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

A doubling-adding method to include multiple orders of rotational Raman scattering 29
2.4 Simulation of multiple inelastic scattering in the Earth’s atmosphere
To study the effect of multiple orders of inelastic Raman scattering in the Earth’s atmosphere, we
used a model atmosphere consisting of two homogeneous layers which represent the troposphere and
the stratosphere. The composition of the two layers is summarized in Table 2.1 and their optical
properties are given in Fig. 2.3. In the ultraviolet and shortwave visible, the scattering properties of
the layers are mainly determined by the well-mixed gases N 2 , O 2 and Ar, whereas ozone is the most
relevant atmospheric absorber in this part of the spectrum. Aerosols, NO 2 and other trace gases were
ignored and the surface was considered to be black.
Three wavelength regions were selected to study the Ring features in further detail. First, the
region 280–290 nm was chosen, where ozone absorption dominates and the single scattering approximation
suffices. Second, the region 320-330 nm was evaluated, where ozone absorption and scattering
have a comparable magnitude. Besides filling-in of solar spectral features, filling-in of the Huggins
ozone absorption structures occurs as well in this range. And last, the radiance for 390-400 nm was
selected, where scattering is less strong than in the range 320-330 nm and where scattering is conservative.
The simulated radiance spectra were convolved with a Gaussian instrument function with a
full width at half maximum (FWHM) of 0.2 nm to represent GOME-type measurements. The effect of
multiple orders of Raman scattering in the three spectral windows can be studied by comparing fillingin
spectra simulated with the DA model and with the DA1 model, respectively (see Section 2.3.1).
The result is shown in the lower panels of Fig. 2.4.
At the shorter wavelengths, from 280 to 290 nm, the filling-in spectra f DA1 and f DA agree very
well, because both multiple elastic and multiple inelastic scattering are of minor importance in this
spectral range. In the second spectral range 320–325 nm, where the multiple scattering contribution
makes up more than half of the total signal, we observe an offset in the difference f DA1 − f DA of
about 0.5 %. This is explained by the fact that the DA1 approach ignores photons which are multiply
Raman scattered. On top of this, we see that multiple inelastic scattering causes structures of about
±0.1 % which are partly related to extra filling-in of the Huggins ozone absorption bands and partly
to the extra filling-in of moderate solar Fraunhofer lines. For example at the location of a prominent
Huggins absorption band feature at 325.00 nm, about 0.6% of the total filling-in can be attributed to
multiple inelastic scattering. In the range 390-400 nm, two prominent Ring features appear at the
Ca II K and H line in the solar spectrum. The multiple inelastic scattering contribution in the Ca II
K and H line is about 0.7% at a spectral resolution of 0.2 nm. Again, a continuum offset of 0.2%
is observed, which is smaller than in the 320–325 nm range due to less strong multiple scattering.
Due to pronounced Fraunhofer lines, the contribution of multiple inelastic Raman scattering to the
filling-in structures, i.e. 0.5%, is stronger than the contribution to the continuum.
In conclusion, the contribution of multiple inelastic scattering has to be taken into consideration
in the spectral ranges 320-330 nm and 390-400 nm where multiple scattering is important. Although
the multiple scattering contribution reaches its maximum around 320-330 nm, the lack of prominent
Fraunhofer lines in this spectral window prevents a strong impact to be seen in a filling-in spectrum.
In the range 390-400 nm on the other hand, where the contribution of multiple scattering is smaller,