Rotational Raman scattering in the Earth's atmosphere ... - SRON

Introduction 5
and Spurr, 1997, Vountas et al., 1998, Sioris and Evans, 1999, Stam et al., 2002, Langford et al.,
2007]. The same phenomenon is observed when an Earth radiance spectrum is compared to a direct
solar irradiance spectrum that is measured from space by a satellite instrument. This depletion of the
Fraunhofer lines in an Earth radiance spectrum is commonly referred to as the Ring effect after one
of its discoverers.
Due to the Ring effect the Fraunhofer lines in an Earth spectrum and in a solar spectrum do not
cancel out in a reflectivity spectrum. A close look at a typical GOME reflectivity spectrum in the
zoom-in of Fig. 1.2 reveals filling-in structures that are typically larger than trace gas absorption
features: in the order of a few nm broad and can be more than 10% in the reflectivity. The Ring
effect structures are most prominent in the ultraviolet wavelength range (λ< 400 nm). There are three
reasons for this. First, the solar spectrum shows more pronounced Fraunhofer lines in this wavelength
range than at longer wavelengths. Secondly, light scattering by N 2 and O 2 molecules is stronger at
these shorter wavelengths. Thirdly, the exact spectral resolution of the satellite instrument plays a
role. GOME, OMI, SCIAMACHY and GOME-2 have a finer instrument spectral resolution in the
ultraviolet wavelength range, which makes the Ring effect structures stronger.
Light scattering by air molecules – The key to understanding the Ring effect is understanding molecular
scattering processes. Near the end of the 19 th century, Lord Rayleigh (John William Strutt;
1842–1919) made a thorough study of the blue color of the sky [Howard, 1964]. He suggested that
this color is caused by the interaction of air molecules with incident sunlight [Lord Rayleigh, 1899].
Light consists of fast-oscillating electric and magnetic fields that force the electric charges inside the
air molecules to oscillate. The oscillating electric charges emit light of their own, which is referred to
as secondary light or as scattered light. Lord Rayleigh assumed that the scattered light always has the
same wavelength as the incident light. He derived that the intensity of the scattered light is inversely
proportional to the wavelength to the fourth power [Lord Rayleigh, 1871]. This relationship is caused
by the increased energy content of light with short wavelengths compared to light with longer wavelengths.
The more energetic short wavelength light imposes a stronger force on the electric charges to
move and therefore drives them to radiate more intensely in all directions (e.g Rybicki and Lightman
[1979]).
In 1923 an Indian scientist named Venkata Raman (1888-1970) discovered a ‘new type of secondary
radiation’ [Raman and Krishnan, 1928]. He found that a fraction of the scattered light has
a different wavelength than the one of the incident light. This type of scattering is called inelastic
scattering and involves an energy exchange between the light and the molecules. This phenomenon
is now referred to as the Raman effect. The Raman effect provides a powerful spectroscopic tool because
the spectrum of the inelastically scattered light provides a unique fingerprint for each molecule
species [Bransden and Joachain, 1996, Atkins and de Paula, 2002, Weber, 1979, Long, 1977]. This
diagnostic tool is used in many active sensing applications today. With the help of a powerful lamp or
laser that has a well-defined incident wavelength various substances can be characterized and quantified
by studying the spectrum of the inelastically scattered light. This tool is used in a wide range of
disciplines, ranging from planetary exploration (e.g. [Ellery et al., 2004]) to applications in biology,
medicine, art, jewelry and forensic science.