Fast Scanning Laser DOAS for Local Monitoring of Trace Gases, in Particular Tropospheric OH Radicals

A method for monitoring tropospheric OH and other tropospheric trace gases is described which makes use of differential absorption spectroscopy and is based on the fast scanning of a narrowbanded and power-stabilized laser. Due to the fast scanning the remaining influences due to atmospheric fluctuations are negligibly small. The modulation and fluctuation of the light source are minimized by a closed loop modulator system. The absorption path is folded by a multiple reflection system with a special design for tropospheric measurements which allows local trace gas measurements in a compartment of only some meters with a well known chemistry. A field campaign was carried out with this instrument on the Schauinsland, Black Forest, Germany. A detection limit better than 10(-5) corresponding to an OH concentration of 5 . 10(5) OH/cm-3 was achieved in local tropospheric air.