosphere (the troposphere).
Tropospheric ozone – what we breathe — is formed primarily from photochemical reactions between two major classes of air pollutants, volatile organic compounds (VOC) and nitrogen oxides (NOX).
Ground level or “bad” ozone is not emitted directly into the air but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight.
Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC.

How it affects?

Breathing ozone can trigger a variety of health problems, particularly for children, the elderly, and people of all ages who have lung diseases such as asthma.
Ground level ozone can also have harmful effects on sensitive vegetation and ecosystems.

Smog

Another most important secondary pollutant is the Smog, which has made up of Smoke and Fog. Traditionally, the smog has resulted from large amounts of coal burning in an area caused by a mixture of smoke and sulphur dioxide. Now-a-days, the Vehicle emissions and Industrial emissions that are acted on in the atmosphere by ultraviolet light from the sun to form secondary pollutants that also combine with the primary emissions to form photochemical smog.

Photochemical smog

Photochemical smog is a mixture of pollutants which includes particulates, nitrogen oxides, ozone, aldehydes, peroxyethanoyl nitrate (PAN), unreacted hydrocarbons, etc. The smog often has a brown haze due to the presence of nitrogen dioxide. It causes painful eyes.

Smog is a kind of air pollution, originally named for the mixture of smoke and fog in the air.

Classic smog results from large amounts of coal burning in an area and is caused by a mixture of smoke and Sulphur Dioxide.
Smoke particles that are trapped in the fog give it a yellow/black colour and this smog is often settled over cities for many days.

How it affects?

It hampers visibility and harms the environment.
It causes respiratory problems
Deaths related to Bronchial Diseases
Heavy smog greatly decrease ultraviolet radiation
Heavy smog results in the decrease of natural vitamin D production leading to a rise in the cases of rickets.

Photochemical oxidants

Photochemical oxidants result from the photochemical reactions involving sunlight with nitrogen oxides, sulfur dioxide, or volatile organic compounds.
They include acids, nitrogen dioxide, sulfur trioxide, and ozone. Ozone is considered a highly dangerous air pollutant. Exposure to ozone can cause many lung diseases such as asthma, emphysema, and bronchitis. Repeated and long exposures to ozone may even permanently scar the lung tissue.

Secondary particulate matter

Secondary particles are the result of the condensation of gases, the chemical reactions involving primary particles with gases, and the coagulation of various primary particles. The main primary pollutants involved in the formation of secondary particulate matter are sulfur dioxide and nitrogen oxides.

PAN

Smog is caused by the interaction of some hydrocarbons and oxidants under the influence of sunlight giving rise to dangerous peroxy acetyl nitrate (PAN).

Aerosols and mists (H2SO4)

Aerosols and mists are very fine liquid droplets that cannot be effectively removed using traditional packed scrubbers. These droplets can be formed from gas phase hydrolysis of halogenated acids (HCl, HF, HBr), metal halides, organohalides, sulfur trioxide (SO3), and phosphorous pentoxide (P2O5).

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