Topical Science
Photochemical Smog July 2018
We are enjoying an exceptional heat-wave in Ireland this month. However, these conditions are conducive to the formation of photochemical smog in urban areas. This is a type of air pollution, which has adverse effects on human health. Warm weather is welcome, but when it continues for extended periods, there are problems. At the moment, Ireland is suffering from a drought, because there has been little or no rain for several Contents.
weeks. This causes particular problems for farmers, as the grass is not growing, so there is a shortage of fodder. There is also a
Page 1: Introduction Page 2: Photochemical smog Page 3: The chemistry of smog
shortage of drinking water for cattle; dairy cows need to drink a lot. The public has been inconvenienced, due to restrictions in the water supply in some areas and there is a nation-wide ban on the use of garden hoses for such activities as watering lawns or washing cars. Hay fever sufferers have experienced an
Page 4: The case for electric vehicles in urban areas.
aggravation of their symptoms. In addition, there are gorse fires in many places. Finally, there is the problem of photochemical smog, which is the main focus in this issue of ‘Topical Science’
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What is Photochemical smog and how is it caused ? In city centres, in dry sunny weather, the burning of petrol and diesel fuel, by motor vehicles, produces photochemical smog.
The burning of fossil fuel, in general, is the principal cause of air pollution. The word ‘smog’ was coined to describe the mixture of smoke and fog, which is produced when fossil fuels are burned. There are two quite different types of smog, which are produced in different weather conditions. Classical Smog is usually associated with damp, cold, wintry weather. This type of smog occurs when solid fuel is burned, particularly coal with a high sulphur and ash content. The burning of this type of fuel, whether by industry, power stations, or in the domestic fire, gives rise to a mixture of fine water droplets, sulphur dioxide (SO2) and solid smoke particles. It was very common in London in the 1950s and caused thousands of deaths each winter.
By its very design, the internal combustion engine is a rather inefficient device for extracting energy from the burning of fuel. An engine is a device for converting heat into mechanical energy. The burning of the fuel produces heat. But the Second Law of Thermodynamics tells us that, even in theory, no engine can be 100% efficient, unless the exhaust is at the absolute zero of temperature. This is clearly impossible to achieve.
The introduction of smoke-free zones and the banning of certain types of solid fuel in cities have helped to alleviate this problem. 2
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The photochemistry of smog
On the previous page, the primary pollutants produced by the inefficient burning of motor fuel, have been described. In summary, they include, unburned hydrocarbons, carbon monoxide, oxides of nitrogen as well as carbon dioxide. There are also fine particles of soot produced, especially by diesel engines. This is known as particulate matter (PM) and is classified according to particle size. Here’s where the photochemistry comes in. Certain wavelengths of ultra violet light, present in sunlight, can split the NO2 molecules to give NO and atomic oxygen, which is highly reactive. This reacts with normal oxygen (O2) molecules to give ozone, a type of oxygen with three atoms in its molecules (O3). Now, we know that ozone in the stratosphere is good, because it filters out the harmful UV rays from the sun. This was described in the May 2018 issue of ‘Topical Science’. However, at ground level, ozone causes respiratory problems and is a pollutant. There are many other complex chemical reactions that occur in photochemical smog, but there is no space to describe them here. You can find a simple summary if you visit this website:
https://energyeducation.ca/encyclopedia/Photoche mical_smog#cite_note-1 3
The primary pollutants produced by motor vehicles react chemically, with the help of sunlight, to produce a cocktail of toxic pollutants, known collectively as ‘photochemical smog’. Among the secondary pollutants produced are ozone, aldehydes and a class of photochemical oxidants, known as PAN. The picture at above left shows photochemical smog in New York City. Source:
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The Case for Electric Vehichles in Cities. The chemistry of photochemical smog is now well understood by scientists. But the message does not seem to have got through to politicians, nor to the general public. Why are private cars, which burn fossil fuels, still allowed to drive in city centres? We have seen that the internal combustion engine is inherently inefficient. It wastes fuel, does not convert all of the heat into energy and causes pollution, leading to poor visibility and a mixture of toxic chemicals, which are a public health hazard. Traffic in cities moves at a much slower speed than on the motorways. At slow speeds, the engine is even more inefficient and because there are so many vehicles, the pollutants are more concentrated than in the countryside. In sunny weather, photochemical smog can be produced. If private motor vehicles were banned from city centres, the air quality would improve. The problem is, people need to go about their business and public transport is not always convenient. Perhaps the use of electric cars might be the answer? Range anxiety may be a deterrent for using electric vehicles on long journeys, but if there were sufficient charging points in city centre car parks, perhaps people could charge their cars there while at work, or doing their shopping? The use of hybrid vehicles may offer a compromise in the short term. At slow speeds, a hybrid car can function as an electric vehicle, while on the open road, at higher speed, it uses the petrol engine, which charges the battery while on the move. Estimates vary as to the number of deaths that can be attributed to air pollution, but in Ireland, it is certainly in excess of 1,000 deaths per year, perhaps as much as 2,000. It is at least ten times higher than the number of deaths caused directly by road traffic accidents. If people were aware of this, surely there would be a public outcry? It is time for us to think about urban air pollution and change the way we get around the city. Wouldn’t it be great if Dublin city centre could be a traffic-free zone, where pedestrians and cyclists could go about in safety? But for the moment, it seems we have to endure the silent killer of photochemical smog.
The Author Margaret Franklin is a retired chemistry lecturer She spent most of her career at Athlone Institute of Technology, in the Midlands of Ireland. For several years, Margaret wrote a column entitled ‘Topical Science’ for a local weekly newspaper. It is now published on-line as a monthly newsletter, covering science topics for the information of the general public. 4
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