Major Chemical Pollutants in Photochemical Smog.

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Major Chemical Pollutants in Photochemical Smog:
Sources and Environmental Effects

This seems like a simple question but it has a rather in depth answer so I’ll give you a concise answer and then some references that you can look at to find out more detail if you need it.

Photochemical smog is the atmospheric haze that is found near many large cities and it is due to the action of sunlight on the hydrocarbons coming mainly from vegetable matter and the nitrogen oxides emittedby factories and car exhausts.

Well, that is the dictionary definition but it doesn’t really say much about how or why the smog occurs.
For photochemical smog to occur there are four necessary conditions:

  • nitrogen oxides (NOx)
  • sunlight
  • hydrocarbons
  • temperatures above about 18°C 

The chemical reactions that are important are:

NO2 -----(energy form sunlight)----> NO + O (1)

O + O
2 -----------------> O3 (2)

NO + O3 ------------>NO
2 + O2

3 --------(energy)---------> O2 + O (3)

O + H
2O ------------------> 2OH (4)

(1) means that in the presence of sunlight nitrogen dioxide splits into nitrogen oxide and oxygen.

(2) means that oxygen spontaneously combines to form ozone, but then ozone is also decomposing back into oxygen (3).

(4) means that oxygen and water in the air are combining together to form hydroxl radicals.

However, things are not as simple as they seem. These reactions are ongoing and involve the production of many different intermediaries and by products such as aldehydes and organic nitrates.

These reactions can be used to explain how photochemical smog is formed during the morning rush hour.

Car emissions cause the levels of unburnt hydrocarbons (from the petrol) and NO to rise. The sun oxidises (converts by adding oxygen) the NO to NO
2. NO2 is photolysed in sunlight which just means that the molecule breaks down to NO again. The production of NO is the first step in the production of ozone, the spare O can join up with the O2 molecules that occur naturally in the air. This means that the levels of ozone start to increase. The photolysis of the ozone leads to the production of OH. These OH molecules then react with other emissions form cars exhausts such as the unburnt hydrocarbons to form by products such as aldehydes and organic nitrates. These compounds form the irritating components found in the smog. (The yellow colour comes from the NO2).

All four of the conditions described above have to be present for smog to be produces and because of this, its occurence is quite predictable. Cars provide the NO
x and the unburnt hydrocarbons so photochemical smog tends to be a city phenomenon. Sunlight and relatively high temperatures are needed so smog occurs throughout the year in places such as Los Angeles but only during the summer in places like London.
Geographical features that inhibit the dispersal of the pollutants also contribute to the prolonged presence of smog.

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Primary Pollutants

injected into the atmosphere directly..... examples include:

  • carbon monoxide (CO) 
  • odorless, colorless, poisonous gas 
  • created by incomplete combustion (especially bad with older cars) 
  • generates headaches, drowsiness, fatigue, can result in death 
  • oxides of nitrogen (NOx, NO) 
  • NO - nitric oxide 
  • emitted directly by autos, industry 
  • sulfur oxides (SOx) 
  • SO2 - sulfur dioxide 
  • produced largely through coal burning 
  • responsible for acid rain problem 
  • volatile organic compounds (VOCs) 

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