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

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Introduction
1.1 Observing skylight
Looking through an airplane window one can admire the deep blue color of the sky. This blue color
arises from molecules in the Earth’s atmosphere, of which the most abundant species in the dry atmosphere
are: nitrogen (N 2 ; volume mixing ratio of 78.09%), oxygen (O 2 ; 20.95%), and argon (Ar;
0.93%). These molecules, which have diameters that are more than a thousand times smaller than
the wavelength of the incident sunlight, scatter sunlight in all directions [Minnaert, 2004]. Light with
short wavelengths (i.e blueish light) is more strongly scattered than light with longer wavelengths.
The human eye contains two groups of light receptor cells, the rods and cones. The rods are sensitive
to dim light and mediate coarse black and white vision. The cones are specialized for daylight and
come in three types 1 that mediate detailed color vision [Stockman and Sharpe, 2000]. When looking
at the sky the cones that are most sensitive to blue light are triggered more than the ones that predominantly
sense blueish-green and yellowish-green light. This information is sent to the brain where the
image of a blue sky is formed [Kandel et al., 1991].
Over the past centuries a plethora of instruments has been developed that complement our eyes and
brains. Sir Isaac Newton (1643–1727) demonstrated with a simple glass prism that light is composed
of different colors. Spectrometers can be built that accurately measure this, i.e. the intensity of light
as a function of wavelength. It was found that visible light covers only a small part (400–700 nm) of
the whole spectrum of electromagnetic radiation. Development of new detector technology enabled
humans to study spectra of invisible radiation as well. During the last decades of the twentieth century
satellites have been launched with spectrometers on board. These space-borne spectrometers act as
artificial ‘eyes in the sky’ that monitor the Earth from above. Unlike our eyes that contain three types
of spectral detectors 2 , many of these spectrometers contain several thousand different spectral detectors
that together cover a similar wavelength range as the human eye. Examples of such space-borne
spectrometers are the Global Ozone Monitoring Experiment and its successor (GOME and GOME-2),
the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY), and
the Ozone Monitoring Experiment (OMI). Table 1.1 lists several properties of these spectrometers.
1 Approximately 8% of the male readers of this thesis has effectively two types of cones and is thus color blind.
2 The spectral responses of the three types of detectors in the human eye (the S, M and L cone) show a maximum near
442 nm, 543 nm and 570 nm, and have a full width at half maximum of 57 nm, 85 nm, and 111 nm, respectively [Stockman
and Sharpe, 2000].