Trace gas absorption spectroscopy

Extracted from Chimot, J., Global mapping of atmospheric composition from space – Retrieving aerosol height and tropospheric NO2 from OMI, PhD book, Delft University of Technology (TU Delft), The Royal Netherlands Meteorological Institute (KNMI), July 2018.

 

The interaction of solar radiation with matter provides a powerful way to investigate the chemical composition of the complex atmospheric mixture. In particular, absorption spectroscopy allows to focus on the spectral regions of interest for tropospheric trace gas analysis (Burrows et al., 2011).

For instance, since UV radiation is absorbed at different altitudes by ozone, exploitation of the UV spectral band contains information of the ozone height distribution. The red-brownish color of NO2 – Nitrogen dioxide illustrates its transmission property in the red spectral domain while it absorbs the Sunlight in the blue band (about 405-470 nm). Green-house gases CO2 – Carbon dioxide and CH4 – Methane interact further in the shortwave and thermal infrared range.

Figure8
SCIAMACHY nadir spectrum expressed as Sun-normalized radiance (Buchwitz et al., 2000; Schneising et al., 2008, 2009; Burrows et al., 2011).

Trace gas research studies focusing on their absorption properties in the UV-visible spectral range (i.e. no thermal emission) focus on the Beer-Lambert law (or Bouguer-Lambert law) describing the light attenuation as a function of the travelled distance s, gas concentration and its spectral absorption intensity:

Screen Shot 2018-07-24 at 21.20.40

with I0(λ,θ,φ) the initial intensity, I(λ,θ,φ) the measured radiation intensity (or radiance), ρ(s) the trace gas density and σ(λ) the absorption cross-section of a given gas measured in a laboratory. This last parameter can be considered as the effective area of the molecule to remove photon’s energy from incoming radiation. Note that the product σ(λ) · ρ(s) · ds gives the optical thickness.