Source of Pollutants

Back to National Air Quality Standards

Where do Air Pollutants come from?

Different types of air pollutants come from different types of source.  In relation to the pollutants for which there are National Air Quality Standards the sources are described below.

Typical Sources of Air Pollutants

Carbon Monoxide

Carbon Monoxide is produced wherever there is incomplete combustion of organic material including fossil fuels. The main source of carbon monoxide in the United Kingdom is road transport, which accounted for 67% of total releases in 2000. Other important sources include large industrial processes, which emit significant quantities of the pollutant to atmosphere.

Benzene

There are no well defined natural sources of Benzene in the Northern Hemisphere. As a result, all ground level incidents of this pollutant in the United Kingdom are likely to be the result of human activities. The main sources of benzene emissions in the UK are petrol-engined vehicles, petrol refining, and the distribution and uncontrolled emissions from petrol station forecourts without vapour recovery systems. It is also important to recognise other sources of human exposure, some of which may have significant impact on individuals. Cigarette smoke for example, contains Benzene and can be a significant source both to smokers and those exposed to passive smoking.

Nitrogen Dioxide

Nitrogen Dioxide and Nitric Oxide are both oxides of nitrogen, and are collectively referred to as Nitrogen Oxides (NOx). All combustion processes produce NOx emissions, largely in the form of Nitric Oxide, which is then converted to Nitrogen Dioxide, mainly as a result of reaction with ozone in the atmosphere. It is Nitrogen Dioxide that is associated with adverse effects upon human health. The principal source of Nitrogen Oxides emissions is road transport, which accounted for about 49% of total UK emissions in 2000. Major roads carrying large volumes of high-speed traffic (such as motorways and other primary routes) are a predominant source, as are conurbation's and city centres with congested traffic.

Within most urban areas, the contribution of road transport to local emissions will be much greater than for the national picture. As an example, road transport is estimated to account for more than 75% of Nitrogen Oxides emissions in London.Other significant sources of Nitrogen Oxides emissions include the electricity supply industry and other industrial and commercial sectors, which accounted for about 24% and 23% respectively in 1999.

Particulate Matter (PM10)

There is no definition of chemical composition for atmospheric Particulate Matter, as it arises from a variety of constituents and sources. In order to assess atmospheric levels it is necessary to define what will be measured. The particulate definition chosen for the purposes of air quality monitoring is mass and, more specifically, particles smaller that 10 microns (1 micron = 1 millionth of a metre).  There is a wide range of emission sources that contribute to PM10 concentrations in the UK.  These sources can be usefully divided into 3 main categories.

• Primary particle emissions are derived directly from combustion sources, including road traffic, power generation, industrial processes etc...
• Secondary particles are formed by chemical reactions in the atmosphere, and comprise principally of sulphates and nitrates.
• Coarse particles comprise of emissions from a wide range of sources, including resuspended dusts from road traffic, construction works, mineral extraction processes, wind-blown dusts and soils, sea salt and biological particles.

Sulphur Dioxide

The main source of Sulphur Dioxide in the United Kingdom is power stations, which accounted for more than 71% of emissions in 2000. There are also significant emissions from other industrial combustion sources. Domestic sources now only account for 4% of emissions, but can be locally much more significant. Road transport currently accounts for less than 1% of emissions partly as a consequence of the introduction of ultra low sulphur fuels. For this reason, road transport is not considered a significant source unless exacerbated by otherwise high levels.

Lead

Leaded petrol was banned in the United Kingdom from the 1^st of January 2000. Emissions of Lead are now restricted to a variety of industrial activities, such as battery manufacture, pigments in paints and glazes, alloys, radiation shielding, tank lining and piping.

1,3-Butadiene

The main source of 1,3-Butadiene in the United Kingdom is emissions from motor vehicle exhausts. 1,3-Butadiene is also an important industrial chemical and is handled in bulk at a small number of industrial premises.

Ozone

While naturally occurring Ozone in the upper atmosphere, 'the ozone layer', protects the Earth, ground level ozone is harmful to health. Ozone is not directly emitted, but is formed by a complex set of reactions involving Nitrogen Oxides and Hydrocarbons in the presence of sunlight. In natural, unpolluted conditions, a cyclic balance is reached where sunlight breaks down Nitrogen Dioxide to form Ozone, which then reacts with Nitric Oxide to reform Nitrogen Dioxide. At night, when there is no sunlight to drive the first part of the cycle, Ozone is destroyed but not replaced.

In polluted conditions where the air contains Hydrocarbons from fuel combustion, the balance is upset. The Hydrocarbons also react to form Nitrogen Dioxide, which then breaks down into more ozone. As a result, both Nitrogen Dioxide and Ozone levels increase leading, in severe cases, to summertime 'smog'. As mentioned above, the majority of Nitrogen Oxides emitted from a vehicle exhaust are in the form of Nitric Oxide. As it is Nitric Oxide that destroys Ozone, Ozone concentrations are actually lower next to busy roads. For the same reason, Ozone annual means are higher in rural locations than in cities.

Current Air Quality Focus

Between 1986 and 2000 the number of cars on the road increased from 17.4 million to 26.7 million. This continuing increase in car ownership has led to a fundamental change in the travelling patterns of ordinary households: 82% of journeys by mileage are made by car. Today it has been estimated that 95% of all current air quality problems are being caused by road transport. Therefore road transport is set to remain the main focus of initiatives to improve local air quality.