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Air Pollution Emissions in the UK

Venhicle Emissions

The relationship between humans and vehicles probably began about 3000 BC with the invention of the wheel. The first linking of wheels with engines occurred in the 18^th century with the appearance of steam power. However, it was not until the invention of the internal combustion engine towards the end of the 19^th Century that a practical horseless carriage was invented. In the early 20^th Century the introduction of assembly line techniques put the motor car within reach of many more people. Motor vehicles now include petrol and diesel-engined cars, motorcycles, light vans, passenger service vehicles (buses and coaches) and heavy goods vehicles (HGVs).

There are currently about 24 million cars on the UK's roads. Petrol is the major fuel type used for cars and light vans, with diesel making up 14% (2001) and other fuel sources making up the remaining 1%. HGVs, light goods vehicles, motorcycles and passenger service vehicles make up around 5 million vehicles, making the total number of vehicles on the UK's roads approximately 29 million.

The transport related problems that we are experiencing now will be made worse by the projected increase in UK traffic. The Department for Transport's (DfT) 10 Year Plan projects an increase in vehicle traffic of 17% from 2000 to 2010 (compared to 21% without the Plan). Road traffic and hence vehicle pollution, tends to be greatest in urban areas. The impacts of exhaust emissions include acid deposition and air pollution, human health effects, global climate change and noise pollution. Since the 1950s, emissions from aircraft have been increasing, posing yet further problems for the environment, both local and global. Today, the growth in air transport is faster than that witnessed for land-based vehicles.

The motor vehicle engine emits many types of pollutants including nitrogen oxides (NO_x), volatile organic compounds (VOCs), carbon monoxide (CO), carbon dioxide (CO₂), particulates, sulphur dioxide (SO₂) and lead.

Emissions are related to use of the engine, mainly the fuel type and the temperature of combustion. If the engine is 100% efficient, then the products of combustion will be CO₂ and water (H₂O). However, at low loads engines are inefficient and therefore the products of incomplete combustion dominate, for example CO and VOCs in petrol engines and carbon monoxide, VOCs and smoke in diesels. As the temperature of combustion increases, the efficiency of conversion to CO₂ and water increases. However, impurities in the fuel such as nitrogen are oxidised to NO₂. At high temperatures atmospheric nitrogen (N₂) is also oxidised to NO₂, hence at higher loads and speeds, NO₂ production dominates.

Emission of VOCs from motor vehicles is a problem that accounts for 40% of anthropogenic (man-made) emissions in Western Europe. Most VOCs are emitted in the exhaust, although they also escape at other points within the fuelling chain. Evaporative losses can occur during filling, the so-called "fuelling loss". Losses can also occur from the engine when the car is being driven and when the engine is cooling down. VOCs are also released from the fuel tank as the temperature goes up and down during the day; this is called the "breathing loss" and is due to vapour evaporating from the petrol as the fuel gets hot.

Buses and Air Quality

Buses are generally recognised as an environmentally friendly form of transport, particularly in relation to the number of car journeys needed to carry the equivalent number of passengers. A bus uses less fuel per person carried and hence produces less pollution than the number of cars it replaces. However, buses do contribute to air quality problems, particularly in cities. Improvements in the emission performance of buses are likely to be needed in the future.

Buses in the UK are predominantly powered by diesel engines. Diesel engines work at a much higher level of compression than a petrol engine, allowing more efficient combustion of fuel within the engine. Therefore, diesels are more fuel-efficient than their petrol counterparts, resulting in lower emissions of CO₂. However, diesel engines emit larger volumes of oxides of nitrogen (NOx) than petrol engines and most importantly, far larger emissions of particulate matter and black smoke. The black smoke component of particulate matter is almost wholly due to diesel emissions and is responsible for the soiling of buildings. Fine particulate matter is also associated with visibility degradation and has been linked with a range of adverse health effects.

Aircraft Emissions

Air transport is one of the world's fastest growing energy use sectors. Most international travel is by air and domestic air travel in developed countries is expanding. Furthermore, whilst per capita demand for air transport is currently very low in poor populous countries, it has the potential to grow considerably. Whilst historically noise has been the major environmental issue associated with airports and aircraft, local and global effects of aircraft emissions on air quality are beginning to dominate the environmental agenda. Although technological advances are helping to reduce emissions, the continued growth in emissions is expected to rise in line with predicted growth in air traffic movements.

Emissions can arise from different modes of aircraft operation, namely idle, taxi, take-off, approach and landing. The mode of operation puts differing demands on the aircraft engines resulting in fluctuating pollution emissions. For example carbon monoxide and hydrocarbons, which arise from incomplete or poor combustion, are generally largest during taxi / idle operations. (Many hydrocarbons are odorous; the typical airport smell of unburned and partially burned kerosene is testament to this.) Emissions of NOx, however, are generated largely by the oxidation of atmospheric nitrogen in the combustion process. As such their production is proportional to the combustion temperature, and emissions of NOx are therefore at their highest during the take-off phase when the engine is generally producing maximum power. Emissions of carbon dioxide are directly related to the amount of fuel burned. During the landing phase the combustion is delivering some 30% power; at such a setting NOx is still an important pollutant, whilst CO and hydrocarbon emissions become increasingly important as the combustion thrust output falls.

Industrial Emissions

Industries contribute significantly to the air pollution problems in the UK. During the Industrial Revolution industries were often located in urban areas. Following the Clean Air Acts and with the decline in heavy industry, few large industries and power stations are located in towns and cities today. Many large industries are now located in the more rural areas of the UK. However, most urban areas have some smaller industries and possibly a power station. The larger industrial sources, even though located out of town, also have an impact on urban air quality.

Industrial Smoke Pollution

Smoke pollution can be defined as particulate material less than 15mm in diameter, which arises from the incomplete combustion of fuel. Estimates for emission of particulates which are 10 m in diameter or less (PM10) in the UK for 2001 identify that power stations account for 10% of UK emissions and other industries for 21%. The significant sources of smoke pollution are therefore not industries, but rather vehicles (24%), other sources such as construction, agriculture (24%) and domestic and commercial sources (21%).

In the past, urban air pollution was dominated by thick black smoke, which was emitted by industries and power plants burning coal. Levels of smoke in cities and towns during the wintertime in particular were at much higher concentrations than those measured today. In the early 1960s, winter smoke concentrations in Manchester averaged at more than 250mgm^-3. Today the typical urban annual mean for smoke is 10 - 40mgm^-3. The amount has decreased dramatically due to technical industrial air pollution control, the decline in the use of coal for domestic purposes and the general shift of power stations and industries from town and city centres to more rural locations.

Sulphur Dioxide Pollution

Industries are the major source of UK sulphur dioxide pollution. Power stations and all other types of industry account for 90% of all UK SO₂ pollution. Most industries and power stations are now located in rural areas, but urban areas often lie in the prevailing wind path of these industries.

Small industries located within urban areas may greatly affect local SO₂ levels as their emissions may become trapped by temperature inversions in the urban environment.

Sulphur dioxide pollution is one of the main pollutants that cause acid rain, when it combines with water in the atmosphere to form sulphuric acid. Industry is the main source of SO₂ in the UK and therefore also the main contributors to rainfall acidity. Acid rain affects urban areas by causing faster erosion of certain building materials and it can causes damage to urban vegetation.

Industrial Oxides of Nitrogen Pollution

Power stations contribute significantly to the total emissions of nitrogen oxides in the UK. In 2001, 23% of NOx came from this source and a further 13% from other industries, iron and steel and refineries. The major source of NOx pollution in the UK is road transport (50%).

Whilst the majority of NOx emissions arise from road transport, the contribution of industrial NOx pollution is still important. Nitrogen oxides are also converted into nitric acid when combined with water in the atmosphere, hence, like SO₂, contributing to acid rain.

Other Air Pollutants

Industries do not emit large quantities of the other urban air pollutants. The UK contribution of power stations and industrial sources in 2001 for carbon monoxide was 5% and for volatile organic compounds only 1%.

UK Emissions

The following table and graphs give recent information relating to the emissions of the following air pollutants: sulphur dioxide (SO₂), nitrogen oxide (NOx), particulate matter (PM₁₀), volatile organic compounds (VOCs) and carbon monoxide (CO) for 2001. In recent decades, emissions of air pollution have generally declined in the UK in response to the implementation of air quality regulations including the Environment Act of 1995, the European Union Framework Directive on Air Quality (1996) and the UK National Air Quality Strategy (1997).

Emissions and major sources of air pollutants in the UK, 2001