Abstract: An open air analysis apparatus includes an optical source for providing an optical signal, an interferometer for modulating the optical signal, and an optical signal detector. The apparatus further includes a folded path mirror configuration defining a sample path through which the optical signal passes from the interferometer to the optical signal detector. An enclosure encloses and seals the sample path with the enclosure including at least one removable portion for allowing entry into the sample path of a representative sample of ambient air when the at least one removable portion is removed. The method of gas analysis includes providing an open air spectrometry system defining a sample path. The sample path is enclosed and sealed when the open air spectrometry system is calibrated. The sample path is then opened to allow ambient air therein and analysis of a sample of ambient air in the sample path is initiated.