Lidar absorption

LIDAR An instrument that uses a laser-radar to study the concentration and location of particulate matter by the reflection or absorption of a laser beam. 

Flame Photometry and Gas Chromatography (CyTerra) -Aerodynamic Particle Size and Shape Analysis (BIRAL) -Flow Cytometry (Luminex, LLNL) -Semiconductor-Based Ultraviolet Light (DARPA) -Polymer Fluorochrome (Echo Technology) -Laser-Induced Breakdown Spectroscopy -Raman Scattering -Infrared Absorption -Terahertz Spectroscopy -UV LIDAR... 

An example of practical importance in atmospheric physics is the inference of effective optical constants for atmospheric aerosols composed of various kinds of particles and the subsequent use of these optical constants in other ways. One might infer effective n and k from measurements—made either in the laboratory or remotely by, for example, using bistatic lidar—of angular scattering fitting the experimental data with Mie theory would give effective optical constants. But how effectual would they be Would they have more than a limited applicability Would they be more than merely consistent with an experiment of limited scope It is by no means certain that they would lead to correct calculations of extinction or backscattering or absorption. We shall return to these questions in Section 14.2. 

Either IR or mass spectrometry may be used for individual hydrocarbon determination. Laser-induced Doppler absorption radar (LIDAR) can be used to remotely measure chemical concentrations in the atmosphere. Two different laser wavelengths are selected so that the molecule of interest absorbs one of the wavelengths, whereas the other wavelength is selected to be in a region of minimal interference. The difference in intensity of the two returned laser signals is then used to determine the concentration (see Figure 3.16, which illustrates the concept for IR signals). 

In addition to the IR, Raman and LIBS methods previously discussed, a number of other laser-based methods for explosives detection have been developed over the years. The following section briefly describes the ultraviolet and visible (UV/vis) absorption spectra of EM and discusses the techniques of laser desorption (LD), PF with detection through resonance-enhanced multiphoton ionization (REMPI) or laser-induced fluorescence (LIF), photoacoustic spectroscopy (PAS), variations on the light ranging and detecting (LIDAR) method, and photoluminescence. Table 2 summarizes the LODs of several explosive-related compounds (ERC) and EM obtained by the techniques described in this section. 

Simonson et al. [148] demonstrated remote detection of explosives in soil by combining distributed sensor particles with UV/vis fluorescence LIDAR technology. The key to this approach is that the fluorescence emission spectrum of the distributed particles is strongly affected by absorption of nitroaromatic explosives from the surrounding environment. Remote sensing of the fluorescence quenching by TNT or DNT is achieved by fluorescence LIDAR - the emission spectra were excited in field LIDAR measurements by a frequency-tripled Nd YAG laser at 355 nm and the fluorescence collected with a telescope and various detector systems housed in a 10 x 50 trailer. TNT has been detected in the ppm range at a standoff distance of 0.5 km with this system (Fig. 16). An important limitation to this technique is the pre-concentration of the explosives on the sensor particles, which requires the presence of water to facilitate the transport of the explosive from the surface of the soil particles to the sensor particles. 

Measurements of the concentration of OH radicals in the stratosphere in the altitude range 34—37 km have been made by balloon-borne LIDAR. The first post-sunset concentration levels were reported and comparisons were made with current 1-dimensional models. The effects of reduced absorption cross-sections for O2 in the Herzberg continuum on the composition of the stratosphere have been examined by Brasseur et ai., " following the suggestion that laboratory values for the cross-sections might be overestimated by as much as 30—50%. A model for the circulation of atmospheric CO has been described by Pinto et and which includes photochemical production and... 

---