Fraunhofer-Institut für Atmosphärische Umweltforschung IFU

Filters

27
27
27
Reset filters

Settings
Now showing 1 - 10 of 27
  • Publication
    Multipass open-path Fourier-transform infrared measurements for nonintrusive monitoring of gas turbine exhaust composition
    ( 2005)
    Schafer, K.
    ;
    Brockmann, K.
    ;
    Heland, J.
    ;
    Wiesen, P.
    ;
    Jahn, C.
    ;
    Legras, O.
    The detection limits for NO and NO2 in turbine exhausts by nonintrusive monitoring have to be improved. Multipass mode Fourier-transform infrared (FTIR) absorption spectrometry and use of a White mirror system were found from a sensitivity study with spectra simulations in the mid-infrared to be essential for the retrieval of NO2 abundances. A new White mirror system with a parallel infrared beam was developed and tested successfully with a commercial FTIR spectrometer in different turbine test beds. The minimum detection limits for a typical turbine plume of 50 cm in diameter are approximately 6 parts per million (ppm) for NO and 9 ppm for NO2 (as well 100 ppm for CO2 and 4 ppm for CO).
  • Publication
    Nonintrusive optical measurements of aircraft engine exhaust emissions and comparison with standard intrusive techniques
    ( 2000)
    Schäfer, K.
    ;
    Heland, J.
    ;
    Lister, D.H.
    ;
    Wilson, C.W.
    ;
    Howes, R.J.
    ;
    Falk, R.S.
    ;
    Lindermeir, E.
    ;
    Birk, M.
    ;
    Wagner, G.
    ;
    Haschberger, P.
    ;
    Bernard, M.
    ;
    Legras, O.
    ;
    Wiesen, P.
    ;
    Kurtenbach, R.
    ;
    Brockmann, K.J.
    ;
    Kriesche, V.
    ;
    Hilton, M.
    ;
    Bishop, G.
    ;
    Clarke, R.
    ;
    Workman, J.
    ;
    Caola, M.
    ;
    Geatches, R.
    ;
    Burrows, R.
    ;
    Black, J.D.
    ;
    Herve, P.
    ;
    Vally, J.
    Nonintrusive systems for the measurement on test rigs of aeroengine exhaust emissions required for engine certification (CO, NO sub x, total unburned hydrocarbon, and smoke), together with CO sub 2 and temperature have been developed. These results have been compared with current certified intrusive measurements on an engine test. A spectroscopic database and data-analysis software has been developed to enable Fourier-transform Infrared measurement of concentrations of molecular species. CO sub 2, CO, and NO data showed agreement with intrusive techniques of approximately plus/minus 30 %. A narrow, band spectroscopic device was used to measure CO sub 2 (with deviations of less than plus/minus 10 % from the intrusive measurement), whereas laser-induced incandescence was used to measure particles. Future improvements to allow for the commercial use of the nonintrusive systems have been identified and the methods are applicable to any measurement of combustion emissions.
  • Publication
    Solar actinic spectroradiometry: a technique for measuring photolysis frequencies in the atmosphere
    ( 1999)
    Hofzumahaus, A.
    ;
    Kraus, A.
    ;
    Müller, M.
    A spectroradiometer has been developed for direct measurement of the solar actinic UV flux (scalar intensity) and determination of photolysis frequencies in the atmosphere. The instrument is based on a scanning double monochromator with an entrance optic that exhibits an isotropic angular response over a solid angle of 2pi sr. Actinic flux spectra are measured at a resolution of 1 nm across a range of 280-420 nm, which is relevant for most tropospheric photolysis processes. The photolysis frequencies are derived from the measured radiation spectra by use of published absorption cross sections and quantum yields. The advantage of this technique compared with the traditional chemical actinometry is its versatility. It is possible to determine the photolysis frequency for any photochemical reaction of interest provided that the respective molecular photodissociation parameters are known and the absorption cross section falls within a wavelength range that is accessible by the spectroradiometer. The instrument and the calibration procedures are described in detail, and problems specific to measurement of the actinic radiation are discussed. An error analysis is presented together with a discussion of the spectral requirements of the instrument for accurate measurements of important tropospheric photolysis frequenciesJ(O(exp 1)D), J(NO2), J(HCHO) . An example of measurements from previous atmospheric chemistry field campaigns are presented and discussed.
  • Publication
    Method to determine snow albedo values in the ultraviolet for radiative transfer modeling
    ( 1999)
    Schwander, H.
    ;
    Mayer, B.
    ;
    Ruggaber, A.
    ;
    Albold, A.
    ;
    Seckmeyer, G.
    ;
    Koepke, P.
    For many cases modeled and measured UV global irradiances agree to within +/-5% for cloudless conditions, provided that all relevant parameters for describing the atmosphere and the surface are well known. However, for conditions with snow-covered surfaces this agreement is usually not achievable, because on the one hand the regional albedo, which has to be used in a model, is only rarely available and on the other hand UV irradiance alters with different snow cover of the surface by as much as 50%. Therefore a method is given to determine the regional albedo values for conditions with snow cover by use of a parameterization on the basis of snow depth and snow age, routinely monitored by the weather services. An algorithm is evolved by multiple linear regression between the snow data and snow-aIbedo values in the UV, which are determined from a best fit of modeled and measured UV irradiances for an alpine site in Europe. The resulting regional albedo values in the case of snow are in the 0.18-0.5 range. Since the constants of the regression depend on the area conditions, they have to be adapted if the method is applied for other sites. Using the algorithm for actual cases with different snow conditions improves the accuracy of modeled UV irradiances considerably. Compared with the use of an average, constant snow albedo, the use of actual albedo values, provided by the algorithm, halves the average deviations between measured and modeled UV global irradiances.
  • Publication
    Application of antimonide lasers for gas sensing in the 3-4-mu m range
    ( 1999)
    Werle, P.
    ;
    Popov, A.
    Antimonide semiconductor laser devices designed for continuous-wave emission in the 3-4-mu m spectral range have been investigated with respect to spectroscopic applications. Representative data on the mode structure, output power, noise characteristics, far-field pattern, and modulation response are presented. Selected laser devices have been applied for methane (CH4) and formaldehyde (HCHO) measurements by use of a high-frequency modulated diode laser spectrometer. From an Allan variance analysis of experimental data a detection limit for HCHO of 120 pptv (where 1 pptv = 10(exp -12) volume mixing ratio) with a 40-s integration time and for CH4 of 2 ppbv (where 1 ppbv = 10(exp -9) volume mixing ratio) with 20-s integration time were determined. The results show that, for selected gases, InAsSb lasers can be an alternative to lead-salt diode lasers.
  • Publication
    Stark-modulation-enhanced FM-spectroscopy
    ( 1999)
    Werle, P.
    ;
    Lechner, S.
    Diode lasers are a versatile tool for fundamental atomic and molecular spectroscopy and have increasing applications in gas analysis and diagnostics. In order to obtain quantum limited performance, the problem of the suppression of time-dependent unwanted background structures superimposed to the desired signal from the spectral feature under investigation has to be addressed. Sample modulation enhanced FM-spectroscopy as described in this paper combines laser frequency modulation (FM) with a sample modulation, based on the Stark effect in molecular spectra. The transitions 1(10) - 1(11) at 1746.2 cm(exp -1) and 5(33) - 4(32) at 1759.9 cm(exp -1) in the v2-band of H2CO have been used in experiments with static and AC electric fields up to 2 kV cm(exp -1). An intercomparison of the FM technique and the FM-Stark double modulation scheme demonstrates the remarkable potential of this technique for background suppression.
  • Publication
    A review of recent advances in semiconductor laser based gas monitors
    ( 1998)
    Werle, P.
    When first tunable diode lasers were developed in the mid-1960s they found immediate application as much needed tunable sources for high-resolution laser absorption spectroscopy commonly referred to as TDLAS (tunable diode laser absorption spectroscopy). Substantial improvements in sensitivity and detection speed have been achieved since then and an increasing number of laser based gas monitoring applications has been reported. Diode lasers in general continue to find application to research areas requiring very high resolution, accuracy and sensitivity. In this article the main features and applications of tunable diode laser absorption spectroscopy will be reviewed. The main characteristics of the currently available semiconductor diode lasers with respect to spectroscopic applications and sensitive detection techniques will be discussed. The focus will be on high frequency modulation schemes, which have been developed and utilized for a series of gas monitoring applications in the p ast. Recent approaches in sample modulation enhanced high frequency modulation spectroscopy developed to cope with limitations caused by signal instability will be addressed together with the future perspectives of TDLAS.
  • Publication
    Remote sensing of gas emissions on natural gas flares
    ( 1998)
    Haus, R.
    ;
    Wilkinson, R.
    ;
    Heland, J.
    ;
    Schäfer, K.
    Emissions from operational natural gas flares are examined by a remote sensing technique using a commercial high-resolution Fourier-Transform-Infrared Spectrometer. The thermal radiation emitted by the post-combustion gas is analyzed to determine plume temperatures and concentrations of CH4, CO2, CO, NO, and H2O. The Multi-component Air Pollution Software MAPS is applied which is based on radiative transfer line-by-line calculations and least-square fit procedures. Emission rates and combustion efficiencies are calculated which indicate that the environmental impact of methane emissions from natural gas flares is small.
  • Publication
    Infrared spectroscopy of tropospheric trace gases. Combined analysis of horizontal and vertical column abundances
    ( 1997)
    Sussmann, R.
    ;
    Schäfer, K.
    Vibration-rotation absorptions in high-resolution Fourier-transform infrared spectra from a 246-m horizontal path were used to derive local concentrations of trace gases at the Alpine observatory at the Zugspitze summit, Germany (2964 m above sea level). The analysis was performed by using the line-by-line nonlinear least-squares spectral fitting software, SFIT, based on the 1992 HITRAN line parameter compilation. (HITRAN is a high-resolution transmission molecular absorption database.) A comparison to in situ measurements shows an agreement of better than 4.3 per cent for the species CO, CO2, and CH4. Using the same spectrometer and analysis software, we obtained the vertical column density of N2O together with an adjusted vertical volume mixing ratio distribution. This translates to a local N2O concentration at the altitude of Zugspitze that agrees with the horizontal path-derived value to within 1 per cent.