Full Description ADOPTED_FROM:EN 15483:2008 This European Standard is applicable to open-path absorption measurements of \'concentration path length\' product using the Fourier transform infrared (FTIR) technique with an artificial radiation source. It is applicable to the continuous measurement of infrared active organic and inorganic compounds in the gaseous state in ambient air using fixed tropospheric open paths up to approximately 1 km in length and provides a spatial average.