Laboratory based measurements

!! NEWS !!

27 June 2011 - The new JPL evaluation Data Base (JPL 10-6) "Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies" has been released.

This is the seventeenth in a series of evaluated sets of rate constants and photochemical cross sections compiled by the NASA Panel for Data Evaluation. The data are used primarily to model stratospheric and upper tropospheric processes, with particular emphasis on the ozone layer and its possible perturbation by anthropogenic and natural phenomena.

Copies of this evaluation are available in electronic form and may be printed from the following Internet URL: http://jpldataeval.jpl.nasa.gov/ but are also available here.

The following data sets obtained under the IASB-BIRA / ULB collaboration have been selected as recommendations:

Moreover, the following references have been mentioned for the SO2 absorption cross sections : Vandaele et al., JGR 99 (1994) and Hermans et al., JQSRT 110 (2009). However no recommendation could be drawn "due to the highly structured nature of the spectrum and [cross sections] should be obtained from the original literature".

3 June 2011 - GEISA 2009 paper accepted for publication.

Scientists from IASB-BIRA and ULB participated to the writing of the GEISA 2009 paper, and provided data for the GEISA database. Our absorption cross sections for SO2 and BTX have been included in the latest version of the data base.

Cross sections and spectroscopic molecular parameters

Most of the atmospheric experiments rely today on the use of spectroscopic techniques to measure the concentrations of atmospheric constituents. These methods require the accurate knowledge of reference spectra and spectroscopic molecular parameters of the species to be detected.

These reference spectra, or absorption cross-sections, are obtained in the laboratory under controlled conditions of temperature and pressure. The cross-sections are determined using the Beer-Lambert law describing the attenuation of light by a homogeneous absorbing system. For a system containing only one species, this law states that :

I(l)=Io(l) exp(-nd s(l))

where Io is the incident light intensity, I is the intensity measured at a distance d (cm) from the source, n is the concentration of the absorber (molec/cm3) and s(l) is its absorption cross-section, and is expressed in cm2/molec. Absorption cross-sections usually depends on the temperature of the absorber, and can be influenced by its pressure.

The BISA, in collaboration with

  • the Laboratoire de Chimie Physique Moléculaire of the Université Libre de Bruxelles (ULB) and
  • the Groupe de Spectrométrie Moléculaire et Atmosphérique of the Université de Reims (GSMA, France),

has performed measurements of the absorption cross-sections of some atmospheric species, such as SO2, CS2, O2 and its collisonal induced absorption, NO2, and H2O.

bruker

Measurements have been performed using Fourier transform spectrometers (BRUKER IFS120HR and 120M), whose characteristics are the following :

      • Spectral Interval : 650-55000 cm-1
      • Resolving Power : 106
      • Max Resolution : 0.008 cm-1
      • Accuracy on wavenumbers : 0.005 cm-1

Such an instrument presents some advantages against conventional grating spectrometers such as a high sensitivity as more light attains the detector, high accuracy on the wavenumber scale, large spectral intervals are recorded in one run, the instrumental function is well defined.

The BRUKER 120M is a mobile version of the high resolution BRUKER IFS120HR, which can be easily used during measurement campaigns outside the laboratory.

multiple reflection cell

Some of the measurements have been performed using the long path absorption cells of the Groupe de Spectrométrie Moléculaire et Atmosphérique of the Université de Reims. This laboratory has developped two multiple reflections cells.

The first one is 5 m long, can produce path lengths up to 300 m in the visible region, can be cooled down to 220 K and is thermostatized. The second cell is 50 m long and path lengths of a few kms have been used.

 

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