Aethalometer

An aethalometer is an instrument for measuring the concentration of optically absorbing (‘black’) suspended particulates in a gas colloid stream; commonly visualized as smoke or haze, often seen in ambient air under polluted conditions. The word aethalometer is derived from the Classical Greek verb ‘aethaloun’, meaning ‘to blacken with soot’.

The gas stream (frequently ambient air) passes through a filter material which traps the suspended particulates, creating a deposit of increasing density. A light beam projected through the deposit is attenuated by those particles which are absorbing (‘black’) rather than scattering (‘white’). Measurements are made at successive regular time intervals. The increase in attenuation from one measurement to the next is proportional to the increase in the density of optically absorbing material on the filter: which, in turn, is proportional to the concentration of the material in the sampled air stream. The sample is collected as a spot on a roll of filter tape. When the density of the deposit spot reaches a pre-set limit, the tape advances to a fresh spot and the measurements continue. Measurement of the sample gas flow rate and knowledge of the instrument’s optical and mechanical characteristics permit a calculation of the average concentration of absorbing particles in the gas stream during the sampling period. Aethalometers may operate on time-base periods as rapid as 1 second, providing quasi-real-time data. Comparison of aethalometer data with other physical and chemical analyses allows the output to be expressed as a concentration of black carbon.

The Aethalometer principle is based upon the continuous filter-tape sampler developed in the 1950s for the measurement of coefficient of haze. This instrument drew the sample air stream through a filter tape spot for a fixed time duration (usually 1 or 2 hours). The tape was advanced and its gray coloration measured optically by either transmittance or reflectance. However, the data units were arbitrary, and were not interpreted in terms of a mass concentration of a defined material in the air stream until retrospective studies linked the ‘COH unit’ to quantitative analyses of atmospheric trace constituents.

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