by its effect on the resonant frequency of a vibrating support; beta attenuation monitors, which measure the increasing amount of material on a filter by its absorption of electrons emitted by a weak beta-source; and optical monitors, which can gauge the size of individual particles from signals scattered from a light beam and integrate this into a total volume of particles.
Some instruments give different results from the reference method to varying degrees in differing circumstances, and have many variations. New technologies which give ever more accurate results are being investigated by NPL, and other measurement laboratories. A variation of the TEOM called the Filter Dynamic Measurement System (FDMS) is currently receiving a great deal of attention in the UK.
Environmental measurement in the real world In addition to the various commercial services offered by NPL, we are closely involved in a number of UK and European projects to monitor air quality.
Air quality refers to the levels of pollutants in air that are relevant to human health or ecosystems. The most important pollutants have changed over recent decades. Historically, coal burning in towns led to high levels of sulphur dioxide (and particles – see below), but this pollutant is now only a localised problem. The most important gaseous pollutants for human health effects are currently ozone and nitrogen dioxide. Various government initiatives exist to monitor and control levels of these pollutants or their precursors. For example NPL manages four of Defra’s Air Quality Networks. These Networks are based on aspects of particle measurement.
Of all the particulate measurement techniques, black smoke appears to be the one most consistently associated with health effects. The Black Smoke Network provides a valuable historical dataset on the impacts of the Clean Air Acts. It involves the measurement of carbon containing particulate matter from the burning of solid fuel and vehicle emissions. Black smoke sampling uses an eight-port sampler to draw air at a constant flow rate through a paper filter. Suspended particulate matter is collected on the filter forming a stain, the darkness of which is measured by a reflectometer.
Another network is the Particle Counting and Speciation Network, which was established to research the nature of particulate matter in ambient air. Particulates are known to exacerbate symptoms of cardiovascular and respiratory diseases, and are also known to contribute to climate change. Particle number has also been associated with health effects.
The Network provides information on the chemical composition of particulate matter, thereby providing information on sources and allowing the relevant chemical processes to be modelled more reliably. It also
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generates reliable datasets of airborne particle number, concentration and size at selected sites. Long time series measurements of particle number are important for studies into the cause of disease.
The Heavy Metals Network measures ambient levels of arsenic, cadmium, chromium, copper, iron, manganese, nickel, lead, platinum, vanadium, zinc, and mercury, in particulate matter, at twenty-four sites around the UK. Accurately measured concentrations of the most toxic metals are requirements of the new EC Air Quality Directive and the Fourth Air Quality Daughter Directives. Sites are predominantly located downwind of large metal processing facilities and in areas with a high population.
NPL has recently taken over management of the PAHs ( Polycyclic aromatic hydrocarbons) Network. (PAHs) are a group of persistent organic compounds, some of which are toxic and/or possible or proven human carcinogens; they are produced via incomplete combustion of carbon containing fuels especially coal.
NPL is also playing a major role in the new EU co-funded project AirMonTech, which will make recommendations to the European Commission on how to revise legislation covering air quality in Europe, and on what research is needed to help with this decision.
NPL, through its various projects, is constantly striving to deliver benefit to the government and customers through detailed understanding of air quality. Air quality monitoring techniques and strategies lead to improved legislation, reduced environmental impact and better health. But the process is continuingly evolving. New technology and greater understanding of measurement methods will lead to ever more accurate and consistent air quality measurements across the UK and Europe, and will guide decisions from healthcare to climate change mitigation in future. NPL, along with its partners and other measurement institutes across the globe, is continually working to ensure better air quality through measurement.
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