FAST SCANNING LASER DOAS - A VERY PROMISING TECHNIQUE FOR MONITORING OH AND OTHER TROPOSPHERIC TRACE GASES

A very promising technique for time resolved local OH measurements is presented which makes use of differential optical absorption spectroscopy (DOAS). The light source is a rapidly tuned, narrowband, frequency-doubled, and power-stabilized dye laser. Due to fast scanning and power stabilization the fluctuations of the atmosphere and the light source are minimized; thus a detection limit better than 10(-5) can be achieved for atmospheric long-path absorption measurements. In a first test near Frankfurt a. M. tropospheric OH was observed with concentrations ranging from 8 x 10(5) OH cm-3 to 2 x 10(6) OH cm-3. The absorption path-length was 800 m and the acquisition time 5 min. In a second generation system the performance of the apparatus was highly improved. A multiple reflection system with a special design for tropospheric open path measurements was constructed. Local OH-measurements in a compartment of only some meters with a well known chemistry are now feasible. Furthermore, a self-test technique was installed for verifying the results and to improve the reliability of tropospheric data. Further application of the experimental device will be its use in monitoring of trace gases absorbing in the ultraviolet, like SO2, and in the visible, such as NO2, to gain a more complex data set for the use with model calculations and to realize a multicomponent device.