Ozone (O3) is the most abundant of the photochemical oxidants, and it is the one for which an ambient air quality standard has been written. Although it is responsible for many of the undesirable properties of photochemical smog, from chest constriction and irritation of the mucous membrane in people, to the cracking of rubber products and damage to vegetation. Near the ground, ozone is a colourless, gaseous secondary pollutant. It is formed by chemical reactions between reactive organic gases and oxides of nitrogen in the presence of sunlight.
Ozone is one of the irritant secondary pollutants in photochemical smog and is often used as a measure of it. Ozone there is not regarded as a pollutant because it is produced naturally. It is important in absorbing harmful ultraviolet radiation and preventing it from reaching the earth.
Certain conditions are required for the formation of photochemical smog. These conditions include:
1. A source of nitrogen oxides and volatile organic compounds. High concentrations of these two substances are associated with industrialization and transportation. Industrialization and transportation create these pollutants through fossil fuel combustion.
2. The time of day is a very important factor in the amount of photochemical smog present.
Early morning traffic increases the emissions of both nitrogen oxides and VOCs as people drive to work.
Later in the morning, traffic dies down and the nitrogen oxides and volatile organic compounds begin to be react forming nitrogen dioxide, increasing its concentration.
As the sunlight becomes more intense later in the day, nitrogen dioxide is broken down and its by-products form increasing concentrations of ozone.
As the sun goes down, the production of ozone is halted. The ozone that remains in the atmosphere is then consumed by several different reactions.
3. Several meteorological factors can influence the formation of photochemical smog. These conditions include:
- Precipitation can alleviate photochemical smog as the pollutants are washed out of the atmosphere with the rainfall.
- Winds can blow photochemical smog away replacing it with fresh air. However, problems may arise in distant areas that receive the pollution.
- Temperature inversions can enhance the severity of a photochemical smog episode. Normally, during the day the air near the surface is heated and as it warms it rises, carrying the pollutants with it to higher elevations. However, if a temperature inversion develops pollutants can be trapped near the Earth's surface. Temperature inversions cause the reduction of atmospheric mixing and therefore reduce the vertical dispersion of pollutants.
Tropospheric ozone is both a potent greenhouse gas and one of the major atmospheric pollutants. Ozone is produced though a series of reactions involving both hydrocarbons and NOx (NO + NO2).
O3 + NO NO2 + O2
Oxygen (O) formed in the above reaction then reacts with one of the abundant oxygen molecules (which makes up 20.94 % of the atmosphere) producing ozone (O3)
O + O2 O3
Sunlight can break down nitrogen dioxide (NO2) back into nitrogen oxide (NO).
NO2 + sunlight (hv) NO + O
NOx concentrations control whether ozone is produced or destroyed during these reactions. At NOx concentrations below approximately 50 parts per trillion by volume (pptv), the series of reactions involving NOx, atmospheric oxygen and hydrocarbons result in net ozone destruction. When NOx concentrations rise above this "critical value," the reactions lead to net ozone production. Measurements of the relationships between ozone and NOx concentrations in the atmosphere generally result in positive correlations between the two gases.
The best BPEO for Longannet power station was to use the method of sea water scrubbing. Their studies of this found this to be the best option because there are no solid wastes to be put in a landfill which would spoil the environment and there would be no by-products to be marketed off. The advantages of this power station using this method though is that the station is positioned of the Firth of Forth bank where it would have unlimited sea water. This process is done when the gases are passed through sea water (pH 7.5).The sulphur dioxide starts to dissolve in the sea water to become sulphite ions. This sulphite to become less harmful to the environment to which it will be released will have to be aerated.
As a result of Longannet power station using this method it has become one of the cleanest stations in the UK.
The part played by chemists in the research on photochemical smog was and is great. If this research wasn’t done there wouldn’t have been any action taken. This increasing awareness of our human activities has now led to national importance and has then led onto world summits on this topic.
Sources I have used:
Books:
Salters Advanced Chemistry
Understanding Chemistry for Advanced Level
Ted Lister, Janet Renshaw
A level Chemistry. E.N.Ramsden; Stanley Thornes
Website (For the diagrams):
http://www.mtsu.edu/~nchong/Smog-Atm1.htm
