Time-Windowing Fourier Transform Spectroscopy
Fast, accurate measurement of rare yet critical chemical species in the Earth's atmosphere is becoming increasingly important. Reactive halogens such as bromine monoxide (BrO) affect the protective ozone layer in the stratosphere as well as the Earth's radiative budget, which influences climate change. Fourier Transform Spectrometers (FTS) have been used to detect the concentrations of BrO and other halogenated species. In the framework of the THALOZ project, the Institute of Environmental Physics and Remote Sensing sought to improve upon current FTS technology. Using knowledge of the exact position of the scanning mirror, they were able to achieve high spectral resolution in very short time periods. The key to this time-windowing (TW) method is larger interferogram intervals. This means the technique is appropriate for low frequency (0.1-1Hz) applications, for which time resolution on the order of milliseconds or less can be achieved. Measurements of BrO concentration with the TW-FTS following flash photolysis of Br2O3 compared well with the time series predicted from chemical kinetics theory. BrO absorption spectra with TW-FTS were also demonstrated to be equivalent to those obtained with regular FTS. A key feature of the hardware and software developed during THALOZ is that they can be integrated into commercially available Michelson interferometers. Further details can be found in a relevant publication in the Journal of Photochemistry and Photobiology.
