Air Pollution

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Icon objectives.jpg
  1. Give the major air pollutants
  2. Discuss the sources and effects of air pollution
  3. Study major large scale secondary pollution problems:
  • Photochemical smog
  • Acid deposition
  • Ozone depletion

Note: Global warming is in the next section

The Atmosphere

The description of the Earth's Atmosphere is now in its own section: Atmosphere

Air Pollution


Air pollution  
Presence of chemical(s) in the atmosphere in sufficient quantity to cause harm to humans, other organisms, or materials
Air pollutants 
Chemicals which cause air pollution


Primary air pollutants 
Pollutants emitted directly
Secondary air pollutants 
pollutants created from reactions between primary air pollutant and other chemicals in the air


Air Quality Standards 
maximum allowed concentration of a pollutant in the air
Emission Standards 
maximum amount of a pollutant allowed to be released into the air from a given source
Criteria air pollutants 
6 pollutants regulated by the US Environmental Protection Agency (USEPA) using air quality standards
  • carbon monoxide
  • particulate matter
  • sulfur dioxide
  • nitrogen dioxide
  • ozone
  • lead

Major Air Pollutants

Primary pollutants

Secondary pollutants


Sulfur Dioxide and Acid Rain

Sulfur Dioxide

Sulfur is oxidized to sulfur dioxide:

S + O2 → SO2

This can then be combined with water in the atmosphere to form sulfuric acid:

2SO2 + O2 + 2H2O → H2SO4


  • burning of sulfur-containing compounds including coal and oil
  • smelting of metal ores
Cu2S + O2 → 2Cu + SO2 (metal ore)
  • Pulp and paper production
  • Sulfuric acid manufacturing
Doe Run's lead smelter in Herculaneum, Missouri, USA

Direct Effect of Sulfur Dioxide

  • Absorbed by moist respiratory tissue causing restriction of airway
- especially severe in people with asthma

Acid Deposition

Sulfuric acid can removed from the air by rain ("acid rain") or by attaching to dust particles ("dry deposition")

Effects of Acid Deposition

  • Destruction of concrete and stone structures
The sulfuric acid converts calcium carbonate to calcium sulfate which is more soluble: CaCO3 + H2SO4 → CaSO4 + 2H+ + CO32+
Acid rain events on a stone monument
  • Acidification of forests, rivers, and lakes
Some lakes have natural buffering, but many do not.
Acidified water causes death of fish and other organisms, due to both the lower pH directly and the leaching of metal, especially aluminum. It also causes damage to trees and other plants.
Acid rain damage on woodlands in Czech Republic

Nitrogen dioxide, ozone, and smog

Nitrogen oxides

Nitrogen can react with oxygen to form nitrogen oxides

  • N2O (nitrous oxide) - not toxic but important for global warming
  • NO (nitric oxide) - important precursor of ozone and smog
  • NO2 (nitrogen dioxide) - important primary pollutant
  • N2O3, N2O4, N2O5 - these are very rare in the atmosphere

NO and NO2 are together known as NOx (read as nox)

Nitrogen Dioxide

At low temperature combustion nitrogen does not combine with oxygen:

CH4 + O2 + N2 → CO2 + H2O + N2

However; at high temperatures, such as in a burner or internal combustion engine, Nitric oxide (NO) and Nitrogen dioxide (NO2) are formed [1]:

CH4 + O2 + N2 → CO2 + H2O + NO
NO + O2 → NO2

About half the second reaction occurs in the burner/engine, the rest occurs in the air

Health Effects:

  • lung irritation
  • increases susceptibility to lung disease
The flask contains nitrogen dioxide. It is this that gives the characteristic brownish color to smog


Nitrogen dioxide reacts with sunlight to form nitric oxide and atomic oxygen. This oxygen reacts quickly with molecular oxygen (O2) to form ozone. Ozone can then react with nitric oxide to regenerate nitrogen dioxide.

NO2 + hν → NO + O

O + O2 → O3

O3 + NO → NO2 + O2

Note The ozone formed here is tropospheric ozone ("bad ozone"), not to be confused with stratospheric ozone ("good ozone")

Health Effects:

  • lung problems
  • eye irritation

Environment Effects:

  • damage to plants and trees
Ozone damage to plants. The top row shows normal plants, the bottom row show plants exposed to ozone.

Photochemical Smog

Ozone is extremely reactive and reacts with any organic chemicals in the air. The result of this reaction between tropospheric ozone and organic chemicals is a "toxic soup" called photochemical smog

Photochemical smog includes NOx, ozone, peroxyacyl nitrates (PANs), aldehydes, and other organic chemicals.

Smog in Kuala Lumpur, Malaysia

Thermal inversions

Under certain conditions a layer of warm air can occur above cold air. This causes any pollutants to be trapped and not dispersed.

Smoke rising in Lochcarron, Scotland is stopped by an overlying layer of warmer air
Winter smoke in Shanghai

A good animation of thermal inversion can be found here.

Particulate matter (PM) and Aerosols

Particulate Matter 
Small solid and liquid particles which remain suspended in the atmosphere.

Particulate matter can be divided into two size ranges: 10 - 2.5 μm (PM10) and < 2.5 μm (PM2.5).

burning field producing particulate matter (smoke)


  • dust from construction, agriculture, and roads
  • forest fires
  • burning of solid and liquids -- especially from coal, wood, and diesel fuel
  • clearing of land for agriculture


  • The small particles can get trapped in the lining of the lung causing irritation, inflammation, or cancer
  • PM2.5 is much more dangerous as it gets deeper into the lung.
view of the human lung showing where different size particulate matter affects different areas
  • Inflammation
  • Silicosis (silica dust)
  • Black Lung disease (coal dust)
  • Cancer
  • Asbestos
  • Polynuclear aromatic hydrocarbons (PAH)
  • Tar from incomplete combustion and tobacco smoke
Asbestos micrograph showing the fibers

Toxic air pollutants

Carbon Monoxide (CO)

Car exhaust contains carbon monoxide (note CO is colorless, the white color is steam)


complete combustion (burning) of hydrocarbons gives carbon dioxide[2]:
CH4 + 2O2 → CO2 + 2H2O
incomplete combustion of hydrocarbons gives carbon monoxide:
CH4 + 1.5O2 → CO + 2H2O


Carbon monoxide binds with hemoglobin in the blood to form carboxyhemoglobin. This means the blood cannot carry oxygen to the cells.

The physical effects include lightheadness, headache, confusion, and dizziness to unconsciousness and death

Heme - the oxygen carrying part of hemoglobin. Both oxygen and carbon monoxide bind to the Fe(II) ion




  • paint
  • smelters
  • batteries
  • leaded gasoline


  • memory loss
  • learning difficulties
  • nervous system damage
  • damage to bones and kidneys
  • accumulative poison



  • burning of coal
  • small-scale gold mining


  • mental effects ("mad-hatter's" disease)
  • kidney disease
  • accumulative poison
  • biomagification - especially in fish (see diagram below)

Mercury amalgam from small-scale gold mining. This will be heated to evaporate the mercury
biomagnification of mercury in food chain

Volatile Organic Compounds (VOC's)


  • Organic compounds which rapidly evaporate.


  • benzene
  • acetone
  • solvents
  • formaladehyde (from building materials)


varies widely depending on substance

  • eye, nose, and throat irritation
  • dizziness and headaches
  • damage to liver, kidney, and nervous system
  • actual toxicity varies widely

Indoor Air Pollution

Indoor air pollution is air pollutants found in your home, office, etc.


  • formaldehdye
  • chlorinated solvents
  • pesticides
  • tobacco smoke
  • radon



  • Naturally radioactive gas. A decay product of radium (which is a decay product of uranium). Uranium and radium can be found naturally in some rocks especially granite. Radon can then enter houses built on such rocks.


  • lung cancer (due to alpha radiation)

Ozone Depletion

As stated above, the stratosphere contains a layer of ozone which protects the Earth from dangerous ultraviolet radiation.

Chlorinated and brominated hydrocarbons can travel unreacted up to the stratosphere. The most important of these are the chlorofluorocarbons (CFC's).

CCl3F + hν → CCl2F + Cl

This free chlorine then reacts with the ozone.

Cl + O3 → ClO + O2

ClO + O → Cl + O2

net O3 + O → O2 + O2

This depletes the amount of ozone present.

These ozone depleting chemicals include:

  • chlorofluorocarbons (CFC's)
  • hydrochlorofluorocarbons (HCFC's)
  • halons (bromochloroflurocarbons)
  • methyl bromide

Montreal Protocol

Ozone "hole" in 2006. Blue and purple have the least ozone and green, yellow, and red have the most.

In the 1980's, it was found the ozone layer was thinning. In 1987 a treaty called the Montreal Protocol was signed.

It banned the use of ozone depleting chemicals. As of 2012, 98% of ozone-depleting chemicals were phased out. All countries are in compliance. The ozone layer has stabilized and concentrations of ozone in the stratosphere are now increasing.


  1. For simplicity the following equations are not balanced
  2. note that methane is only used as an example, it could be any hydrocarbon