UV spectroscopy

Gas-filter correlation Filtered bandpass absorption Laser absorption Photoacoustic spectroscopy LIDAR

Diode-laser spectroscopy

A broadband source allows collection and analysis of a full IR spectrum. This method compares standard spectra with field readings to determine constituents and concentrations UV spectra are collected over a limited absorption spectral region. This method uses the differential absorptions of the compounds in the air to determine the identity and concentrations of contaminants A sample of the gas to be detected is used as a reference. This method compares the correlation between the spectrum of the sample gas to the gas in the measurement path to determine concentration This method measures absorption of the gas in certain bands to detect composition and concentration

This method uses one or more lasers to measure absorption at different wavelengths

The pressure change from the deactivation of excited molecules is measured in a closed acoustic chamber

This method measures molecular or aerosol backscatter by either differential absorption or Raman scattering to identify gases and determine their concentrations. Unlike the other methods listed, this method provides ranging information on measurements A developing technology for open-air use. This method scans a line feature of the gas spectrally to identify and quantify the compound

Source: G.E. Harris, M.R. Fuchs, and L.J. Holcombe, 1992, A guide to environmental testing, Chem. Eng. (November).

FIG. 5.13.2 Bistatic and unistatic systems. Bistatic systems (top), which place the IR source and receiver in different positions, require two power sources and precise alignment to record properly. Unistatic systems (bottom) are simpler to operate. Most installations use four separate bi- or unistatic source-receiver pairs—one for each side of a property. The next step may be to use one centrally located IR source on a tower, with retroreflectors around it.

Receiver Telescope

Spectral Analyzer

Pollution Gases


IR Source

Transmitter-Receiver Telescope

Spectral Analyzer

Pollution Gases

Retro Reflector Array

UV-DOAS systems are more selective. They have a lower limit of detection and fewer problems with interferences from water vapor and carbon dioxide. For the best features of both systems, these complimentary systems can be integrated.

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