DOAS for flue gas monitoring—I. Temperature effects in the U.V./visible absorption spectra of NO, NO2, SO2 and NH3

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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