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- Mellqvist, Johan, 1965, et al.
(författare)
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DOAS for flue gas monitoring—I. Temperature effects in the UV/visible absorption spectra of NO, NO2, SO2 and NH3
- 1996
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Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer. - 0022-4073. ; 56:2, s. 187-208
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Tidskriftsartikel (refereegranskat)abstract
- The temperature dependence of the absolute and the differential absorption cross-sections for NO, SO2, NO2 and NH3 were studied by recordings of spectra in a heat-pipe cell and by simulations of theoretical spectra for NO. A review and comparison of the present results with other relevant works were also made. The experimental results showed that the differential absorption features for some of the studied species change dramatically with temperature. For SO2 and NO2 the quantitative change in differential structure was very large with a relative change in magnitude of 70% between 300 and 700 K. For the two other species studied, NO and NH3, the change in magnitude of the differential structure was only 15–20%, over the same temperature range. Simulations for NO showed that the temperature effect was strongly dependent on the spectral resolution of the instrument and that it became smaller at lower resolution. The qualitative change in the spectral features was a continuous lowering of absorbance peaks and an increase in valleys which made the band integral of the absorbance quite insensitive to the temperature. Hot bands also appeared for SO2 and NH3 around 220 nm. The temperature affected the spectral features more in a quantitative than in a qualitative manner.
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| 3. |
- Mellqvist, Johan, 1965, et al.
(författare)
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DOAS for flue gas monitoring—II. Deviations from the Beer-Lambert law for the UV/visible absorption spectra of NO, NO2, SO2 and NH3
- 1996
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Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer. - 0022-4073. ; 56:2, s. 209-224
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Tidskriftsartikel (refereegranskat)abstract
- Deviations from the Beer-Lambert law were studied for the differential absorption cross-sections for NO, SO2, NO2 and NH3. This was performed by simple calculations, computer simulations of spectra and by recordings of spectra for the above mentioned species at various total columns. The linearity studies for the DOAS instrument displayed large variations for the molecules studied and for different wavelength bands. In a calculation it was shown that the optical depth deviated from a linear concentration dependence by a term which was directly proportional to the statistical variance of the true absorption cross sections and proportional to the square of the total column, under the assumption of a boxcar instrument lineshape. Species exhibiting little variance or fine structure in their spectra, for instance NO2, displayed a larger linear region compared with molecules exhibiting a rich structure, i.e., NO. The former species was linear to a total column of 3150 mg/m2, which correspond to a maximum optical depth of 0.7, while the latter was linear to only 6 mg/m2, corresponding to a maximum optical depth of 0.024, in the resolution range studied. The linear regions for the other species studied were 90 mg/m2 for SO2 at 230 nm, 180 mg/m2 for SO2 at 300 nm and 36 mg/m2 for NH3. The main effect of the nonlinearity was to cause a reduction in the peak height of the absorption. It was shown that the nonlinearity effect is independent on the spectral resolution when a large number of absorption lines are covered by the bandpass of the instrument. It was also shown that the largest change in linearity occurs when the resolution is similar in magnitude to the absorption linewidth. The nonlinear behavior for NO varied less than 2% in the temperature range 300–1000 K and the spectral resolution range 0.25–1 nm. The nonlinearity effect caused quantitative rather than qualitative changes of the spectral features and typical relative errors can be as high as 35% in a flue gas.
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| 4. |
- Mellqvist, Johan, 1965, et al.
(författare)
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DOAS for flue gas monitoring—III. In-situ monitoring of sulfur dioxide, nitrogen monoxide and ammonia
- 1996
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Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer. - 0022-4073. ; 56:2, s. 225-240
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Tidskriftsartikel (refereegranskat)abstract
- A methodology is described for the in-situ detection of NO, NH3 and SO2 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 SO2 retrieved from DOAS spectra that were recorded in a flue gas at 413 K. The measurements of SO2 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 1%, with a maximum random error of 5%, for concentrations between 60 and 160 ppm. The measurements of NH3 demonstrated the versatility of the DOAS technique for time resolved in-situ measurements (
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