  SAOZ (Systeme d'Analyse par Observation Zenitale) is a balloon born
experiment which determines the column density of several molecular species
from the visible spectrum of sunlight. We will use sequence of spectra
collected during a sunset to discuss atmospheric extinction, and the nature of
the radiation field in the atmosphere. The radiation field in the atmosphere
is, from daylight to sunset, and with a clear sky, dominated by light coming
from the direction of the sun. This light is composed of direct sunlight
(extinguished by the gas), and of sunlight forward-scattered by aerosols. As
the sun sets, aerosol scattering is first perceived towards the UV. It
progressively replaces direct sunlight over all of the spectrum. Our analysis
permits fixing the main parameters of each component of the radiation field at
any time. The fits we find for the extinction of sunlight in the atmosphere
must also apply to starlight. Thus, the present work can be used in astronomy
to correct ground-based spectral observations for extinction in the atmosphere.
