During the 1990s, a novel approach to air quality monitoring using spectroscopy - known as open-path methods - gained ground. Instead of measuring pollution at a specific location, open-path methods record the average pollutant concentration along the path length of a light beam. Measurements are made in situ and there is no disturbance in the airflow due to pump sampling as with point analysers. Open-path methods are used for the remote sensing of the atmosphere and provide better estimates of average pollutant concentrations over a given area. Furthermore, a single instrument can measure several components, both organic and inorganic. Open-path techniques were first tried in the 1970s, but commercial instruments have been available only since the mid-1980s. Open-path systems include Differential Optical Absorption Spectroscopy (DOAS), Fourier Transform Infrared Spectroscopy (FTIR), Light Detection and Ranging (Lidar), Differential Absorption Lidar (Dial), and Tuneable Diode Laser Spectroscopy (TDLAS). DOAS is the most widely applied open-path method used for air quality monitoring. The DOAS method is based on the difference in absorption between different wavelengths. It uses advanced computer technology, with software including the spectrographic fingerprint of the pollutants to be determined, to evaluate pollutant concentrations. Using Beer-Lambert's law (A = -log10(Io/I ), where A is the absorbance, Io is the intensity of the incident beam, I is the intensity of the transmitted beam, e is the molar absorptivity of the substance, c is its concentration, and l is the path length) the instrument detects and measures the reductions caused by pollutants in the spectrum of ambient air. The advantage of the DOAS method is that it does not use a sample cell and the path length can be increased considerably, to hundreds of metres and even to several kilometres, allowing the determination of much lower concentrations than with conventional instruments.

                              Source: Department of Environment, Riga City Council

In the DOAS method, the computer rapidly sums the reference spectra while varying the proportions of the different components until a match is obtained. In the DOAS system, a narrow beam of light from a high-pressure Xe lamp is focused onto a receiver up to 1 km away from the source. The light detected by the receiver is transmitted through an optical fibre to a computerised spectrum analyser. The spectrometer in the analyser splits the light into narrow wavelength bands using an optical grating. A narrow slit sweeps past the detector at high speed, allowing a large number of instantaneous measurements to be made, building up the entire spectrum. The scan is repeated 100 times each second and the registered spectrum is stored in the computer's memory. The measured spectrum is compared with various spectra calculated by the computer based on different proportions of individual pollutants that may be present in ambient air. (Reference spectra of individual pollutants are stored in the computer's memory.) When the best possible match is obtained, the computer calculates the concentrations of the different gases present in air. The major supplier of DOAS monitoring systems is OPSIS, a Swedish company that took the initiative in marketing open-path technology in the late 1980s.