In the production of photochemical smog, there are primary and secondary sources that contribute to its production.
In the production of photochemical smog, there are primary and secondary sources that contribute to its production. Primary sources are injected directly into the atmosphere, whereas secondary sources are formed in the atmosphere through chemical and photochemical reactions. As the graph shows, motor vehicles (the red and pink columns) contribute largely to most of the primary pollutants. The main pollutants though are Carbon Monoxide and oxides of nitrogen (Nox). Motor vehicles also contribute largely to the secondary pollutant Nitrogen Dioxide.
During the combustion of coal, primary pollutants are formed. Oxides of Nitrogen and Sulphur are formed because is there is both nitrogen and sulphur in coal – initially from the decomposed organisms. This becomes oxidised by burning. Oxides of Nitrogen are also created because there is such a high temperature of combustion, atmospheric nitrogen and oxygen form what is called thermal NOx (1).
Photochemical smog is a condition that develops when primary pollutants interact under the influence of sunlight (2) to produce secondary pollutants.
Several reactions lead to the production of the constituents of photochemical smog, taking place in the troposphere. Ozone is present in all levels of the atmosphere, even in unpolluted air. In unpolluted air, Nitrous Oxide reacts with the ozone, producing Nitrous Dioxide and Oxygen:
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03 + NO NO2 + O2 (a)
However, natural sunlight breaks down the Nitrous Dioxide when the wavelength is less than 435nm: NO2 + sunlight NO + O (b)
This atomic oxygen (from (a)) then reacts with the abundant oxygen (from (b)) to form ozone:
O2 + O O3 (c)
The ozone created and broken down soon reaches a steady equilibrium. However if this air becomes polluted with hydrocarbons and oxides of nitrogen, ozone levels increase. This is because Nitrogen oxide can become Nitrogen Dioxide without destroying Ozone. To demonstrate this let us represent the hydrocarbon with RCH3:
RCH3 + OH RCH2 + H20 (d)
RCH2 + O2 RCH2O2 (e)
The peroxy radical RCH2O2 then reacts with NO to form NO2 (3):
RCH2O2 + NO RCH20 + NO2 (f)
As this converts NO into NO2, there is now a method that does not involve breaking up ozone. This causes ozone concentration to rise. In reaction (d) there is an OH group that is essential to the breaking down of hydrocarbons – they remove hydrogen from compounds to form water. It is formed when ozone absorbs light and breaks down into oxygen and a high energy oxygen atom (O*): 03 + sunlight O2 + O* (g)
The high energy atom can react with water to produce the OH groups:
O* + H2O 2OH (h)
Each high energy atom produces 2 OH groups. If these OH groups then follow the second process to form NO2 (reactions (d), (e) then (f)), there will be double the amount of oxides of nitrogen in the atmosphere. If the NO2 then gets destroyed by sunlight (reaction (b)), it can lead to the creation of ozone (reaction (c)). As there is now twice as much NO2, then twice as much ozone can be produced. This means for every ozone molecule that absorbs sunlight and is broken down, two more ozone molecule can be produced. This all happens because there is an increase of hydrocarbons and oxides of nitrogen in the atmosphere, and as ozone can lasts for copious days and travels the breadth of countries, gaining impetus whenever pollutants are added, the concentration is continually rising. This whole process is highly dependant on a number of factors. There must be lots of sunlight present, as this breaks up both Nitrogen Dioxide and Ozone. Weather factors can alter the concentration of the smog. The time of day and season are important. Research shows that during the middle of the day and summer, smog concentration is higher. There must also be a light wind. Geographical aspect can also affect the intensity of the smog. If the is a large plane in between two high areas of land, concentration will be higher as the smog will get “caught” there.
To lower sulphur dioxide emissions, Longannet have chosen to use a process called the sea water scrubbing process. In this process, the flue gases are passed through sea water. This causes the sulphur dioxide to dissolve the naturally alkaline solution and form sulphite ions. The water is then aerated, causing the sulphite to be oxidised into less harmful sulphate, and then disposed of into the sea. The slight pH change caused by this process becomes negligible as it is placed back into the sea. To reduce the amount of Nitrogen oxides produced, Longannet have adopted the new technique of gas reburn. The boiler is split into three boiling zones. In the primary combustion zone (which is the hottest and has lowest air concentration), the powdered coal is oxidised in less air than normal, so combustion rate is lowered leading to less NOx formed. The reburning zone causes the NOx that is formed to be reduced onto Nitrogen because it reacts with the alkanes:
CH4 + 4NO 2N2 + CO2 + 2H20
If there are any excess products, such as alkanes or carbon monoxide, then the cooler burning zone oxidises them. This process is also useful as the oxidation of the natural gases is exothermic which contributes to the generation of electricity. The management of Longannet choose these approaches because they suit their BPEO’s. The sea water scrubbing process is used to control Sulphur Dioxide emissions because there is a supply of sea water readily available from the banks of Firth of Forth. Also, this process creates no solid wastes or by products which have to be disposed of. Longannet has adopted the reburn technique because this is the cleanest and environmentally acceptable.
Chemists are doing their utmost to research into photochemical smog. They are monitoring the pollutants of the troposphere, studying the reactions leading to ozone creation in the laboratory, modelling the reactions and simulating a smog chamber.
(1) – Article 1
(3) – Article 2
Microsoft Encarta 2000
Article 1 and 2
“process of photochemical smog”
Electricity and Thermal Physics