The Atmosphere and Air Pollution

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  1. Describe the Atmosphere
  2. List the major air pollutants
  3. Look at some special topics
  • Photochemical smog
  • Acid deposition
  • Ozone depletion

Note: Global warming is in the next section


The Atmosphere

Atmospheric layers

The atmosphere is composed of layers as shown in the diagrams below:

Layers of the atmosphere
Temperature profile of atmosphere


The stratosphere is the layer above the troposphere. Here the temperature increases slightly versus height.

Contained in the lower part of the stratosphere is the ozone layer.

Concentration of ozone in atmosphere versus height. You can see clearly the ozone layer in the stratosphere.

Ozone cycle in the stratosphere

This ozone absorbs ultraviolet (UV) radiation:

O3 + hν → O2 + O → O3

Thus the ozone protects the Earth from dangerous ultraviolet radiation. If it wasn't for this layer life could not exist -- too much radiation would reach the surface.


The mesosphere is where most meteors burn up in the atmosphere. It also has some interesting, but poorly understood, phenomena such as noctilucent clouds, blue jets, and sprites.


In the thermosphere, the temperature starts to increase again due to solar radiation.

An important part of the thermosphere is the ionosphere, a region where the solar radiation is strong enough to split electrons off atoms to form ions.

A spectacular aspect of the ionosphere are the auroras

Aurora Borealis
Aurora Timelapse.ogv
Aurora video (timelapse)

Another important aspect of the ionosphere is that it allows radio waves to travel long distances. The waves refract off the ionosphere directing them back to Earth.

Radio wave propagation by the ionosphere

Atmospheric composition

Composition of the atmosphere

The numbers in the diagram consider a dry atmosphere. Water vapor accounts for 0-4%.

Air Pollution

Primary air pollutants  
pollutants emitted directly
Secondary air pollutants 
pollutants created from reactions between primary air pollutants
Criteria air pollutants 
6 pollutants regulated by the US Environmental Protection Agency (USEPA)
  • carbon monoxide
  • particulate matter
  • sulfur dioxide
  • nitrogen dioxide
  • ozone
  • lead

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 sulfur dioxide:

2SO2 + O2 + 2H2O → H2SO4


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

Direct Effect

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

Environmental Effect

  • Acid Deposition

Acid Deposition

The fallout of sulfuric acid either in rain or as dry particles is called acid deposition or, more commonly, acid rain.


  • Destruction of concrete and stone structures
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. It also causes damage to trees and other plants.
Acid rain damage on woodlands in Czech Republic

Nitrogen dioxide, ozone, and smog

Nitrogen Dioxide

Nitric oxide (NO) and nitrogen dioxide (NO2) are formed from high temperature combustion. [1]

low temperature combustion: CH4 + O2 + N2 → CO2 + H2O + N2
high temperature combustion:
CH4 + O2 + N2 → CO2 + H2O + NO
NO + O2 → NO2

Nitric oxide and nitrogen dioxide are together called NOx (read as nox).

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

Photochemical Smog

Tropospheric ozone reacts with organic chemicals to produce photochemical smog.

Smog is a toxic soup of 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

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

Sizes are up to 10 micrometers in diameter.


  • 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
  • Inflammation
  • Silicosis (silica dust)
  • Black Lung disease (coal dust)
  • Cancer
  • Asbestos
  • Polynuclear aromatic hydrocarbons (PAH))

Toxic air pollutants

Carbon Monoxide (CO)

Source: incomplete burning of organic materials

complete combustion: CH4 + 2O2 → CO2 + 2H2O
incomplete combustion: CH4 + 1.5O2 → CO + 2H2O

Effects: binds with hemoglobin to form carboxyhemoglobin. This carboxyhemoglobin cannot transfer oxygen to the cells.



  • 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

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



  • 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.

Montreal Protocol

File:160658main2 OZONE large 350.png
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 bans the use of chlorofluorocarbons (CFCs). This has stabilized the ozone layer.


  1. For simplicity the following equations are not balanced
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