Differential Optical Absorption Spectroscopy, Air Monitoring by
Environment: Trace Gas Monitoring
Published Online: 15 SEP 2006
Copyright © 2000 John Wiley & Sons, Ltd. All rights reserved.
Encyclopedia of Analytical Chemistry
How to Cite
Platt, U. 2006. Differential Optical Absorption Spectroscopy, Air Monitoring by. Encyclopedia of Analytical Chemistry. .
- Published Online: 15 SEP 2006
Differential optical absorption spectroscopy (DOAS) allows the quantitative determination of atmospheric trace gas concentrations by recording and evaluating the characteristic absorption structures (lines or bands) of the trace gas molecules along an absorption path of known length in the open atmosphere. The DOAS technique is characterized by the following: (1) measuring the transmitted light intensity over a relatively (compared to the width of an absorption band) broad spectral interval; (2) high-pass filtering of the spectra to obtain a differential absorption signal and eliminating broad-band extinction processes such as Rayleigh and Mie scattering (RS and MS); and (3) quantitative determination of trace column densities by matching the observed spectral signatures to prerecorded (reference) spectra by, for instance, least-squares methods.
DOAS shares the advantages of most other spectroscopic techniques, including inherent calibration, sub-parts per trillion (ppt) to ppt sensitivity and precision (1–10%), good specificity, wall-less operation, and the capability for remote measurements. In particular, the concentration of very reactive species such as the free radicals OH, NO3, ClO, BrO, and IO are determined with DOAS. Other species of interest to atmospheric chemistry are also measurable such as SO2, CS2, O3, NO, NO2, HONO, NH3, CH2O, and most monocyclic aromatic hydrocarbons.
A description of the technique is given, and the various modes of operation and light-path configurations are explained. The emphasis of this article is on the practical aspects of instrument design, components of DOAS systems, operation of DOAS instruments, and state-of-the-art techniques for the evaluation of DOAS spectra and realistic determination of detection limits. Finally some examples of DOAS applications are given.