DOAS stands for Differential Optical Absorption Spectroscopy. It is the fundamental principle for measuring gas concentrations in most of OPSIS’ gas analysers.

The acronym DOAS gives us good clues to what it’s about: “Spectroscopy” means the experimental investigation of the nature of light. “Absorption” indicates that we look at the “disappearance” of specific wavelengths. “Optical” limits us to study the optical wavelengths, ranging from ultraviolet via visible to infrared light. Finally, “differential” is the mathematical process applied to the recorded optical absorption spectra.

DOAS utilizes a broad band light source; in OPSIS’ case a xenon lamp. The light is made to form a narrow and intense beam which is sent across the volume where the gas to measure resides; often referred to as the “measurement path”. The ray of light looks white to the human eye but also includes “invisible” infrared and ultraviolet wavelengths.

A specific type of gaseous molecule (and in some cases also atom) usually has narrow-band absorption features in certain wavelength regions. It can be thought of as a fingerprint of the molecule type. A fraction of the light emitted from the light source will be absorbed by the molecules present in the path. The higher concentration of the gas or the longer measurement path, the more absorption will occur. The remaining light that arrives at the end of the measurement path is forwarded to a spectrum analyser, the key component of the gas analyser. The analyser can then detect the features of the absorption. Dividing the recorded spectrum by a reference (i.e. a “clean” spectrum, this division is the “D” in DOAS) and comparing it with known absorption spectrum of the sought-for gas yields the average gas concentration along the measurement path.

Each type of gaseous molecule has its own characteristic absorption abilities in specific wavelengths, and thereby its own specific fingerprint. This makes it possible to optically separate different gases from each other, and to use the same measurement system to detect multiple gaseous compounds in the light path. It is a matter of knowing in what wavelength region the respective type of molecule has its unique fingerprint, and then look for that.

Today, DOAS is a well-established principle for gas concentration measurements, as evidenced by numerous approvals and thousands of DOAS systems deployed throughout the world.

OPSIS AB can provide monitoring solutions for all the following substances.

1,3-Butadiene – C4H6
Acetaldehyde – C2H4O
Acetone – C3H6O
Acetylene – C2H2
Ammonia – NH3
Benzene – C6H6
Benzaldehyde – C7H6O
Bromine – Br
Butane – C4H10
BTX
Carbon dioxide – CO2
Carbon disulfide – CS2
Carbon monoxide – CO
Chlorine – Cl2
Chlorine dioxide – ClO2
Chlorobenzene – C6H5Cl
Cresol – C7H8O
Cyanogen – (CN)2
Dichlorobenzene – C6H4Cl2
Dimethylamine – (CH3)2NH
Dioxins
Ethane – C2H6
Ethanol – C2H5OH
Ethylbenzene – C8H10
Ethylene – C2H4
Formaldehyde – CH2O
Heptane – C7H16
Hexane – C6H14
Hydrocarbons
Hydrogen bromide – HBr
Hydrogen chloride – HCl
Hydrogen cyanide – HCN
Hydrogen fluoride – HF
Hydrogen sulfide – H2S
Iodine – I
Mercury – Hg0 and THg
Methane – CH4
Methanol – CH3OH
Nitrate – NO3
Nitrogen dioxide – NO2
Nitrous oxide – N2O
Nitric oxide – NO
Oxygen – O2
Ozone – O3
Phenol – C6H5OH
Phosgene – COCl2
Particulate Matter
Propane – C3H8
Silane – SiH4
Styrene – C8H8
Sulfur dioxide – SO2
Sulfur trioxide – SO3
Tetrafluoromethane – CF4
Toluene – C7H8
Trimethylbenzene – C9H12
Water vapour – H2O
Xylene – C8H10

To measure the ambient air quality at airports is a challenge. The monitoring site needs to be representative for background levels and not dependant on changes in local traffic. A large number of gaseous components need to be measured with high accuracy and high availability.
The OPSIS DOAS system provides the user with a fast system that gives high availability at low cost. The impact of aircraft take-offs and landings can easily be followed on-line. The OPSIS system can also monitor fugitive emissions from other sources at the airport.

Several gaseous compounds can be monitored with one system, such as Nitrogen oxides NOx, Sulphur dioxide SO2, Ozone O3, Benzene, Toluene, Formaldehyde, 1,2,4-trimethylbenzene, and Ammonia NH3.

Airport Monitoring by OPSIS

High data availability
Cost-effective, open-path technology
Direct monitoring of NO2
Representative path-integrated data
Gas calibration only once per year
Low energy consumption
Operates with a minimum of maintenance
Approved by MCERTS, TÜV, U.S. EPA, Chinese EPA, and others

Benzene is an aromatic hydrocarbon liquid that evaporates quickly if exposed to ambient air. It is one of the most commonly used substances in the chemical industry and it is known to cause cancer. For this reason, refineries in the United States must start monitoring fugitive emissions of benzene from January 30, 2018. This is part of the 2015 U.S. EPA Petroleum Refinery rule. In case a yearly rolling average of benzene concentrations exceeds 9 μg/m³, actions have to be taken to reduce the fugitive emissions.

Benzene emissions can be conveniently and accurately monitored in real time using OPSIS UV-DOAS systems. The UV-DOAS technology utilizes a beam of light to measure gas concentrations. By arranging the light paths along the fence-line of a refinery, the benzene emissions from within the fence can be isolated and detected. In combination with meteorological sensors, it is also possible to calculate the locations and strengths of the benzene emission sources.

General Specifications

Gas Monitoring Systems

Monitoring Techniques

Fence-line monitoring

Aluminium Smelters