UV DOAS is a method used to measure the concentration of various gases in the atmosphere by studying the amount of electromagnetic energy they can absorb. The spectral region used by this technique is the infrared, visible light and UV light. The basic principle applied is that light loses some of its intensity as it travels from one point to another. The extent by which it loses the intensity depends on the particles in between the distance and their specific absorption behavior. Apart from losing intensity due to absorption, some of the light is reflected or deflected by the particles along its path. All these factors are studied to determine the gases and chemicals present in the atmosphere.

The UV DOAS instrument is usually made of three main parts. These are the spectrometer, optics and source. In most cases, the source employs xenon-arc or tungsten halogen lamps to produce light. In rare cases, deuterium light or scattered light from the sun is used. The light is transmitted to a receiving end through a telescope. Light on the receiving end is passed through fiber optics cables before it is focused on the spectrometer. Some industries use a telescope for transmission and on the receiving end as well. However, this means that they are limited to mono-static configuration.

Once light on the UV DOAS instrument reaches the spectrometer, it is diffracted and focused on a detector. Some systems make use of multi-channel analyzers that are able to analyze multiple spectra at once. The information from the spectrometer is sent to a computer where it is analyzed and evaluated. Different users use different spectrometers based on the problem at hand. The DOAS system can be used in a variety of commercial systems for gas detection and analysis. The length to be measured can extend to tens of kilometers. Some of the gases that can be analyzed by this instrument include HONO, NO3, NO2 and H2O among others.



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