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’. 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 timebase 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. Work in the 1970s at Tihomir Novakov's lab at the Lawrence Berkeley National Laboratory established the quantitative relationship between the optical attenuation of a deposit of particles on a fibrous filter, and the carbon content of that deposit. Improvements in optical and electronic technology permitted the measurement of very small increases in attenuation, such as would occur during the passage of typical ambient air through a filter on a 5- or 10-minute timebase. The development of personal computers and analog-digital interfaces permitted the real-time calculation of data, and mathematical conversion of the signals to a concentration of black carbon expressed in units of nanograms or micrograms of Black Carbon per cubic meter of air. The first-ever Aethalometer was developed at the Lawrence Berkeley National Laboratory by Anthony D. A. Hansen (who would later found Magee Scientific), Hal Rosen and Tihomir Novakov, and was utilised in an EPA visibility study at Houston in September 1980, with the first real-time data chart of black carbon concentrations in ambient air published in 1981. The instrument was first flown on board a NOAA research aircraft in the Arctic in 1984, and coupled with previous ground-level work showed that the Arctic haze contains a strong component of soot. The aethalometer was commercialized in 1986 and an improved version patented in 1988. Its earliest uses were in geophysical research at remote locations, using black carbon as a tracer of the long-range transport of air pollution from industrialized source areas to remote receptor regions. In the 1990s, increasing concerns about the health effects of diesel exhaust particulates led to increasing need for measurements using the blackness of the carbon content as an indicator. In the 2000s, increasing interest in the role that optically absorbing particles play in climate change led to expanded measurement programs in both developed and developing countries. The effect of these particles is believed to contribute to the accelerated melting of the Arctic and the thawing of glaciers in the Himalayas. A comprehensive summary of black carbon (including a review of aethalometer data) was submitted to the U.S. Congress by the U.S. Environmental Protection Agency in 2012. The Aethalometer has been developed into rack-mounted instruments for use in stationary air quality monitoring installations; transportable instruments which are often used at off-grid locations, operating from batteries or photovoltaic panels in order to make measurements at remote locations; and hand-held portable versions for measurements of personal exposure to combustion emissions. The main uses of aethalometers relate to air quality measurements, with the data being used for studies of the impact of air pollution on public health; climate change; and visibility. Other uses include measurements of the emission of black carbon from combustion sources such as vehicles; industrial processes; and biomass burning, both in wild fires and in domestic and industrial settings.