e-Digest Statistics about: Air Quality
Definitions
- 1, 3-butadiene
- a VOC and suspected human carcinogen
- Acidity
- is expressed on pH scale of acidity/alkalinity. A neutral substance has pH 7.0 and the lower the pH the greater the acidity.
- Ammonia
- NH3 contributes towards acidification
- Annual Mean
- average of the concentrations measured for each pollutant for one year. Usually this is for a calendar year, but some species are reported for the period April to March, known as a pollution year. This period avoids splitting a winter season between 2 years, which is useful for pollutants that have higher concentrations during the winter months.
- Benzene
- a VOC and known human carcinogen
- Black smoke
- fine suspended particulate air pollutants arising from incomplete combustion of fossil fuels. Concentrations of smoke in the air are measured by determining their soiling capacity; a calibration curve is then used to give concentrations in terms of "equivalent standard smoke" per cubic metre of air.
- Carbon monoxide
- CO, a colourless odourless gas produced by the incomplete combustion of carbon. It can accumulate in the blood with toxic effects though the effect produced depends on the length of exposure.
- Carcinogen
- a substance which causes cancer.
- Congener
- a substance of the same kind or category as another.
- Cracking
- the refinery process of chemically breaking up the heavier fractions of petroleum to form lighter, more volatile compounds which can be used in petrol, etc.
- Data capture
- gives the percentage of all the possible measurements for a given period that were validly measured.
- Dioxin
- a powerful poison which is released during the production of the disinfectant, hexachlorophene.
- Furans
- One of a group of colourless, volatile, organic compounds obtained from wood oils and used in the synthesis of furfural and other organic compounds.
- Ecosystem
- an ecological system composed of inter-linked species and their physical resources.
- exceedance
- a period of time where the concentration of a pollutant is greater than, or equal to, the appropriate air quality standard.
- Days with exceedances
- number of days in which at least one period has a concentration greater than, or equal to, the relevant air quality standard (the averaging period will be that defined by that standard). Since the national air quality standards cover different time periods (15 min average, 24 hour running mean etc.) this gives a useful way of comparing data for different pollutants
- Hydrocarbon
- a chemical compound of hydrogen and carbon. The basic compounds in fossil fuels.
- Hydrogen chloride
- HCl is an acid gas.
- Hydrogen fluoride
- HF is an acid gas.
- Lead
- Pb, a metallic element which, in the form of its compounds, can be retained in the human body with clinical and sub-clinical effects. Lead in the air arises mainly from anti-knock additives to petrol.
- Nitrogen oxides
- there are a number of nitrogen oxides of different composition, emissions normally being expressed in terms of nitrogen dioxide equivalent. Nitrogen dioxide (NO2) is a brown toxic gas which is involved in photochemical reactions with other air pollutants to give irritant products.
- Ozone
- O3, a strong pungent blue gas, which is an irritant to eyes, nose and throat and toxic by inhalation. It is formed in the upper atmosphere by ultraviolet light and is produced by photochemical reactions involving volatile organic compounds and nitrogen oxides in the lower atmosphere. It contributes to photochemical smog and is toxic to life.
- Particulate
- a fine solid (or liquid) particle found in air or emissions, such as dust, smoke or smog.
- Percentile
- a value that is the rank at a particular point in a collection of data. For instance, a 98th percentile of values for a year is the value that 98 per cent of all the data in the year fall below, or equal.
- PM10
- the resulting mass of particles collected by a size-selective sampler which selects 50 per cent of particles 10 µm (10 thousands of a millimetre) in diameter, more than 95 per cent of particles 5µm in diameter, and less than 5 per cent of particles 20µm in diameter.
- Running mean
- a mean - or series of means - calculated for overlapping time periods, and is used in the calculation of several of the national air quality standards. For instance, an 8 hour running mean is calculated every hour, and averages the values for eight hours. The period of averaging is stepped forward by one hour for each value, so running mean values are given for the periods 00:00-07:59, 01:00-08:59 etc.
- Sulphur dioxide
- SO2, a colourless gas with a choking smell, the main product of the combustion of sulphur contained in fuels. Globally, much of the sulphur dioxide in the atmosphere comes from natural sources, but in highly developed and heavily populated regions the greater part comes from combustion of sulphur-containing fossil fuels (coal and oil).
- Volatile organic compounds (VOCs)
- consist of a large number of compounds including hydrocarbons and oxygenated and halogenated organics.
Further Information:
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Page last modified: 16 September 2003
Page published: 10 September 2003