How to quantify gases in air with an open-path FTIR or a variable pathlength cell / Comment mesurer des gaz dans l'air à l'aide un FTIR à parcours ouvert ou une cellule à parcours variable

Since the middle of the 90’s several FTIR instruments are available to measure and quantify gases in the open atmosphere. Before going outside, it is easier to get spectra in the lab and make sure of the results. When not respecting the Beer-Lambert law, spectra may be distorted and care was taken to avoid it. A usual feature in open atmosphere spectra is the presence of bands in the 1400-1800, 2250 and 3500-4000 cm-1 regions. These bands are due to the ubiquitous presence of H2O and CO2 which were avoided in the lab setup. NH3, SO2 and NO2 were quantified; only the two last ones would lead Einstein A coefficient comparable to the data in the HITRAN public database. NH3 showed almost three orders of magnitude difference mainly due to distorted spectra from saturation of the signal.\u