1. What are primary pollutants?
Primary pollutants are injected directly into the atmosphere, these include:


  • carbon monoxide (CO)
  • oxides of nitrogen (NOx, NO)
  • sulfur oxides (SOx)
  • volatile organic compounds (VOCs)
  • particulate matter (dust, ash, salt particles)
2. What secondary pollutants?
Secondary pollutants form in the atmosphere through chemical and photochemical reactions from the primary pollutant, these include
  • sulfuric acid H2SO4
  • nitrogen dioxide NO2
  • ozone O3
    • colorless gas
    • has a sweet smell
    • is an oxidizing agent - lung tissue to rubber products
    • irritates the eyes

3. Source of primary pollutants
CO
  • Unvented kerosene and gas space heaters;
  • leaking chimneys and furnaces;
  • back-drafting from furnaces,
  • gas water heaters,
  • wood stoves, and
  • fireplaces;
  • gas stoves;
  • generators and other gasoline powered equipment;
  • automobile exhaust from attached garages;
  • and tobacco smoke.
  • Incomplete oxidation during combustion in gas ranges and unvented gas or kerosene heaters may cause high concentrations of CO in indoor air. Worn or poorly adjusted and maintained combustion devices (e.g., boilers, furnaces) can be significant sources, or if the flue is improperly sized, blocked, disconnected, or is leaking. Auto, truck, or bus exhaust from attached garages, nearby roads, or parking areas can also be a source.
NO2

  • Kerosene heaters,
  • un-vented gas stoves and heaters.
  • Environmental tobacco smoke.
4. Conditions necessary for formation of photochemical smog and
Formation of secondary pollutants in photochemical smog
  • Smog is produced by a set of complex photochemical reactions involving volatile organic compounds (VOCs), nitrogen oxides and sunlight, which form ground-level ozone.
  • Smog-forming pollutants come from many sources such as automobile exhaust, power plants, factories and many consumer products, including paint, hairspray, charcoal starter fluid, chemical solvents, and even plastic popcorn packaging. In typical urban areas, at least half of the smog precursors come from cars, buses, trucks, and boats.
  • Major smog occurrences often are linked to heavy motor vehicle traffic, high temperatures, sunshine, and calm winds. Weather and geography affect the location and severity of smog. Because temperature regulates the length of time it takes for smog to form, smog can occur more quickly and be more severe on a hot, sunny day.
  • When temperature inversions occur (that is, when warm air stays near the ground instead of rising) and the wind is calm, smog may remain trapped over a city for days. As traffic and other sources add more pollutants to the air, the smog gets worse. Ironically, smog is often more severe farther away from the sources of pollution, because the chemical reactions that cause smog take place in the atmosphere while pollutants are drifting on the wind.

Photochemical smog is a type of air pollution produced when sunlight acts upon motor vehicle exhaust gases to form harmful substances such as ozone (O3), aldehydes and peroxyacetylnitrate (PAN).
Photochemical smog formation requires the following conditions:
  • a still, sunny day
  • temperature inversion (pollutants accumulate in the lower inversion layer)
Ozone causes breathing difficulties, headaches, fatigue and can aggrevate respiratory problems.
The peroxyacetylnitrate (CH3CO-OO-NO2) in photochemical smog can irritate the eyes, causing them to water and sting.

Ozone Production

Motor vehicles produce exhaust gases containing oxides of nitrogen such as nitrogen dioxide (NO2) and nitric oxide (NO).
At the high temperatures of the car's combustion chamber (cylinder), nitrogen and oxygen from the air react to form nitric oxide (NO):
N2(g) + O2(g) -----> 2NO(g)
Some of the nitric oxide (NO) reacts with oxygen to form nitrogen dioxide (NO2):
2NO(g) + O2(g) -----> 2NO2(g)
The mixture of nitric oxide (NO) and nitrogen dioxide (NO2) is sometimes referred to as NOx.
When the nitrogen dioxide (NO2) concentration is well above clean air levels and there is plenty of sunlight, then an oxygen atom splits off from the nitrogen dioxide molecule:
NO2(g)
sunlight
---------->
NO(g)
+
O(g)
This oxygen atom (O) can react with oxygen molecules (O2) in the air to form ozone (O3):
O + O2 -----> O3
Nitric oxide can remove ozone by reacting with it to form nitrogen dioxide (NO2) and oxygen (O2):
NO(g) + O3(g) -----> NO2(g) + O2(g)
When the ratio of NO2 to NO is greater than 3, the formation of ozone is the dominant reaction. If the ratio is less than 0.3, then the nitric oxide reaction destroys the ozone at about the same rate as it is formed, keeping the ozone concentration below harmful levels.
The reaction of hydrocarbons (unburnt petrol) with nitric oxide and oxygen produce nitrogen dioxide also in the presence of sunlight, increasing the ratio of nitrogen dioxide to nitric oxide.

Peroxyacetylnitrate Production

Nitrogen dioxide (NO2), oxygen (O2) and hydrocarbons (unburnt petrol) react in the presence of sunlight to produce peroxyacetylnitrate (CH3CO-OO-NO2):
NO2(g) + O2(g) + hydrocarbons
sunlight
---------->
CH3CO-OO-NO2(g)
Discussion question:

What can we do in our everyday life to reduce the amount of sulfur dioxide production?

Bibliography:
  1. AUS-e-TUTE n.d.,
    Chemistry Tutorial : Photochemical Smog,
    viewed 29 April 2011,
    http://www.ausetute.com.au/photsmog.html