FTIR spectroscopy of the atmosphere. I. Principles and methods

In recent years Fourier transform infrared (FTIR) spectroscopy has been the dominant technique used for measuring the infrared (IR) absorption and emission spectra of most materials, with substantial advantages in signal-to-noise ratio, resolution, speed, and detection limits. The major advantage of the FIFIR technique over other spectroscopic methods is that practically all compounds show characteristic absorption/emission in the IR spectral region and based on this property they can thus be analyzed both quantitatively and qualitatively. The quest for highly sensitive detection methods for atmospheric trace gas samples, either in laboratory setup or in outdoor remote sensing, has been on agenda for several decades. The fast and intensive development of FTIR spectroscopic techniques has propelled the progress of trace gas analysis of the atmosphere. Since the early 1970s the number and scope of FTIR atmospheric measurements has increased steadily. In this review article we are making an attempt to summarize the results of the most significant contributions to the field of FTIR spectroscopy to trace gas analysis of the atmosphere. Beside the basic description of the extractive and open-path measurement methods, the difficulties connected with collection of appropriate background reference spectra, the ways of qualitative analysis and quantitative evaluation of measured spectra, the problems of calibration, and the effects of spectral resolution on detection sensitivity are discussed. The techniques reviewed include in situ IR absorption measurements over open paths in the field, such as remote sensing using the sun, the sky, or natural hot objects as IR sources of radiation, and also IR emission measurements of hot trace gas sources e.g., stack emissions, exhaust gases of combustion sources, and other industrial effluents.