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6.2.17 Scattering/absorption LIDAR

Use: Light detection and ranging (LIDAR) measurements of atmospheric trace gases have historically employed two basic techniques: elastic scattering differential absorption LIDAR (DIAL) and inelastic scattering Raman LIDAR.


Several techniques have been used to demonstrate the DIAL technique including wavelength modulation and FM spectroscopy. Wavelength modulation usually employs a low frequency dither (5 kHz) and derivative spectroscopy techniques to lock the laser to the molecular absorption line for the on-line measurement. An additional measurement is then made off-line. The ratio of on-line to off-line LIDAR measurements is used to infer the absorber concentration. FM spectroscopy utilizes a high frequency small signal modulation applied to the laser. The modulation frequency must be greater than the spectral width of the absorption feature in order to probe the molecular absorption line with one of the FM sidebands. As one sideband is tuned through an absorption line, a balanced receiver generates a beat frequency as the upper and lower sidebands become unbalanced. The quadrature component of the detected signal is proportional to the differential absorption. FM spectroscopy techniques using tunable diode lasers provide the most sensitive means to measure trace gas concentration using differential absorption LIDAR.

The Raman LIDAR technique involves detecting transmitted laser radiation which has been shifted in wavelength due to interaction with the scattering molecule. This wavelength shift is equal in energy to a vibrational-rotational or rotational transition in the scattering molecule. Thus, the primary advantage of Raman LIDAR compared to DIAL is that it offers a direct measure of species concentration or mixing ratio by comparing the Raman signal of the scatterer to the Raman signal of N2 or O2. However, Raman scattering is a very weak process and the signal can be two to four orders of magnitude weaker than the elastic backscattered signal


1. Non-Halogenated VOCs
3. Halogenated VOCs


Soil/Sediment Water Gas/Air
Requires extraction to a liquid or gas phase Requires extraction to a liquid or gas phase BETTER
Selectivity: Technique measures a part of the compound.
Susceptibility to Interference: High.
Detection Limits : 500+ ppm (soil); 100+ ppm (water).
Turnaround Time per Sample: Minutes.
Applicable To:
Screen/Identify Characterize Concentration/Extent Cleanup Performance Long-Term Monitoring
Quantitative Data Capability: Data become quantitative with additional effort.
Technology Status: Commercially available technology with limited field experience.
Certification/Verification: Technology has not participated in CalEPA certification and/or CSCT verification program.
Relative Cost per Analysis: Mid-range expense.


ASTM Standards/EPA Methods:

No applicable ASTM standards or EPA methods are cited for this technology.

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