Das Projekt "Airborne instrumentation for HOx measurement in the low stratosphere" wird vom Umweltbundesamt gefördert und von Max-Planck-Institut für Kernphysik durchgeführt. Objective: It is well recognised that the main oxidation processes in the atmosphere are driven by free radicals which are photochemically generated. In the stratosphere as well as in the troposphere the most important species appear to be the OH and HO2 radicals. Due to their very low level of concentration, measurements are very difficult and most of the model calculations of the photochemistry in the atmosphere are still based on theoretical values for the HOx family. However some techniques like Laser-Induced Fluorescence (LIF) or Mass Spectrometry (MS) become reliable and have proved recently to be sensitive enough for locally measuring OH and HO2 mixing ratios down to very low levels The final objective is to make available an airborne instrument to the European scientific community of the atmosphere, for in-situ measurements of free OH and HO2 radicals in the low stratosphere and upper troposphere. The HOx family responds rapidly to any change in the local solar flux and active species concentration. Associated with complementary variable measurements like O3, H2O, NO NO2, CO and solar fluxes these measurements provide a useful basis to understand the photochemistry at all altitudes. In a first step the objective of the present project will be limited to the development and validation of concept of a LIF prototype. General Information: Two subprojects are considered: - Development of an instrument based on the LIF technique for airborne application. The instrument to be built, OHLIF, will be designed to measure OH, in situ, in a multipass pressurised cell or in ambient air, enabling measurement in the upper troposphere at altitude attainable to an aircraft (about 12 km). - Validation of concept by cross comparison of the results obtained with OHLIF and the results of an existing HOXMAS ion mass spectrometer. This subproject includes the development of a calibrated OH source and the adaptation to an aircraft platform. An airborne experiment performed around 10-12 km will be carried out with both instruments, simultaneously measuring OH. In the first step OH will be formed from HO2 by conversion with NO, taking advantage of the large concentration of HO. A calibrated and validated airborne instrument using the LIF technique for HO2 detection and measurement in the upper troposphere will be available at the end of the project. This instrument will be used as a prototype for the next step. i.e. the development of an automated balloon borne instrument for simultaneous measurement of OH and HO2. Prime Contractor: Office National d Etudes et de Recherches Aerospatiales, Department of Physics; Chetillon; France.