Aethalometer

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;[12] 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.

The Aethalometer Model AE-31 was tested by the Environmental Technology Verification Program administered by the U.S. Environmental Protection Agency, and a validation report was issued in 2001.[13] The Aethalometer Model AE-33 was tested under the same program in 2013, report pending.

The pollutant species black carbon appears gray or black due to the absorption of electromagnetic energy by partially mobile electrons in the graphitic microstructure of the black carbon particles. This absorption is purely ‘resistive’ and displays no resonant bands: consequently, the material appears gray rather than colored. The attenuation of light transmitted through a deposit of these particles increases linearly with the frequency of the electromagnetic radiation, i.e. inversely with respect to wavelength. Aethalometer measurements of optical attenuation on a filter deposit will increase at shorter wavelengths as λ^(-α) where the parameter α (the Angstrom exponent) has the value α = 1 for ‘gray’ or ‘black’ materials. However, other species may be co-mingled with the black carbon particles. Aromatic organic compounds associated with tobacco smoke and biomass smoke from wood-burning are known to have increased optical absorption at shorter wavelengths in the yellow, blue and near-ultraviolet portions of the spectrum.

Aethalometers are now constructed to perform their optical analyses simultaneously at multiple wavelengths, typically spanning the range from 370 nm (near-ultraviolet) to 950 nm (near-infrared). In the absence of aromatic components, the aethalometer data for black carbon concentration is identical at all wavelengths, after factoring in the standard λ⁻¹ response for ‘resistive’ gray materials. The angstrom exponent of the attenuation for these materials is 1. If aromatic components are present, they will contribute increased absorption at shorter wavelengths. The aethalometer data will increase at shorter wavelengths, and the apparent angstrom exponent will increase. Measurements of pure biomass smoke may show data represented by an angstrom exponent as large as 2. Due to different artifacts, the angstrom exponent measured by aethalometers might be biased but comparison with other techniques have found that the aethalometer model AE-31 provides fair absorption angstrom exponent results.[14] Many areas of the world are impacted by emissions both from high-temperature fossil fuel combustion, such as diesel exhaust, which has a gray or black color and is characterized by an angstrom exponent of 1; together with emissions from biomass burning such as wood smoke, which is characterized by a larger value of angstrom exponent. These two sources of pollution may have different geographic origins and temporal patterns, but maybe co-mingled at the point of measurement. Real-time aethalometer measurements at multiple wavelengths are claimed to separate these different contributions and can apportion the total impact to different categories of sources. This analysis is an essential input to the design of effective and acceptable public policy and regulation.

The accuracy, and even the ability, of the aethalometer to differentiate smoke sources is disputed.[15]

The aethalometer measurement principle is based upon air filtration, optics, and electronics. It does not require any physical or chemical support infrastructure such as high vacuum, high temperature, or specialized reagents or gases. Its only consumable is a filter which needs to be replaced every one or two days in portable models, but larger units have a roll of filtration tape which usually lasts from months to years. Consequently, the instrument is rugged, miniaturizable and may be deployed in research projects at remote locations, or at sites with minimal local support. Examples include:

• measurements at South Pole Station,[16] the location at which the cleanest air has been measured with an aethalometer, showing black carbon concentrations on the order of 30 picograms per cubic meter in winter;
• measurements in urban locations in China[17] and Bangladesh,[18] at which the concentrations of black carbon can often exceed 100 micrograms per cubic meter;
• measurements at rural locations in Africa, with installations operating from solar photovoltaic panels and registering high concentrations of black carbon due to agricultural burning;
• measurements at high-altitude installations in both the Indian Himalayas[19] and Tibet[20] at heights exceeding 5,000 metres (3.1 miles), operating from solar photovoltaic panels and registering the impact of combustion emissions from adjacent densely populated lowland areas;
• measurements on board commercial aircraft flights using a hand-held aethalometer, in which the in-cabin presence of black carbon is derived from the external concentrations in the stratosphere: in which manner, it is possible to map the dispersion of black carbon on a global scale at 10 km (6.2 mi) altitude without the need for extremely expensive dedicated research aircraft;
• measurements taken from automobiles, trains, light aircraft[21] and tethered balloons,[22] from which the real-time data may be converted to horizontal and vertical mapping;
• measurements at a station in the midst of the Taklimakan Desert of Central Asia,[23] a location almost as remote and inhospitable as the South Pole.
• measurements taken with a micro-aethalometer while cycling in traffic Bangalore, India.[24]
• measurements combined with heart rate and minute ventilation sensors to study inhalation exposure.[25]

Some measurements are available as Open Data:

• personal exposure measurements with microaethalometers from Belgium[26]