Formation of Photochemical Smog
Photochemical smog usually occurs during summertime during high pressure, anticyclonic periods, (in the UK, from as early as May to August.) Ozone mixes with primary pollutants in the lower troposphere. Smog tends to form over cities due to higher pollutant concentration, but can disperse over large areas.
Although there is a background concentration of ozone in the troposphere, these naturally occurring pollutants are a negligible contribution.
Production of high concentrations of Tropospheric ozone
For the production of photochemical smog, the troposphere symbolises a huge reaction vessel, where the primary pollutants mix with ozone, forming secondary pollutants and photochemical smog. Many of the chemical reactions involve radicals. Such reactions absorb sunlight, to change their energy levels. Ozone is still present in unpolluted air, due to being transported down from the stratosphere. Other pollutants are produced in the following processes:
(Ref 1) NO2 can also react with radicals produced from VOCs in a series of reactions to form toxic products such as PAN.
NO2 + R∙ → PAN
(Ref 5) “The World Health Organisation advise that people’s lungs and breathing system can be damaged if the ozone concentration rises above 50 parts per billion (ppb).” (Ref 4) “On 3rd May 1995, this concentration reached 71 ppb over 8 hours in Yorkshire, UK.” Many health experts believe this concentration of ozone to be carcinogenic, and periods of smog lasting for 5 days, such as in 1991, can have long lasting negative effects on health.
Although its composition varies greatly due to the nature of the primary and secondary pollutants, local geography, time of day, and weather conditions, a photochemical smog can still have devastating effects for those who breathe in the air. Healthy humans can suffer breathing difficulties and soreness in the eyes and nose; if vulnerable people such as the elderly, small children or people with respiratory problems such as asthmatics breathe in smoggy air, the effects can be much more dangerous.
Controlling Emissions
Sulphur compounds in coal originate from living organisms. When burned, these sulphur compounds turn into Sulphur Oxides (SOx). The amount of SOx produced varies according to the amounts of sulphur in the coal, and in the organisms that make it up. At Logannet, a coal with less than 1% sulphur is used; this is lower than other British coals. However, Logannet still produces 20 tonnes of SO2 every hour; this can have devastating effects on the environment.
One method of controlling emissions is to react the SOx in the flue gas with limestone, making calcium sulphate (CaSO4) which can be used in the construction industry:
Another method is the ‘seawater scrubbing process.’ This involves passing the flue gases through seawater with a slightly alkaline pH. The SO2 dissolves in the water, forming sulphite ions. These are oxidised during aeration, to form sulphate ions, which can be safely disposed at sea. The following diagram shows how the process works: (ref 3)
Flue gases also contain NOx, (mainly thermal NOx.) Many power stations including Logannet use ‘low NOx burners,’ which burn at a lower temperature, decreasing the production of NOx. Also, finely-ground coal can be controlled to give lower temperature burning, which gives significantly lower NOx emissions.
Using ‘gas reburn’ techniques, NOx formed are chemically turned into nitrogen with the addition of natural gas into the boilers, above the flame. NOx react to form nitrogen, carbon dioxide and water.
The Logannet management chose to implement these procedures to improve efficiency in the power station, to cut waste products, e.g. Wasted Sox can now be turned into useful building products and reduce pollutant release.
Research Into Photochemical Smog
Scientists have set up monitoring stations in over 20 locations nationwide, to monitor the concentration of ozone and NOx in the troposphere. Chemists measure the rate of formation of photochemical smog, and make predictions about pollution in the future. Using computer modelling, chemists can simulate the behaviour of pollutants.
Huge smog chambers (volume 1500m3 to minimise any surface effects) contain primary pollutants which are mixed and exposed to sunlight to measure the formation and concentration of photochemical smog.
References
Ref 1 :
Human and Natural History of the Thompson Okanagan Region of British Colombia, Atmosphere and Climate Section, ‘Photochemical Smog’
Ref 2 :
‘Chemical Storylines’ (2000). Ed : Burton, G., Holman, J. et al. 2nd Edition. Heinemann, Oxford. Pages 33 – 35.
Ref 3 :
Miller, Donald (2000) ‘Environmental Pressure’, Chemistry Review 9(4) Pages 3 – 6.
Ref 4 :
Pilling, M., Pilling, G., (1996) ‘What is photochemical smog’, Chemistry Review 5(5).
Ref 5 :
World Health Organisation Official Website