Primary and Secondary Air Pollutants

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Primary & Secondary Air Pollutants

Primary pollutants enter the atmosphere directly from a process like combustion of fuel in an engine releasing carbon monoxide and carbon dioxide out of the exhaust. Another example is sulfur dioxide released in a power plant or factory from combustion of fossil fuel. A volcanic eruption releases smoke and ash which are considered primary pollutants. Nitrogen oxides are produced by combustion of fossil fuels at high temperature or by thunderstorms.

Secondary pollutants form when primary pollutants interact with something else in the air and it forms something else entirely. For example, when nitrogen oxides react under conditions of high temperatures, with volatile organic compounds, ground level ozone is created. This is a secondary pollutant that can lead to the production of smog and poor air quality. Smog, a combination of smoke and fog can be the result of burning coal which releases smoke and sulfur dioxide. Or, it can be the result of cars and factories releasing emissions that interact with UV light from the sun to form secondary pollutants.


Primary Air Pollutants

Oxides of Sulphur

Sulfur dioxide is a gas. It is invisible and has a nasty, sharp smell. It reacts easily with other substances to form harmful compounds, such as sulfuric acid, sulfurous acid and sulfate particles.

About 99% of the sulfur dioxide in air comes from human sources. The main source of sulfur dioxide in the air is industrial activity that processes materials that contain sulfur, eg the generation of electricity from coal, oil or gas that contains sulfur. Some mineral ores also contain sulfur, and sulfur dioxide is released when they are processed. In addition, industrial activities that burn fossil fuels containing sulfur can be important sources of sulfur dioxide.

Sulfur dioxide is also present in motor vehicle emissions, as the result of fuel combustion. In the past, motor vehicle exhaust was an important, but not the main, source of sulfur dioxide in air. However, this is no longer the case. The highest concentrations of sulfur dioxide in the air are found around petrol refineries, chemical manufacturing industries, mineral ore processing plants and power stations.



Oxides of Nitrogen

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.


Oxides of Carbon

Carbon dioxide, a greenhouse gas, is the main pollutant that is warming Earth. 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.

Other greenhouse gases include methane—which comes from such sources as swamps and gas emitted by livestock—and chlorofluorocarbons (CFCs), which were used in refrigerants and aerosol propellants until they were banned because of their deteriorating effect on Earth’s ozone layer.



Particulate Matter

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 inorganicparticles, 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.



Other Primary Pollutants

Heavy metals such as cadmium, lead and mercury are common air pollutants. Chlorofluorocarbons (CFCs) are nontoxic, nonflammable chemicals containing atoms of carbon, chlorine, and fluorine. They are used in the manufacture of aerosol sprays, blowing agents for foams and packing materials, as solvents, and as refrigerants.

Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and halons destroy the earth’s protective ozone layer, which shields the earth from harmful ultraviolet (UV-B) rays generated from the sun. Hydrofluorocarbons (HFCs) also act to warm the planet.

Secondary Air Pollutants

Ground level Ozone

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.

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 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 sulfur dioxide.

Ground Level Ozone

High concentrations of ozone near ground level can be harmful to people, animals, crops, and other materials. Ozone can irritate your respiratory system, causing you to start coughing, feel an irritation in your throat and/or experience an uncomfortable sensation in your chest. Ozone can aggravate asthma, and can inflame and damage cells that line your lungs.

Ozone may also aggravate chronic lung diseases such as emphysema and bronchitis and reduce the immune system’s ability to fight off bacterial infections in the respiratory system. Lastly, ozone may cause permanent lung damage. These effects can be worse in children and exercising adults.

Unlike stratospheric ozone, which forms naturally in the upper atmosphere and protects us from the sun’s harmful ultraviolet rays, ground-level (or tropospheric) ozone is created through the interactions of man-made (and natural) emissions of volatile organic compounds and nitrogen oxides in the presence of heat and sunlight. Cars and gasoline-burning engines are large sources of volatile organic compounds (VOCs).

VOCs also come from consumer products such as paints, insecticides, and cleaners as well as industrial solvents and chemical manufacturing. Nitrogen oxides (NOx), the other chemical precursor of ozone, are produced whenever fossil fuels are burned and are primarily produced by motor vehicles and power plants. View charts on sources of VOC and NOx emissions in New England. The sun’s direct ultraviolet rays convert these emissions into ground-level ozone, which is unhealthy to breathe.

Many factors impact ground-level ozone development, including temperature, wind speed and direction, time of day, and driving patterns. Due to its dependence on weather conditions, ozone is typically a summertime pollutant and a chief component of summertime smog.

Formation of Ground Level Ozone

Ozone has the same chemical structure (O3) whether it occurs miles above the earth or at ground level.  At ground level, “bad” ozone is formed when certain compounds react in the presence of direct sunlight.

VOCs + NOx + Sunlight = Ozone

VOCs, (volatile organic compounds) are widely used as ingredients in household products including; paints, varnishes, wax, fuels, cleaning, disinfecting, cosmetic, degreasing, and hobby products.  Some VOCs are safe to handle and have little known health effects, while other VOCs are highly toxic.  In addition to all of the man made sources of VOCs, natural sources of VOCs exist.  For example, trees naturally release small amounts of VOCs.

NOx, (nitrogen oxide gases) is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts.  Many of the nitrogen oxides are colorless and odorless.  The primary sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels. When high levels of VOCs and NOx are present in the air, they can react.  When they react in the presence of sunlight and hot weather, ground level ozone forms.

There are other factors involved with the formation of “bad” or ground level ozone, including; cloud cover, wind direction, and low wind speeds.  If the weather conditions are conducive, and there are ample amounts of NOx and VOCs, harmful concentrations of ground level ozone can form in the air. Often industry is blamed entirely for emissions that cause ground level ozone air pollution, but actually private citizens are responsible for a significant percentage of the air pollutants that lead to ground level ozone production. Motor vehicle emissions are the single greatest contributor to ground level ozone pollution.

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