The Infrared Working Group (IRWG) manages twenty solar viewing Fourier-Transform Infrared Spectrometers (FTIR) that are distributed on all continents spanning the latitude range from 80ºS to 80ºN. These instruments record mid-infrared solar transmission spectra at high spectral resolution. Some, especially at remote sites operate autonomously, others are operated remotely or manually. The spectra contain the spectral signatures of vibrational-rotational transitions of numerous trace gases in the terrestrial atmosphere as they absorb solar radiation. The spectra are analysed to measure the concentration of these trace gases in the atmosphere using the method of optimal estimation. Gases of primary interest are O3, HCl, HF, ClONO2, HNO3, N2O, CH4, CO, C2H6, and HCN. Concentrations of these species all vary with altitude, this information along with retrieval characterization metadata are provided to allow optimal use of the data. Our aim is to record long-term high quality inter-calibrated spectra that provide trends of trace gases that can be used to understand tropospheric (https://en.wikipedia.org/wiki/Troposphere) and stratospheric chemistry (https://en.wikipedia.org/wiki/Stratosphere) and transport.