Condensed from an OPSIS AB article by Bengt Löfstedt, first published 2017-11-21.
DOAS stands for differential optical absorption spectroscopy. The idea is old — researched for air quality work in the 1970s and commercialised in the mid-1980s — and rests on one fact: each gaseous molecule absorbs light at its own set of wavelengths, a fingerprint that stays recognisable even where fingerprints overlap. A xenon lamp emits continuous light from the ultraviolet through the visible to the infrared. That light crosses the measurement path; molecules absorb part of it; a spectrometer, or an interferometer in some models, separates the wavelengths and a window of interest is recorded. The measured spectrum is divided by a stored reference spectrum of the lamp, then by a polynomial, which removes broadband attenuation that does not come from the target gas. What remains is the differential spectrum. Compared mathematically with reference spectra of known concentration, it yields the average concentration along the light path, together with an estimate of the fit quality.
A DOAS result is an average over the whole light path, not a point sample. That is an advantage in a duct or an open-path application, because the reading represents the volume the light actually crossed — but it also means the path has to be chosen so that it represents the process. In our projects the path definition is agreed at design stage, together with the mounting flanges and the purge arrangement, because moving it later is expensive. On Chinese emission sources the design, acceptance and operation of a CEMS follow the HJ 75 and HJ 76 series, which set out calibration, span checks and data-validity rules; whatever the analyser principle, those routines decide whether the data survives an audit. Open-path and in-situ arrangements are chosen most often where dust load, moisture or corrosive components make an extractive sample line difficult to keep clean, and where several gases are wanted from one instrument.
DOAS Explained — read the original at opsis.se ↗
Written by Bengt Löfstedt, OPSIS AB, first published 2017-11-21. 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.
Send the gases, ranges, duct or path dimensions and required certification — we will respond with a technical proposal.