Spelthorne Borough Council

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Ashford, Laleham, Shepperton, Staines, Stanwell & Sunbury

Air quality - glossary of terms

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Further information on the eight air pollutants identified by the national air quality strategy.
Bias adjusted It is widely acknowledged that diffusion tubes have a tendency to over or under read, sometimes by a significant amount. Inaccuracies in the region of +/-20-30% are not uncommon in diffusion tubes. In addition to this inaccuracies can vary between laboratories that analyse diffusion tubes and even between different individual technicians in the same laboratory. Therefore, to produce accurate data a ‘bias adjustment’ of the raw data is often required. This is done through co-location with other tubes or with an continuous monitoring station.
Background A diffusion tube monitoring position typically in a residential housing area at least 50 metres in any direction from any busy road with continuous moving traffic.
Continuous monitoring station These measure levels of pollutants each second.
% Data capture The percentage of maximum available data that was collected
Diffusion tube Passive gas collection devices consisting of a small tube (7cmx1cm) containing a chemical absorbent. After exposure, theabsorbed gas is determined by chemical analysis. Diffusion tubes (samplers) are used to determine relatively long period average concentrations, typically weekly, fortnightly or monthly.
Kerbside A diffusion tube monitoring position where a tube is located on or close to a pavement of a busy road where people walk. Typically a worst-case position where highest levels of pollutants are likely to be found.
NAQS National air quality strategy
NO A chemical compound formed during the combustion of compounds containing nitrogen (N), from any high-temperature combustion in the presence of air, and by secondary processes in the atmosphere. A component of oxides of nitrogen. Nitric oxide is the predominant oxide of nitrogen produced in the combustion process.
NO2 Nitrogen dioxide is calculated from direct measurements of NOx and NO.
NOx A group of chemical compounds which may be interconverted in the atmosphere. The principal oxides of nitrogen are nitric oxide and nitrogen dioxide.
PM10 Particulate matter consists of very small liquid and solid particles floating in the air. These particles are less than 10 microns in diameter - about 1/7th the thickness of a human hair.
PM2.5 Particulate matter consists of very small liquid and solid particles floating in the air. Increasingly, attention has been turning towards monitoring the smaller particle fraction which is capable of penetrating deepest into the lungs. These particles are less than 2.5 microns in diameter
Roadside A site sampling between 1m of the kerbside of a busy road and the back of the pavement. Typically this will be within 5m of the road, but could be up to 15m.
Sensitive location A location (or receptor) where people might be subject to a change in air quality. In this context people would include susceptible people in locations such as schools and hospitals.
Urban Located in an area representative of typical population exposure in town or city centres.
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