Abstract-A methodology is described for the in-situ detection of NO, NH, and SO, in flue gases by DOAS (Differential Optical Absorption Spectroscopy). In order to perform accurate measurements of the concentration it is necessary to compensate for the temperature dependence of the absorption cross-sections as well as for potential deviations from the Beer-Lambert law (nonlinearity effects), From the experimental data in two previous papers, empirical equations were derived for the compensation of the nonlinearity and temperature effects. These were used to compensate obtained concentration values of NO and SO1 retrieved from DOAS spectra that were recorded in a flue gas at 413 K. The measurements of SO, showed that in a concentration interval of 500-1600 ppm at 413 K, the resulting systematic discrepancies between the DOAS and a conventional reference system decreased from 40 to only 2% when compensating the DOAS data. The maximum random difference was approximately 15%. In the same manner the systematic difference for NO decreased from 23 to l%, with a maximum random error of 5%, for concentrations between 60 and 160 ppm. The measurements of NH, demonstrated the versatility of the DOAS technique for time resolved in-situ measurements (~20 set), and also the feasibility of the technique for measuring several species simultaneously. The measurement methodology developed for NH, was more complicated than for NO and SO, and required a larger amount of laboratory calibrations. In the spectral evaluation procedure of NH, hot bands were utilized for flue gas temperatures above 450K.
Long-path DOAS (differential optical absorption spectroscopy) in the ultraviolet spectral region has been shown to be applicable for lowconcentration measurements of light aromatic hydrocarbons. However, because of spectral interferences among different aromatics as well as with oxygen, ozone, and sulfur dioxide, the application of the DOAS technique for this group of components is not without problems. This project includes a study of the differential absorption characteristics, between 250 and 280 nm, of twelve light aromatic hydrocarbons representing major constituents in technical solvents used in the automobile industry. Spectral overlapping between the different species, including oxygen, ozone, and sulfur dioxide, has been investigated and related to the chemical structure of the different aromatics. Interference effects in the DOAS application due to spectral overlapping have been investigated both in quantitative and in qualitative terms, with data from a field campaign at a major automobile manufacturing plant,
The differential absorption structure of the ozone spectrum between 250 and 330 nm has been investigated in order to determine the optimal wavelength region to be utilized for active differential optical absorption spectroscopy (DOAS) measurements. Considering aspects of atmospheric attenuation and interference from other species as well as the magnitude of the differential absorption cross section, an interval around 283 nm was found to be a good candidate for this application. This result was also verified during 12 months of continuous ozone monitoring in an urban environment.
DOAS (Differential Optical Absorption Spectroscopy) is a remote sensing technique in which light is transmitted over kilometer distances with a transmitter, then collected by a receiver and analyzed in a special spectrometer. Thus an absorption spectrum of the air between transmitter and receiver is obtained (Platt o Perner 1979). The method has proven to be powerful and a number of substances have been successfully measured in applications ranging from background monitoring to urban air and emission studies (Platt, Perner, Pitts, Biermann). Traditionally light is transmitted by a parabolic mirror with the lamp mounted in its focus, thus emitting a parallel beam of light. In the receiving end, light is collected by a similar mirror and focused into an optical fiber connected to the spectrometer. Alternatively the converging light may be coupled directly to the spectrometer by means of a folding mirror in Newtonian or Cassegrain arrangement.
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