J. Quanr. Specrrosc.Radior. Trons/k Vol. 56, No. 2, 187-208,1996 pp. CoDvrieht0 1996 ElsevierScienceLtd Pergamon PII: SOO22-4073(%)00042-8 Printed’& GreatBritain. All rights reserved 0022-4073/96 %I 5.00 + 0.00 DOAS FOR FLUE GAS MONITORING-I. TEMPERATURE EFFECTS IN THE U.V./VISIBLE ABSORPTION SPECTRA OF NO, N02, SO2 AND NH, JOHAN MELLQVISp* and ARNE ROSEN” “Department of Physics, Chalmers University of Technology and Gijteborg University, S-412 96 Giiteborg and *The Swedish Environmental Research Institute (IVL), P.O. Box 47086, S-402 58 GGteborg, Sweden zyxwvutsrqpo (Received 11 December 1995) Abstract-The temperature dependence of the absolute and the differential absorption cross-sections for NO, S02, NOz 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 SO, and NO, 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 NH,, the change in magnitude of the differential structure was only 1 S-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 SO, and NH3 around 220nm. The temperature affected the spectral features more in a quantitative than in a qualitative manner. Copyright 0 1996 Elsevier Science Ltd 1. INTRODUCTION Differential Absorption Spectroscopy in the u.v./visible has been utilized for more than half a century in Dobson spectrometers’ which are used to monitor total columns of atmospheric ozone and in Brewer-photometers’ which are used to monitor total columns of NO,, SO? and 0,. It was Platt et al3 who first started to utilize differential absorption for tropospheric gas composition measurements by introducing DOAS (Differential Optical Absorption Spectroscopy). The tech- nique is based on the recording of differential absorption, i.e., the difference between local maxima and minima in the absorption spectrum of the probed gas species. In the tropospheric application, light from a broadband xenon high-pressure lamp is transmitted through the atmosphere for distances up to several kilometers. The light is received and analyzed using a fast scanning device to eliminate the influence of air turbulence. The most straightforward atmospheric molecules to detect with DOAS are probably N02, 0, and SO,, and these species are also routinely monitored in many cities around the world.4,S Other species that have been monitored, but which require more advanced instrumentation are: CH,O, CS,, Hg, HN02, NH,, NO, NO, and OH.“9 It is also feasible to measure light aromatic hydrocarbons such as benzene, toluene and xylenes.“*” The absorption features of the latter molecules are however very similar and interfere severely. By using scattered sunlight it has also been possible to perform total column measurements on the atmosphere for species such as NO, and 0, but also for OClO, BrO and NO3 .“-16It has furthermore been possible to obtain information on aerosol density and size distribution in the stratosphere.” An interesting application of the DOAS technique is to use it for in-situ detection, across the stack, of flue gases, since many of the main pollutants from fossil fuel combustion can be detected with the technique. The feasibility of using the technique in-situ is attractive in order to minimize response times and in order to detect polar and reactive gases which can be very difficult to measure 187