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

Introduction 3
Extracting the wealth of information that is contained in the measured reflectivity spectra requires
models that accurately describe the measurement with all its spectral features. In this thesis we focus
on improving the modeling of the spectral features that are associated with light scattering by
molecules. Before we discuss the details of molecular light scattering processes (Section 1.3), we
will point out some of these spectral signatures. The inset in Fig. 1.2 shows a zoom-in on a typical
clear-sky GOME reflectivity spectrum in the wavelength range 350–400 nm. The fact that light
with short wavelengths is more strongly scattered by molecules than light with longer wavelengths
is clearly seen in this particular wavelength range. The spectral continuum is commonly described
by a purely elastic light scattering process called Rayleigh scattering. However, a closer inspection
shows spectral fine-structure that is also caused by scattering by molecules. In each scattering event
approximately 4% of the light is inelastically scattered by molecules due to a process called Raman
scattering (e.g. [Joiner et al., 1995]). This type of inelastic scattering causes spectral fine-structure in
the reflectivity spectrum, which is also referred to as the Ring effect as will be explained in the next
section.
This thesis deals with the development of accurate radiative transfer models that adequately describe
the full reflectivity spectrum, with emphasis on including the Ring effect. With the help of such
accurate models the retrieval of trace gas concentrations from satellite-based reflectivity spectra can
be improved.
1.2 The Ring effect: a short historical background
Filling-in of Fraunhofer lines – In 1802 William Wollaston noticed lines of low intensity in a solar
spectrum that was measured on Earth. He figured – incorrectly – that these had to be the division
lines between the different colors. Joseph von Fraunhofer rediscovered the lines in 1814 and studied
them in greater detail. By 1815 he had catalogued more than 300 of them, assigning Roman letters to
the most prominent ones. Each Fraunhofer line, as was found later by Kirchhoff and Bunsen (1861),
can be attributed to a specific chemical element. Most Fraunhofer lines in a solar spectrum that is
measured on the ground are absorption lines of chemical elements in the upper layers of the Sun, but
some of them turned out to be of terrestrial origin. For example, the A Fraunhofer line near 760 nm
is a strong absorption band of O 2 , which was later referred to as the O 2 A absorption band.
In 1961 Grainger and Ring compared a diffuse skylight spectrum (i.e. a spectrum of the sky
measured during the day, but pointing away from the solar disk) with a direct lunar spectrum. They
discovered that the H Fraunhofer line near 397 nm was less deep in the diffuse skylight spectrum
than in the direct lunar spectrum [Grainger and Ring, 1962]. Grainger and Ring proposed that daylight
airglow might be responsible for this, but this hypothesis was later rejected [Brinkmann, 1968,
Kattawar et al., 1981]. Since then several mechanisms have been proposed to explain the filling-in,
for example by aerosol fluorescence [Noxon and Goody, 1965] and surface reflection effects such as
plant fluorescence [Sioris et al., 2003]. Nowadays scientists concur that the filling-in of Fraunhofer
lines in a daytime spectrum is predominately caused by inelastic scattering by N 2 and O 2 molecules
in the Earth’s atmosphere [Joiner et al., 1995, Fish and Jones, 1995, Burrows et al., 1996, Chance