Objectives 1. To develop the measurement technique further, providing more accurate measurements and extend the method to a larger number of trace species 2. To monitor the presence of CFC:s and other longlived anthropogenic tracers in the stratosphere 3. To use long-lived anthropogenic species as tracers of atmospheric motion, in particular for comparison with atmospheric models Reserarchers: Descartes is a joint research programme currently involving N.R.P Harris and J.A. Pyle, Centre for Atmospheric Science at the Department of Chemistry, University of Cambridge, U.K., and Hans Nilsson and Johan Arvelius, Swedish Institute of Space Physics, Kiruna, Sweden
General principle. Samples are taken by pumping air through a sample tube containing an adsorbent (Carboxen 569). Samples are taken at predefined atmospheric pressure levels (which can be programmed for each flight). The flow through the sample tube is measured and the average flow stored in the onboard computer memory. The sample tubes are mounted on a 16 position valve, and 15 of those sample tubes are used for samples. The sixteenth is a 'waiting position' used while passing up and down through the troposphere, to prevent pollution of the stratospheric samples. A bottle containing a standard of trace species can be used for taking standard samples at different pressure levels for calibration and instrument performance monitoring. To prevent pollution of the samples the sample tubes and the 16 position valve are kept in an airtight 'sample box', which is kept under inert nitrogen atmosphere at about 100 hPa above ambient pressure. Analysis. The trapped trace gases are thermally desorbed by heating the sample tube to about 200°C. The heating is done in a number of 5 second cycles. For each heating cycle the desorbed trace gases are passed into a gas chromatograph (GC) using an electron capture detector (ECD). The setup is described in Danis et al. . The response of the ECD is monitored by taking standard samples with tropospheric concentrations of N2O, CFC-11, CFC-12 and CFC-113 between the analysis of each sample tube. Calibration of the sample tube - GC system is achieved by passing known amounts of the above mentioned gas standard through the sample tubes in the laboratory, and then using the standard analysis procedure for stratospheric samples. Initial investigations indicate that the ECD response is non-linear, and most of the uncertainty in the quantitative estimate of CFC-11 and CFC-12 is due to uncertainties in the calibration of the ECD response (Stacey ).
University of Cambridge, Centre for Atmospheric Science at the Department of Chemistry
NILU database within the EU funded Theseo projects