Fourier transform infrared photoacoustic spectroscopy (FT-IR/PAS) has been used to investigate the reaction of SO2, in He with CaO and CaCO3 particles at temperatures between 25 and 900°C. The reaction of SO2 with CaO occurs at 25°C, while the reaction of CaCO3 with SO2 is first evident at 550°C. The initial product of both of these reactions is CaSO3. The CaSO3 then further reacts with SO2 to form CaSO4, CaS2O3, and CaS at higher temperatures. At 900°C and above, the net production of SO4= and S2O3= is decreased because of the decomposition of SO3=. FT-IR/PAS depth-profiling experiments indicate the formation of a reaction-limiting product layer of SO3= and SO4= from the above reactions on the surface of the CaO and CaCO3 particles.

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The sections in this article are Introduction Historical Perspective Current State of the Art Principles of OP / FT ‐ IR Monitoring Optics Transfer Optics Telescopes Retroreflectors IR Sources Detectors Monitoring Configurations Monostatic Configurations Bistatic Configurations Siting and Selection of Monitoring Paths Data Acquired B eer's Law Interferograms and Single‐Beam Spectra Background Spectra Absorption Spectra Concepts of Path‐Averaged and Path‐Integrated Concentrations Instrumental Parameters Measurement Time Spectral Resolution Apodization and Zero Filling Atmospheric Effects Data Analysis Preparation of Field Spectra for Analysis Correcting Single‐Beam Spectra for Stray Light or Ambient Radiation Baseline Correction Correcting Absorption Spectra for Wavenumber Shifts Generating Reference Spectra Analysis Methods Nonlinear Response Quality Control Issues Measurement of the Random Noise Relative Intensities in Single‐Beam Spectra Time Series Plots Correlation Plots Statistics from Multivariate Analyses Estimation of Detection Limits Accuracy and Precision Measurement of Permanent Gases Short Cell Comparability Applications Future Directions