Ozone molecule is formed by an oxygen molecule reacting with oxygen biradical:
The oxygen biradical, needed for the reaction above, is formed by photodissociation of a nitrogen dioxide molecule, which the nitrogen-oxygen bond is broken homolytically:
Concentration of ozone remains constant (or in a steady state) by the rate of reactions A and B equals the rate of reaction C, which removes ozone by reacting with nitrogen oxide radical to form an oxygen molecule and a nitrogen dioxide molecule:
High concentrations of tropospheric ozone are produced by hydrocarbons, RCH3, reacting with hydroxyl radical, OH, to form water molecule and RCH2 radical (reaction D). Then the RCH2 radical reacts with dioxygen molecule to form peroxy radical, RCH2O2 (reaction E):
Then the peroxy radical reacts with nitrogen oxide radical to form nitrogen dioxide:
This means that there are higher concentrations of nitrogen dioxide. Concentration of ozone will increase because via reaction B, more NO2 molecules are photo dissociated to form nitrogen oxide radicals and oxygen biradicals. Then the oxygen biradicals react with dioxygen molecule to form an ozone molecule (reaction A).
In order for reactions D, E and F to take place, the hydroxyl radical is needed to react with hydrocarbon molecules, as stated in reaction D. The hydroxyl radical is formed by photodissociation of ozone molecules, which the oxygen bond is broken homolytically, to form an oxygen molecule and an oxygen biradical:
Then the oxygen radical reacts with a water molecule to form two hydroxyl radicals:
At Longannet, the following processes are chosen to reduce sulphur dioxide and nitrogen oxides emissions.
Sea Water Scrubbing Process
Adapted from Fig. 1 (Article 1 ‘Longannet: clean coal power?’)
Gas Reburn
The sea water scrubbing process is chosen because there are no by-products to be handled and marketed and no solid wastes to be placed in landfills, which increases costs and causes further environmental damage. Sea water, a key raw material, is available freely from the Firth of Forth and sulphate ions can be dumped at sea without significant change in the pH. Unlike the limestone process, limestone does not need to be quarried, preventing spoiling of the environment.
The gas reburn process is chosen because electricity is generated at a lower cost and no nitrogen oxides are produced, which makes Longannet one of the cleanest coal-fired power stations in the world.
Chemists are researching photochemical smog in the following ways: they monitor the atmosphere at various places to know which pollutants are present in the troposphere and how their concentration levels vary.
They study individual reactions in the laboratory by measuring the rate at which each reaction takes place under a variety of controlled conditions. This means that the chemists can predict the rate of these reactions for any condition.
They use computer simulation models and information on reaction rates to reproduce and predict the behaviour of pollutants during a photochemical smog episode. The more accurate the information used, the better predictions can be made and results the model provides.
Finally, smog chamber simulations are used conduct large scale experiments. Primary pollutants are mixed in a smog chamber, which is a large clear plastic bag, and exposed to sunlight under carefully controlled conditions. Probes are used to monitor concentrations of pollutants as the photochemical smog builds up. The smog chamber has to be large to minimise reactions taking place on the container walls instead of in gaseous phase.
The information collected by chemists meant that action has been taken by politicians to reduce emissions of primary pollutants.
Bibliography
Reference
Article 1 ‘Longannet: clean coal power?’ (Adapted from ‘Environmental Pressure’ by Donald Miller, Chemistry Review, Volume 9, Number 4, March 2000) - Pages 3 to 7
Article 2 ‘Photochemical smog: the killer on a summer’s day’ (Adapted from ‘What is ‘Photochemical smog’ by Gwen and Mike Pilling, Chemistry Review, Volume 5, Number 5, May 1996) - Pages 10 to 13
Chemical Ideas by George Burton, John Holman, Gwen Pilling and David Waddington - First Edition - Heinemann
Chapter 6 Radiation and matter - 6.3 Radiation and radicals - Page 103
Chemical Storylines by George Burton, John Holman, Gwen Pilling and David Waddington - First Edition - Heinemann
Developing Fuels - DF5 Trouble with emissions - Page 31
The Atmosphere - A3 Ozone: A vital sunscreen - Pages 58 and 59
Websites
ScienceNet - site distributor - Broadcasting Support Services (BSS)
http://www.sciencenet.org.uk
Web Page - Environmental Science - What causes photochemical smog?
Atmosphere, Climate & Environment Information Programme - site owner - ARIC
http://www. www.ace.mmu.ac.uk
Web Page - Industrial Emission Controls: Sulphur Dioxide
