The Atmosphere and Air Pollution
The atmosphere is composed of layers as shown in the diagrams below:
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.
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.
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
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.
The numbers in the diagram consider a dry atmosphere. Water vapor accounts for 0-4%.
- 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
Sulfur Dioxide and Acid Rain
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)
- Absorbed by moist respiratory tissue causing restriction of airway
- - especialy severe in people with asthma
- 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
- 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.
Nitrogen dioxide, ozone, and smog
Nitric oxide (NO) and nitrogen dioxide (NO2) are formed from high temperature combustion. 
- low temperature combustion: CH4 + O2 + N2 → CO2 + H2O + N2
- high temperature combustion:
- CH4 + O2 + N2 → CO2 + H2O + NO
- NO + O2 → NO2
- high temperature combustion:
Nitric oxide and nitrogen dioxide are together called NOx (read as nox).
- lung irritation
- increases susceptibility to lung disease
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")
- lung problems
- eye irritation
- damage to plants and trees
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.
Under certain conditions a layer of warm air can occur above cold air. This causes any pollutants to be trapped and not dispersed.
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
- Silicosis (silica dust)
- Black Lung disease (coal dust)
- 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.
- 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.
- 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)
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.
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.
- ↑ For simplicity the following equations are not balanced