The mobile laboratory will mostly carry on equipments based on optical principles in detecting the nature and quantity of pollutants (water state, gas state, solid state) from the surrounding air, water and soil. The devices used are based on one or several optical phenomena tacking place when light interacts with matter, phenomena considered to be dominant, any other kind of interaction is taken as a side effect and eliminated.
The devices used on the mobile laboratory use one of the following phenomena:
Mie/Raman backscattering: the emitted radiation is scattered (on the aerosols- elastic scattering, on water vapors- inelastic scattering) and the backscattered radiation received is detected and analyzed. The signal (coming from hundreds, thousands of meters away) contains information about the particle’s concentration and optical properties. Using this method, aerosol optical profiles can be retrieved in real time.
Differential UV-VIS absorption: For most substances, the amount of light absorption varies with the wavelength of the light depending on the spectral signature of that substance. Knowing the initial spectra of the light emitted and investigating the light emerging from the medium of interest, information about types and concentrations of different molecules can be extracted. Using this method, several gas types can be determined in a single measurement.
fluorescence (LIF) is a spectroscopic method used for studying structure
of molecules, detection of selective species, flow visualization and measurements.
The species to be examined is excited with a laser. The wavelength is often selected to be the one at which the species has its largest cross section. The excited species will after some time, usually in the order of few nanoseconds to microseconds, emit light at a wavelength larger than the excitation wavelength. This light, fluorescence, is measured.
The signal-to-noise ratio of the fluorescence signal is very high, providing a good sensitivity to the process.
IR absorption: is a spectroscopic method used mostly in gas detection. Devices using this kind of technique (not open path based) have as main components an IR source (lamp), a sample chamber or light tube, a wavelength filter and an infrared detector. The gas is pumped (or diffuses) into the sample chamber and gas concentration is measured electro-optically by its absorption of a specific wavelength in the infrared (IR).
Path: this technique relates the quantity of light absorbed to the
number of gas molecules in the light path. This technology is used by instruments
that can measure a number of different pollutants along a single light beam
which may be up to 800 meters long.
Using this technique, small traces of pollutant can be detected (ppb), and errors introduced by the point monitoring due to the local variations are eliminated.
Monitoring: is based on the direct contact between the sensor and the
investigated media altering certain properties of that medium and changing
chemical and physical properties of the sensor.
An advantage of this method is that local characterization of the air is well determined. Point monitoring should not be used when local variations are strong and spread area measurements are needed.