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Air Pollutants, Types and Classification

Air pollutants can present a real danger to living organisms as well as the wider environment.
air pollutants air pollution

Air pollutants come in the form of gases and finely divided solid and liquid aerosols.

Aerosols are loosely defined as “any solid or liquid particles suspended in the air” (1).

Air pollutants can also be of primary or secondary nature.

Primary air pollutants are the ones that are emitted directly into the atmosphere by the sources (such as power-generating plants).

Secondary air pollutants are the ones that are formed as a result of reactions between primary pollutants and other elements in the atmosphere, such as ozone.

Possibly one of the most important characteristics of air pollutants is their transboundary nature - they can easily travel and affect the areas far away from their points of origination.

Gaseous Air Pollutants

Renowned author Jeremy Colls identifies the following three main types of gaseous air pollutants: (2)

  • Sulfur dioxide (SO2)
  • Oxides of nitrogen (NOx = NO + NO2)
  • Ozone (O3)

Sulfur dioxide and nitric oxide (NO) are the primary air pollutants, and ozone is a secondary pollutant (though there are negligible direct emissions of the gas itself).

Nitrogen dioxide (NO2) is both a primary and secondary air pollutant.

Other important gaseous pollutants are: ammonia, carbon monoxide, volatile organic compounds (VOCs) and persistent organic pollutants (POPs) which we discuss below.

Air Pollutants: Sulfur Dioxide (SO2)

Sulfur dioxide is a colorless gas with a pungent, suffocating odor. It is a dangerous air pollutant because it is corrosive to organic materials and it irritates the eyes, nose and lungs. (3)

Anthropogenic Sources of Sulfur Dioxide Emissions

Sulfur is contained within all fossil fuels, and is released in the form of sulfur dioxide (SO2) during fossil fuel combustion. Fossil fuel combustion accounts for almost all anthropogenic (human-caused) sulfur emissions. (4)

Sulfur contents in fossil fuels range between 0.1% and 4% in oil, oil by-products and coal, and up to 40% in natural gas (5) (when immediately extracted from the well; however, the sulfur is efficiently removed during the processing of gas before distribution (6); therefore, combustion of natural gas is not a major source of sulfur emissions (7)).

say no to coal Say No To Coal Photo: SayYesToCleanEnergy.org

Historically, the use of coal in domestic heating was a major source of sulfur dioxide emissions (at least in the UK), but it has declined substantially over time. (8)

Over the last several decades the industrial use of coal in the UK has also declined, whereas the use of oil and natural gas has gradually increased. (9)

In the UK, the sulfur dioxide emissions declined significantly since 1970, thanks to the introduction of low sulfur fuels, the switch from coal to gas and increased energy efficiency. (10)

This trend is possibly true for other industrialized countries as well, though the US as the most important economy in the world is still a large consumer of energy derived from coal.

Below is a breakdown of all the significant sources of sulfur dioxide emissions (based on sources of the UK emissions) (11):

  • Energy Production

    • Electric power generation
    • Petroleum refining
    • Other combustion
  • Commercial and residential use
  • Combustion for industry use
  • Production processes
  • Extraction and distribution of fossil fuels
  • Transport

    • Road transport
    • Other Transport (such as aviation, ships, trains).

Currently, the most important sources of sulfur dioxide emissions (as a result of fossil fuel combustion) are electric power generating plants.

For example, as of 1998, 66% of all sulfur dioxide emissions in the UK came from power plants. In contrast, transport contributions of sulfur dioxide emissions are among the smallest ones. (12)

The biggest sulfur dioxide emitters: US, China and Russia. (13)

In fact, you may be surprised to learn that just one Siberian city in Russia – Norilsk – produces 1% of the total global emissions of sulfur dioxide. In 2007, Norilsk was considered to be one of the most polluted places on Earth. (14)

Natural Sources of Sulfur Dioxide Emissions

There are also significant sulfur emissions generated by natural sources.

The main natural sulfur emissions come in the reduced forms of sulfur compounds such as (15):

  • hydrogen sulfide (H2S)
  • carbon disulfide (CS2)
  • carbonyl sulfide (COS)

and in the organic forms of:

  • methyl mercaptan (CH3SH)
  • dimethyl sulfide (DMS) (CH3SCH3)
  • dimethyl disulfide (DMDS) (CH3SSCH3)

Most of these compounds get oxidized to sulfur dioxide or to sulfate aerosols in the atmosphere. (16)

Marine phytoplankton produce dimethyl sulfide (DMS) which is then oxidized to SO2 in the atmosphere; decay processes in soil and vegetation produce H2S (as one of sulfur compounds); and SO2 is emitted into the atmosphere by volcanoes. (17)

Around 90% of all natural sulfur emissions come in the form of DMS. (18)

Most recently the natural sources have been by far surpassed by anthropogenic sources. Natural sources have been estimated to produce around 24% of all sulfur dioxide emissions, whereas human-caused emissions made up around 76%. (19)

Effects of Sulfur Dioxide Emissions

Sulfur dioxide found in the air produces following effects (20, 21):

  • Irritates eyes, nose, throat
  • Damages lungs when inhaled
  • As part of acid rain:

    • acidifies lakes and streams
    • destroys plant and fish life in lakes and streams
    • may deplete mineral nutrients in the soil
    • may cause reduction of forest and agricultural yields
    • corrodes metals
    • damages surfaces of buildings.

Air Pollutants: Nitrogen Oxides

Author Andrew Farmer points out that oxides of nitrogen are produced by combustion of all fossil fuels including coal- and gas-fired power stations and motor vehicles. (22)

Whereas fuel itself can produce some nitrogen (for example, oil and coal contain around 0.5 – 1.5% of nitrogen, and natural gas contains less than that (23)), most of nitrogen oxides' production comes from the reaction of atmospheric nitrogen and oxygen within the combustion chamber. (24)

The two main nitrogen oxides are nitric oxide (NO), or nitrogen monoxide, and nitrogen dioxide (NO2) the sum of which is equal to NOx.

Nitric oxide (NO) is a colorless gas. (25)

Nitrogen dioxide (NO2) is a gas of reddish-brown color with a distinct sharp, biting odor. (26)

Combustion of fuels always produces both NO2 and NO.

But almost 90% of the NOX combustion product is in the form of NO which is then oxidized to nitrogen dioxide (NO2) in the air (27, 28).

Therefore, only a small percentage of NO2 found in the atmosphere is directly emitted there in this form. The rest has been formed as a result of chemical reactions in the atmosphere itself. (29)

Anthropogenic Sources of Nitrogen Oxide Emissions

Road transport (motor vehicles) is by far the largest contributor of nitrogen emissions (in contrast, it contributes a very small proportion to sulfur dioxide emissions, as discussed above).

bus pollution
Bus Pollution in Guatemala
Doron Laor

For example, based on 1998 UK figures, road transport contributed nearly half of all nitrogen emissions, followed by contributions from electric power generating plants which only contributed around 20% of total nitrogen emissions. (30)

Below is a breakdown of the significant sources of emissions of nitrogen oxides (based on sources of the UK emissions) (31):

  • Road transport
  • Other Transport
  • Energy Production

    • Electric power generation
    • Petroleum refining
    • Other combustion
  • Combustion for industry use
  • Production processes
  • Extraction and distribution of fossil fuels.

Natural Sources of Nitrogen Oxide Emissions

Nitric oxide (NO) is also emitted by soils but there is very little data available for the proper assessment of this area.

Some estimates suggest that the soil production of NO in the UK may be around 2 – 5% of its production from fossil fuel combustion. (32)

Effects of Nitrogen Dioxide (NO2) Emissions

When inhaled, nitrogen dioxide becomes a serious air pollutant which may (33):

  • Cause pulmonary edema (accumulation of excessive fluid in the lungs)
  • Be part of acid rain (destroying fish and plant life in lakes, damaging surfaces of buildings etc)
  • Contribute to photochemical smog.

Air Pollutants: Ammonia

Ammonia is a colorless, pungent, hazardous caustic gas composed of nitrogen and hydrogen. Ammonia emissions are also grouped as NHy which is a sum of NH3 and NH4. (34)

Sources of Ammonia Emissions

Agriculture is by far the biggest source of ammonia emissions.

Livestock farming and animal waste account for the biggest percentage of total ammonia emissions which are due to the decomposition of urea from large animal wastes and uric acid from poultry wastes. (35)

Based on Jeremy Colls’ table of global ammonia emissions, below is a breakdown of their major sources:

  • Livestock – contributes more than 50% of all emissions
  • Fertilizer application
  • Oceans
  • Vegetation
  • Biomass burning

Effects of Ammonia Emissions

Exposure to very high concentrations of gaseous ammonia in the air may result in lung damage and even death. (36)

Air Pollutants: Carbon monoxide (CO)

Carbon monoxide is a colorless, odorless gas which is highly toxic to humans.

The combustion of carbon-based fuels produces carbon dioxide (CO2).

But not all such combustion is complete, and this leads to the production of carbon monoxide (CO). (37)

Motor vehicles and industry are among the largest anthropogenic sources of carbon monoxide emissions. (38)

Effects of Carbon Monoxide Emissions

Carbon monoxide is the most common type of fatal poisoning in many countries around the world. (39)

Exposures to carbon monoxide may lead to (40):

  • Toxicity of the central nervous system and heart
  • Severe effects on the baby of a pregnant woman
  • Headaches and dizziness
  • Problems with getting oxygen supplied to some body parts which may be life-threatening.

Air Pollutants: Volatile Organic Compounds (VOCs)

Volatile organic compounds (VOCs) are defined as organic compounds which easily evaporate and enter the atmosphere.

VOCs may include a wide range of organic air pollutants, from pure hydrocarbons to partially oxidized hydrocarbons to organic compounds containing chlorine, sulfur, or nitrogen. (41)

Historically, the definition of VOCs did not include methane compounds (non-methane VOCs: NMVOCs) since the atmospheric concentration of methane was considered to be a stable natural background. But it was ultimately recognized that methane is also an anthropogenic air pollutant that comes from intensive animal and rice production. (42)

Though some of these compounds can have direct toxic effects, they have been grouped together because of their role in ozone formation. (43)

Anthropogenic Sources of Volatile Organic Compounds

The major anthropogenic sources of VOCs include (44):

  • Solvent Use (including paints, adhesives, aerosols, metal cleaning and printing)
  • Road transport (emissions from fuel / petroleum use)
  • Production processes
  • Extraction and distribution of fossil fuels

For example, in the UK the biggest emissions of NMVOCs are due to solvent use and road transport. (45)

Jeremy Colls points out that substantial NMVOC emissions occur during the following processes (46):

  • Painting (evaporation of solvents)
  • Oil production (flaring and venting of gas)
  • Oil refining (flaring and fugitive emissions)
  • Distribution of oil or refinery products (evaporation from storage, displacement losses when venting tanks)
  • Dry cleaning (final drying of clothes)
  • Production of alcoholic drinks (breweries and distilleries)
  • Arable farming (crop growing, silage manufacture, sludge spreading)

Natural Sources of Volatile Organic Compounds

Not a lot is known about the natural emissions of VOCs.

But we know that forests are the primary natural sources of VOC emissions. (47) And tropical forests are estimated to produce about half of all global natural non-methane VOC emissions. (48)

Plants synthesize many organic molecules and release some VOCs (including a range of terpenes) into the atmosphere. (49)

In total, around 1000 different compounds (with some of which themselves being families with thousands of their own members) are known to be emitted by natural sources. (50)

Effects of Volatile Organic Compounds

VOCs may produce the following effects (51):

  • Some aromatic compounds such as benzene, toluene and xylene are potential carcinogens and may cause leukemia
  • Contribute to sick building syndrome indoors
  • As facilitators in ozone formation, VOCs may indirectly contribute to respiratory problems and other ozone-related problems

Air Pollutants: Ozone (O3)

Ozone (O3) is a colorless, poisonous gas with a sharp, cold, irritating odor. (52)

Ozone can be found in (53, 54):

    • the stratosphere, one of the upper layers of the atmosphere, where it occurs naturally, and
    • the troposphere, the lowest layer of the atmosphere, where it occurs both naturally and as a result of human-generated emissions.

The natural stratospheric ozone is considered to be of beneficial nature – it keeps harmful excessive ultraviolet sunlight from reaching the surface of the Earth.

Ozone which is formed in the troposphere as a result of anthropogenic emissions of primary pollutants, has negative effects on humans and the natural environment. And from this point of view it is an air pollutant.

This human-caused ozone in the troposphere is a secondary pollutant because it is produced by the reaction of primary pollutants, nitrogen oxides and hydrocarbons [including VOCs], in the presence of sunlight. (55)

The tropospheric ozone is the main component of the photochemical smog.

los angeles smog
Smog in Los Angeles
Photo: Bob Travis

A photochemical smog (of brown-yellow color) is a product of the chemical reaction between sunlight, nitrogen oxides and VOCs, which results in the formation of ozone and airborne particles. (56)

The process of ozone formation may take several days to complete, and ozone itself may turn out to be far from the sources of original primary pollutant emissions. (57)

Effects of Ozone as an Air Pollutant

Ozone in the troposphere can have the following negative effects on animals (including humans) and the natural environment (58, 59):

  • Irritation of the respiratory system causing coughing, throat irritation and an uncomfortable sensation in the chest
  • Susceptibility to respiratory infections
  • Compromised lung function harming the breathing process which may become more rapid and more shallow than normal
  • Inflammation and damage to the lining of the lungs
  • Aggravation of asthma
  • Reduction in agricultural yields
  • Interference with photosynthesis and suppression of growth of some plant species

Air Pollutants: Persistent Organic Pollutants (POPs)

Persistent organic pollutants are compounds which are resistant to degradation and persistent in the environment, with half lives of years in the soil or sediment and days in the atmosphere. (60)

Such compounds may include dioxins, furans, polychlorinated biphenyls (PCBs) and organochlorine pesticides such as DDT. (61)

They enter the food chains via the process of biomagnification, get accumulated in human and animal tissue, and are capable of long range transport through being attached to airborne particles. (62)

Sources of Persistent Organic Pollutants

Some POPs are used as pesticides. (63)

Others are used in industrial processes as well as in the production of goods such as solvents, polyvinyl chloride and medicines. (64)

Effects of Persistent Organic Pollutants

Exposure to persistent organic pollutants takes place through diet (specifically, consumption of animal fats), environmental exposure or accidents. (65)

POPs may lead to (66) :

  • Death and illness including disruption of endocrine, reproductive and immune systems
  • Neurobehavioral disorders
  • Cancers

Please note that when POPs are present in the atmosphere in the form of aerosols, they may be classified as airborne particles (see below) rather than gaseous pollutants.

Airborne Particles as Air Pollutants

Airborne particles present one more type of air pollutants.

They are tiny fragments of solid or liquid nature suspended in the air (aerosols).

Particles may be primary – when emitted directly into the atmosphere by sources, or secondary – when particles are formed in the atmosphere through the interaction of primary emissions. (67)

Solid particles between 1 and 100 μm (micrometres) in diameter are called dust particles, while solid particles less than 1 μm in diameter are called fumes, or smoke. (68)

Anthropogenic Sources of Airborne Particles

Anthropogenic particles account for around 10% of the total amount of particles in the atmosphere. (69)

Fossil fuel combustion is one of the main processes which causes vast amounts of particles to be emitted into the atmosphere. (70)

The major anthropogenic sources of airborne particles are (71, 72):

  • Road transport
  • Power generating plants
  • Production processes (such as dust blown away by winds from construction sites)

Natural Sources of Airborne Particles

Main natural sources of particles are (73):

  • Erosion of soil by wind which generates dust particles that travel around the globe
  • Evaporation of droplets of sea water resulting in sea salt crystals being suspended in the air
  • Volcanoes
  • Forest fires
  • Living vegetation

Effects of Airborne Particles

Particles less than 10 μm in diameter are of biggest concern to human and animal health as they can be easily inhaled and get trapped in the respiratory system. (74)

Jeremy Colls also notes that particles of this size have very low gravity-related sedimentation rates and may therefore remain in the atmosphere for days before being washed out by rain or attached to vegetation or buildings. (75)

Airborne particles may cause (76):

  • Asthma
  • Lung cancer
  • Cardiovascular problems

Air Pollutants: Other Notes

Most recently, climate scientists have also started recognizing greenhouse gases such as carbon dioxide as important air pollutants. (77)

Written by: Irina Gray of Tropical-Rainforest-Animals.com
Publication Date: April 2008
© Irina Gray & Tropical-Rainforest-Animals.com
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