Condensed from the OPSIS knowledge base page on measurement methods.
The ultraviolet band is where OPSIS monitoring began: when differential optical absorption spectroscopy is applied in the ultraviolet the technique is called UV-DOAS, and the company's first instruments were based on it. A high-pressure xenon lamp projects light along the measurement path; the lamp is intense and emits across the visible, ultraviolet and infrared. A receiver at the far end captures the light and passes it over an optical fibre to the analyser, which can therefore be installed away from the receiver and out of a harsh environment. The analyser consists of a spectrometer with a light detector, a computer and control electronics. The spectrometer separates wavelengths with an optical grating; because different molecules need different wavelength ranges, the grating can rotate so that the target band is measured with high resolution. Spectrometers perform best in the ultraviolet, so UV-DOAS mainly addresses molecules that absorb there and uses ultraviolet detectors; where the spectrometer can also handle molecules absorbing in the infrared, an infrared detector is added and the technique becomes UV/IR-DOAS. A narrow slit sweeps rapidly in front of the detector and the signal is sampled repeatedly to build the spectrum, hundreds of times per second.
UV-DOAS is the workhorse of Chinese CEMS: SO2 and NOx absorb in the ultraviolet, and one analyser with a xenon lamp and a rotating grating covers both, replacing a chemiluminescence NOx analyser and a fluorescence SO2 analyser. Three points from our installations. The light path must stay clear — on a dusty duct the transmitter and receiver windows need purge air, and on an open path the alignment has to survive thermal movement between day and night. The reference spectrum belongs to the instrument and its lamp, so replacing a lamp invalidates it and the analyser has to be re-referenced before the next calibration. And because the measurement is made across a duct or the open air, the reported value is a path average; when it is compared with a point-sampling reference method, the difference in sampling volume is part of the result, not a fault in the instrument.
UV-DOAS Technology — read the original at opsis.se ↗
Source: OPSIS knowledge base, monitoring methods. The summary and the A.Johnson note above are our own text; the OPSIS article remains the authoritative version. Chinese reader? See the full Chinese article.
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