The difference between a primary and secondary source of air pollution 

The below article talks about air pollution, air quality, the difference between primary sources of air pollution, and secondary sources of air pollution, along with some frequently asked questions about air pollution and its effects on the world around us. 

What is the difference between a primary and secondary source of air pollution? 

Primary and secondary air pollution is directly linked to the kind of pollutants that are released. Primary air pollution is caused by primary air pollutants, which include particulate matter, carbon monoxide, nitrogen oxides, sulfur oxides, etc. Secondary air pollution, caused by secondary air pollutants such as ozone or organic aerosol is caused in the lower strata of the atmosphere due to chemical reactions. The below article elaborates on the differences between primary and secondary pollutants and provides you with everything you need to know about them and the impact they could have. 

Air pollution and air quality

Poor air quality due to increasing pollution is an extensively studied field. The effects of exposure to bad air quality are inevitable and researchers have taken up various measures to identify the different impacts air-borne pollutants could have. Air pollution has been found to impact human health severely and if exposed to it on a long-term basis, it could cause irreversible damage including respiratory conditions and neuro system damages amongst other illnesses. 

Research into air pollution in the top 30 cities in the world shows that Indian cities hold the first 21 positions and that in 2019, approximately 1.67 million deaths were attributed to long-term exposure to poor air quality. The long-term exposure to high levels of air pollution caused heart attacks, chronic lung diseases, strokes, along with neonatal diseases which were accelerated due to the poor air quality (Hadley S., 2020). 

Various countries have taken measures to ensure clean air. For example, cities like Birmingham and Madrid aim to create “clean air zones” by enforcing strict regulations within the inner-city borders, China has acknowledged the need to speed up the construction and usage of electric vehicles to ensure that decent levels of air quality can be maintained in the country, Bangladesh high court has put together several directives aimed at ministries across the country to take actions to counter the effects of air pollution on its citizens (Hadley S., 2020).

Primary and Secondary air pollutants: What do you need to know?

In the most generic sense, air pollution is the contamination of the air quality due to certain airborne contaminants. When the contaminants rise above a certain recommended concentration, they are said to be toxic and are capable of impacting the lives of all kinds of organisms in the vicinity. The pollutants could be solid, liquid droplets, or gaseous particles, and depending on where they originate from, they are classified as being a primary or a secondary pollutant, and their sources are referred to as primary or secondary sources of pollution (Chahine-Böhme L., n.d.). Primary and secondary air pollutants are quite dangerous and could have devastating impacts on the surrounding ecosystems. 

A primary pollutant is an airborne contaminant that is directly released into the atmosphere as a result of anthropogenic or other natural events, this could be pollutants emitted from volcanoes, sandstorms, vehicular emissions, combustion of fossil fuels, industrial activities, etc. Examples of primary pollutants would include sulfur dioxide, carbon monoxide, nitrogen oxides, and particulate matter. Most primary pollutants are traced back to human activities and some of them include:

  • Carbon oxides are released during the combustion of fossil fuels. 
    • Carbon monoxide is released during the incomplete combustion of fossil fuels and is mainly emitted from traffic and from other areas that use fossil fuel as a source of energy. 
  • Formation of nitrogen oxides at high temperatures, mostly occurring as a result of emissions from vehicles, industrial activities, power plants, etc. 
  • Sulfur oxides, particularly sulfur dioxide are emitted when fuel sources such as coal and oil are used. These compounds can also be released by naturally occurring events such as erupting volcanoes. Sulfur dioxide is one of the leading causes of ocean acidification, acid rain, etc. 
  • Nitrogen oxides are a growing concern and are mainly emitted from motorized vehicles, along with some parts contributed via volcanoes and fires. It is one of the reasons for smog and is capable of forming nitric acid in the instance of rainfall, leading to acid rains in the area. 
  • Emissions of heavy metals such as mercury and lead as a result of industrial activities mainly put into the atmosphere via smokestacks
  • Volatile organic compounds such as methane from various activities including agricultural activities, raising livestock, industrial activities, etc. 
  • Solid particulate matter concentrations in the atmosphere could be ash, soot, dust, animal waste particles, pollutants emitted during the combustion of fossil fuels, etc. 

Secondary pollutants are formed from primary pollutants, most of them occur and form what is called a cloud of photochemical smog. It is not emitted into the atmosphere directly from a source, rather it originates as a result of chemical reactions between existing pollutants in the atmosphere. It is usually seen as a brown haze that is capable of covering large distances and reducing visibility drastically. Photochemical smog is formed as a result of certain chemical reactions of pollutants in the presence of sunlight. Secondary pollutants are majorly ozone and coagulated particulate matter, also known as secondary particulate matter. These are formed as a result of the condensation of gases, chemical reactions involving gases and other solid particles, coagulation of various solid particulate matter present in the air, etc. The main contributors to this kind of pollutants would be sulfur dioxide and nitrogen oxides. 

The below image is a depiction of the most dangerous primary and secondary pollutants, and a brief illustration of where they usually originate from. 

Source: Envira IoT, 2022

Secondary pollutants are much harder to control, as it is hard to form a mitigation strategy for any number of pollutants that could be synthesized and formed at any given set of circumstances. Since they form naturally in the environment, and some of them are even quite useful, for example, ozone is a pollutant in the lower levels of the atmosphere, however, the presence of the ozone in the stratosphere ensures that the earth is protected from the harsh ultraviolet radiation from the sun, and keeps the earth in a habitable temperature. Below is an illustration of the formation of ozone at ground level. 

Source: CK-12, n.d.

Frequently Asked Questions (FAQs): What is the difference between the primary and secondary sources of pollutants?

What is photochemical smog? 

A photochemical smog refers to a mixture of several airborne contaminants and is a thick haze that is formed as a result of the various chemical interactions of these airborne contaminants with sunlight. The smog is usually formed from the chemical interactions of nitrogen oxides or volatile organic compounds with sunlight. It occurs more during summers and is mainly formed as a result of rash anthropogenic activities that lead to a large volume of toxic concentrations of air pollutants into the atmosphere (AU EPA, 2004). 

Can HEPA filters remove disease-causing viruses such as the COVID-19 virus? 

Various studies conducted under different settings such as hospitals with patients or controlled circumstances show that HEPA filters, HVAC filters, etc. are capable of removing airborne disease-causing viruses such as the coronavirus. A study conducted found that the coronavirus is sized at 0.125 microns in diameter (Heffernan T., 2020), which falls squarely into the filtering capacity of HEPA filters, as proven by NASA through their recent studies that showed that HEPA filters are capable of removing approximately 100% of particulates of 0.01 microns, and this proves effectively that it has startling efficiency to remove airborne viruses that can cause COVID-19 as well as absorb any aerosolized droplets from the carriers of the virus. 

Other FAQs about Air quality that you may be interested in.

What are the current air quality levels in Sedona?

The country with the worst air quality

Air Quality in Japan and America


AU Environmental Protection Agency (US EPA). (2004 March). Photochemical smog – what it means for us. EPA Information. Government of South Australia. Viewed on 02-14-2022. 

Chahine-Böhme L. (n.d.). Difference between primary pollutants and secondary pollutants. Viewed on 02-14-2022. 

CK-12. (n.d.). 10.19 Types of Air Pollution. Viewed on 02-14-2022. 

Envira IoT. (2022, January 10). Primary and secondary pollutants: these are the most dangerous. Viewed on 02-14-2022. 

Hadley S. (2020, December 21). Air pollution is causing permanent damage to eye health. Viewed on 02- 14- 2022. 

Heffernan T. (2020, November 18). Can HEPA Air Purifiers capture Coronavirus? Wirecutter. NY Times. Viewed on 02-14-2022. 

UC Riverside. (n.d.). Primary vs. Secondary Pollutants. Air Pollution in California. Viewed on 02-14-2022.,lower%20atmosphere%20by%20chemical%20reactions

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