unité COH

CONT: Certains paliers de gouvernement utilisent des sous-indices de visibilité et de particules inhalables à partir de mesures de la visibilité de 1 h et de 2 h en unités COH. Le Comité considère que cette méthode n'est pas aussi fiable que les mesures des concentrations en temps réel de particules inhalables.
OBS: Le COH («coefficient of haze unit») est le coefficient de transmission ou l'indice d'opacité (quantité de matières solides donnant une densité optique de 0,0l).

CONT: coefficient of haze (COH) (in atmospheric chemistry): One technique of measurement of the amount of filterable particulate matter suspended in air which has been used in the past depends upon drawing a measured sample of air (usually 1 000 linear feet) through a paper or membrane filter. A measurement is made of the intensity of light transmitted through the dust spot formed relative to that transmitted through an identical clean filter. The dirtiness of the air is reported in terms of the COH unit. This relates to the quantity of particulate material which produces an optical density, lg (I 0 /I) of 0.01 when measured by light transmission at 400 nm and relative to the transmission of an identical dust-free filter taken as 100%. Thus a filter which transmitted 50% relative to the clean filter has an absorbance of 0.301 or 30.1 COH units. This is not a recommended measure of filterable particulate matter since the size, colour and other properties of the aerosol and the air in which it is suspended affect the results. [Source: IUPAC Compendium of Chemical Terminology, 2nd Edition (1997).]