On path length, beam divergence, and retroreflector size in open path FTIR spectroscopy

Abstract

Open-Path Fourier Transform InfraRed (OP-FTIR) spectroscopy is an established technique used to measure boundary layer trace gas concentrations, consisting of a spectrometer and a retroreflector separated by a measurement path. The detection limit is directly proportional to the optical path, which controls target gas spectral absorption feature depth; however, depending on the specifics of the spectrometer and telescope optics, beam divergence can begin overfilling the distant retroreflector array for paths greater than ~300 m, resulting in decreased returning radiation. In this case, the absorption signature of the target gas increases, but the signal to noise ratio of the recorded spectrum does not, making detection difficult. The results of an experiment where the retroreflector array area was increased to collect a larger fraction of returning radiation at path lengths ranging from ~200 m – 1000 m are discussed, including an analysis of both underlying spectra and quality indicators for retrieved concentrations of carbon monoxide. The results show that the larger retroreflector array results in smaller decreases in the signal-to-noise ratio as a function of the measurement path when compared to a smaller array. Next, the effectiveness of cleaning retroreflector arrays after extended field use is presented using quantitative information, including 1) a measured 10% increase in infrared intensity and 3) surface characterization of a single retroreflector cube corner array before and after the cleaning process, which resulted in no notable changes, but revealed corrosive processes and contaminants present on the cube. Lastly, the results of theoretical spectral simulations are discussed in detail for CO and HCHO (formaldehyde) showing how path length, water concentration, and target concentration affect the differential absorption spectrum of the target, also considering random and systematic noise levels. It was determined that path lengths > 300 m are necessary for robust HCHO measurements. A further 10 common atmospheric species are explored in Appendix B. Finally, spectral simulations also explore the relationship between absorbance and transmittance by increasing the target gas concentration, from which it is seen that for less abundant trace gases (i.e., HCHO at 1 ppb) concentration and transmittance are approximately linearly related.