Beta attenuation monitoring (BAM) is a widely used air monitoring technique employing the

absorption Absorption may refer to: Chemistry and biology * Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which ...

of beta radiation by solid particles extracted from air flow. This technique allows for the detection of PM₁₀ and PM_2.5, which are monitored by most air pollution regulatory agencies. The main principle is based on a kind of Bouguer ( Lambert–Beer) law: the amount by which the flow of beta radiation (

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

s) is attenuated by a

solid matter Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structural ri ...

is exponentially dependent on its

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...

and not on any other feature (such as

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

, chemical composition or some

optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...

or electrical properties) of this matter.A. Liberti. Modern Methods for Air Pollution Monitoring. ''Pure and Applied Chemistry'' 1975, ''44''(''3''). – pp. 519–534. So, the air is drawn from outside of the detector through an "infinite" (cycling) ribbon made from some filtering material so that the particles are collected on it. There are two sources of beta radiation placed one before and one after the region where air flow passes through the ribbon leaving particles on it; and there are also two detectors on the opposite side of the ribbon, facing the detectors. The sources'

intensity Intensity may refer to: In colloquial use *Strength (disambiguation) *Amplitude * Level (disambiguation) * Magnitude (disambiguation) In physical sciences Physics *Intensity (physics), power per unit area (W/m2) *Field strength of electric, ma ...

and detectors' sensitivity being the same (or corrected with appropriate calibration lookup table), the intensity of beta rays detected by one of detectors is compared to that of the other. Thus one can deduce how much mass has the ribbon acquired upon being exposed to air flow; knowing the drain velocity, actual particle mass concentration in air could be assessed. The radiation source can be a gas chamber, filled with ⁸⁶ Kr gas, or a pieces of ¹⁴ C-rich polymer plastic, such as

PMMA PMMA may refer to: * para-Methoxymethamphetamine, a stimulant drug * Philippine Merchant Marine Academy The Philippine Merchant Marine Academy ( fil, Akademiya sa Bapor Pangkalakalan ng Pilipinas) also referred to by its acronym PMMA) is a ma ...

.''Compendium of Methods for the Determination of Inorganic Compounds in Ambient Air. Chapter IO-1. Continuous measurement of PM10 suspended particulate matter (SPM) in ambient air.'' EPA:Cincinnati, 1999. – 7 p.Enviro Technology Services. Product overview: OPSIS SM200 PM10 PM2.5 Sampler. Online: http://www.et.co.uk/products/air-quality-monitoring. Detector is simply a Geiger–Mueller counter. The particulate matter content measured is affected by the

moisture Moisture is the presence of a liquid, especially water, often in trace amounts. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some commercial products. Moisture also refers to the amount of water vapo ...

content in the air, unfortunately.''UK Equivalence Programme for Monitoring of Particulate Matter. Final report.'' Bureau Veritas: London, 2006. – 126 p. To discriminate between particle of different sizes (e. g., between PM₁₀ and PM_2.5), some preliminary separation could be accomplished, for example, by

cyclone In meteorology, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an anti ...

battery. A similar method exists, where instead of beta particle flow an X-ray Fluorescence Spectroscopic monitoring is applied on the either side of air flow contact with the ribbon. This allows to obtain not only cumulative measurement of particle mass, but also to detect their average chemical composition (technique works for

potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...

and elements heavier than it).Enviro Technology Services. Product overview: CES Xact 625 Monitoring System. Online: http://www.et.co.uk/products/air-quality-monitoring.

References

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Literature

* ''List of Designated Reference and Equivalent Methods''. EPA: Research Triangle park, 2013. Online: http://www.epa.gov/ttn/amtic/criteria.html . Air pollution Aerosols Detectors Measuring instruments Radioactivity Meteorological instrumentation and equipment