The long-path differential optical absorption spectroscopy (LP-DOAS) instrument is an active measurement system that uses a high pressure Xenon arc lamp as light source. This lamp is placed at the focal point of a Newtonian telescope that acts as transmitter and receiver unit at the same time. The collimated light beam is directed towards a corner cube array reflector (accuracy < 5 arc seconds) which is placed several kilometres away from the source. The reflected beam is then focused onto an optic quartz fibre bundle which feeds the light into a Czerny Turner spectrometer with a focal length of 0.5 m. The refraction grating disperses the light onto the chip of a cooled charge-coupled device (CCD) detector. The spectrometer is temperature-stabilised to avoid wavelength drifts with changes of the ambient temperature. The spectral resolution of this system obtained from the full-width half-maximum of a representative line is 0.3 nm.