Differential Optic Absorption Spectrometry or DOAS is a process that is used to determine concentrations of trace gases. It measures their narrow band absorption structures in the ultraviolet and visible spectral area. Columns of trace-species are derived from the measurements of electromagnetic radiations in a specific spectral interval. DOAS has actually become a universal technique of measuring the concentrations of atmospheric trace gases.

A typical Differential Optic Absorption Spectrometry instrument is made up of an optical setup and a source of light such as a Xe-arc lamp. The optical setup is necessary for sending and receiving light through the atmosphere. Aside from artificial lighting, however, the sun can also be used as a source of light. The usual length of the path of light in the atmosphere is hundreds of meters up to several kilometers.

In order to perform Differential Optic Absorption Spectrometry, a couple of spectra are needed: the one in which the light has already passed through none or little amount of the absorber, and the one in which the light has already passed through a huge amount of the absorber. ACD or apparent column density is the quantity retrieved. If the instrument used is pointed toward the sun, a signal coming from the sunlight is measured.

If this is the case, the ACD seems straightforward, representing the difference in the absorber's column density between that in the reference and along the line of sight. Then again, the viewing direction is far from the sun; hence, the scattered light becomes the source of the signal measured. The ACD loses its clear physical interpretation because of the complex path of light. Nonetheless, the ACD remains a representation of the difference in absorber column density.

Today, Differential Optic Absorption Spectrometry is being used to measure concentrations of atmospheric ammonia. In the past, however, direct contact with ammonia was involved in the techniques used. Such old technique has lead to memory effects and inaccurate measurements; therefore, proving to be inefficient. Ammonia, after all, is highly adhesive and direct contact with it is not a good option.

Fortunately, optical remote sensing techniques have been developed and used. In fact, the National Institute for Public Health and Environment make use of this system for measuring the concentrations of atmospheric ammonia. It has actually developed the RIVM Ammonia Measurement System. Not only is this system effective and has a high up-time, but it is also very easy to maintain.

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