DOAS stands for Differential Optical Absorption Spectroscopy. The DOAS technique was conceived almost a century ago, and it was applied on research level to air quality monitoring already in the 1970s. In the mid-1980s, the technology was then commercialised and turned into practical and wide-spread use.
The acronym DOAS itself reveals quite a lot of what it is about. “Spectroscopy” is the study of interaction between matter and light, and that is indeed what happens. The gaseous molecules of the air pollutants (the matter) interacts with certain wavelengths of the light that passes through the gas. “Absorption” specifies the optical effect utilized: some wavelengths are absorbed by the molecules. “Optical” tells which wavelengths are in use. The technology operates in the optical wavelength range, covering infra-red (IR), visible and ultraviolet (UV) light, where certain ranges (or “windows”) of wavelengths (i.e. spectra) are detected. Finally, “differential” is the mathematical method applied to the detected spectra, allowing gas concentrations to be calculated.
Each type of gaseous species, such as molecules of NO2 or SO2, has its own specific absorption spectrum. It can be thought of as the fingerprint of that type of molecule. Some “fingerprints” overlap, but knowing where to look it is still possible to find clear-enough and isolated-enough fingerprints of a large number of gaseous species that are of interest in ambient air quality monitoring and continuous emissions monitoring.
A xenon lamp is utilized as a light source. It emits a white light, effectively representing every wavelength from UV to IR. As the light passes in the air, different molecule types absorb different wavelengths in the specific absorption patterns (fingerprint) of those molecules. The more molecules of a certain type, the more absorption in that particular pattern. The partially absorbed light is then captured and lead to a spectrometer (or in some analyser models an interferometer), i.e. a device with the ability to separate the different wavelengths of the light. The wavelength window of interest is selected, and the measurement spectrum can be recorded.
The rest is math. First, the measurement spectrum is divided by a previously stored “reference”, holding the fixed emission spectrum of the xenon lamp. A further processing is then applied by dividing by a polynomial to remove any remaining and undesired broadband absorption, not originating from the gaseous species of interest. This is possible due to the nature of the molecular absorption, which shows rather sharp “absorption lines”. The result is the differential spectrum, i.e. the “D” in DOAS.
The differential spectrum is then mathematically compared to one or more pre-recorded differential absorption spectra of known gases of known concentrations. The combination resulting in the best fit to the just recorded differential spectrum yields the actual average gas concentration along the light path. The number crunching also results in an estimation of the error in the calculations: if an unexpected and spectrally interfering gas would appear in the monitoring path, a quality indicating “deviation” number increases which indicates an increased uncertainty in the reported result.