Energy and Emissions: Data Definitions and Caveats

The emissions for which data are provided are oxides of nitrogen, particulates and carbon dioxide. The measures of fuel used vary between countries, but generally cover tonnes of petrol and diesel used, and total energy used as tonnes of oil equivalent.

The main difference between source and end user emissions comes from the treatment of emissions from combustion of fossil fuels, the largest source of carbon dioxide in most countries. To derive emissions by end user, emissions from power stations and other fuel processing industries have been re-allocated to end users on an approximate basis according to their use of the fuel. Emissions by end user are subject to more uncertainty than emissions by source and should only be used to give a broad indication of emissions by sector.

Non-transport end users include industry, residential, commercial, and agriculture. Exports are emissions arising from the production of fuel which is then exported (including fuel for international marine and aviation bunkers).

Data is far from complete, and some countries provide no information at all.

Conversion factors:

Motor spirit: One tonne = 298 gallons or 1,354 litres provides 47.0 GJ energy

Diesel: One tonne = 265 gallons or 1,203 litres provides 45.7 GJ energy

1 tonne of oil equivalent (toe)	= 107 kilocalories
	= 396.83 therms
	= 41.868 GJ
	= 11,630 kilowatt hours

Carbon emissions

Motor spirit: 2.344 kg CO2 per litre fuel

Diesel fuel: 2.682 kg CO2 per litre fuel

Carbon monoxide:

Derived from the incomplete combustion of fuels containing carbon. It is one of the most directly toxic of substances and interferes with respiratory biochemistry and can affect the central nervous and cardiovascular systems.

NOx (Oxides of Nitrogen):

A number of nitrogen compounds including nitrogen dioxide and nitric oxide formed in the combustion of fossil fuel. Nitrogen dioxide is directly harmful to human health causing respiratory problems and can reduce lung function. Nitrogen oxides contribute to the formation of ozone, which is a harmful secondary pollutant in the lower atmosphere and also an important greenhouse gas contributing to global warming. High levels of ozone increase susceptibility to respiratory disease and irritate the eyes, nose, throat and respiratory system. Oxides of nitrogen can have adverse effects on plants, reducing growth and can contribute to acid rain. The quantity of nitrogen oxides is expressed as nitrogen dioxide equivalent.

Lead:

Of concern because of its effects on health, particularly that of children. The main sources of lead in air stem from lead in leaded petrol, coal combustion, and metal works. The maximum amount of lead permitted in petrol was progressively reduced from 0.45 grams per litre before 1981. The general sale of leaded petrol in the United Kingdom was banned at the end of 1999.

Sulphur dioxide:

An acid gas, sulphur dioxide can affect health and vegetation. It affects the lining of the nose, throat and airways of the lung, in particular among those who suffer from asthma and chronic lung disease. The United Nations Economic Commission for Europe's (UNECE) Second Sulphur Protocol sets reduction targets for total SO2 emissions from a 1980 baseline of 50% by the year 2000, 70% by 2005 and 80% by 2010. Road transport emissions in UK have fallen by over 87% since 1998, following a reduction in the sulphur content of fuel.

Hydrocarbons or Volatile Organic Compounds (VOCs):

Consist of a large number of compounds including hydrocarbons and oxygenated organics but excluding methane, which are released from vehicle exhausts, oil refining, petrol distribution, various industrial processes and from solvents such as those used in paint. Some hydrocarbons are carcinogenic, others cause unpleasant effects such as eye irritations and coughing. VOCs are also, together with nitrogen oxides, involved in the formation of ozone in the lower atmosphere. In UK, the figures for emission from road transport include evaporation of motor spirit during production, storage and distribution, and evaporative emissions from the petrol tanks and carburetors of petrol-engined vehicles. From 2005, volatile organic compounds have been removed from the list of air pollutants published by the British Department for Transport, and benzene and 1,3-butadiene added.

Particulates (PM10):

Emitted by the exhausts of motor vehicles, particularly those with diesel engines, and produced as a result of tyre and brake wear. Airborne particles may be measured in a number of ways. For quantifying the particles produced by transport (especially motor traffic), the most commonly used indicator relies on the use of a size-selective sampler which collects smaller particles preferentially, collecting more than 95% of 5um (0.005 mm) particles, 50% of 10um aerodynamic particles, and less than 5% of 20um particles. The resultant mass of material is known as PM10.

A number of naturally occurring gases including water vapour, carbon dioxide, nitrous oxide and methane, that increase the effectiveness of the Earth's atmosphere at trapping the heat it receives as radiation from the Sun. The concentrations of some of these have increased since 1750 as a result of human activities, and in addition, new industrial gases such as CFCs (chlorofluorocarbons) have been released into the atmosphere. These increased concentrations and additional gases have increased the amount of heat trapped by the Earth's atmosphere.

Carbon dioxide:

The most important greenhouse gas and estimated to account for about two thirds of anthropogenic global warming. Although its global warming potential is much less per tonne than the other greenhouse gases, it is present in the atmosphere in much greater quantities. Once emitted, carbon dioxide persists in the atmosphere for 50 - 200 years. Its concentration in the atmosphere has increased by 33% since 1750.

Carbon dioxide emissions can be measured either in terms of the weight of carbon emitted, or as the weight of carbon dioxide emitted. To convert the figures, 44/12 (3.667) tonnes of carbon dioxide contain 1 tonne of carbon.

Methane:

A gaseous hydrocarbon, CH 4, that remains in the atmosphere for approximately 9 - 15 years. Methane is over 20 times more effective in trapping heat in the atmosphere than carbon dioxide. It is mainly produced through anaerobic decomposition of organic matter in biological systems. Agricultural processes such as wetland rice cultivation, the raising of livestock and the decomposition of animal wastes emit methane, as does the decomposition of municipal solid wastes in land fill sites. Methane is emitted during the production and distribution of natural gas and petroleum, and is released as a by-product of coal mining and incomplete fossil fuel combustion.

Methane is a primary constituent of natural gas and an important energy source. Atmospheric concentrations of methane have increased by about 150% since 1750, although the rate of increase has been declining. The IPCC has estimated that slightly more than half of the current CH 4 flux to the atmosphere is anthropogenic.

Nitrous Oxide:

Anthropogenic sources of N2O emissions include agricultural soils, especially the production of nitrogen-fixing crops and forages, the use of fertilizers and manure deposition by livestock; fossil fuel combustion, especially from mobile sources; nylon and nitric acid production; wastewater treatment and waste combustion; and biomass burning. The atmospheric concentration of N2O has increased by 17% since 1750.

Energy Consumption by Transport:

This is the energy content of fuels delivered to consumers or users. For electricity, it does not include the energy lost in generation and distribution. Detailed data for individual fuels are converted from original units to tonnes of oil equivalent using gross calorific values and conversion factors appropriate to each category of fuel: 1 tonne of oil equivalent (toe) = 107 kilocalories = 396.8 therms= 41.87 gigajoules (GJ) = 11.63 megawatt hours (MWh). This unit should be regarded as a measure of energy content rather than a physical quantity. There is no intention to represent an actual physical tonne of oil, and actual tonnes of oil will normally have measurements in tonnes of oil equivalent which differ from unity.

Energy used and carbon dioxide emissions can be quoted on the basis of source or end user. The main difference between source and end user emissions comes from the treatment of emissions from the combustion of fossil fuels, the largest source of carbon dioxide in the UK. To derive emissions by end user, emissions from power stations and other fuel processing industries have to be re-allocated to end users on an approximate basis according to their use of the fuel. Emissions by end user are subject to more uncertainty than emissions by source and should only be used to give a broad indication of emissions by sector.

Petroleum

Motor spirit: One tonne = 300 gallons or 1,362 litres Calorific value 47.1 GJ per tonne

Diesel: One tonne = 265 gallons or 1,203 litres Calorific value 45.6 GJ per tonne