Glossary of Terms / Units Annual mean An annual mean is a mean which is calculated on a yearly basis, yielding one annual mean per calendar year. Annual means will be based on 366 days for leap years. Data capture  Gives the percentage of all the possible measurements for a given period that were validly measured. Diffusion tubes Tubes used to measure the amount of a particular pollutant in the atmosphere, usually oxides of nitrogen. Hourly mean An hourly is a mean calculated every hour. The hourly mean for a particular substance at a particular location for a particular hour is the mean of the levels recorded for that substance at that location during that hour. µg/m3 Microgrammes per cubic metre. A measure of concentration in terms of mass per unit volume. A concentration of 1µg/m3 means that one cubic metre of air contains one microgramme (millionth of a gramme) of pollutant. PM10 Particles of organic or inorganic substances present in the atmosphere that are less than 10 microns in diameter ppb Parts per billion. The concentraion of a pollutant in air in terms of volume ratio. A concentration of 1ppb means that for every billion (109) units of air, there is one unit of pollutant present. NAQS A national programme to monitor and manage air pollution levels and air quality in the UK. Published in spring 1997 following the Environment Act of 1995.