
AirMass Factor using Lookup Tables
There are two approaches taken to convert the SO_{2} slant column density
(SCD), retrieved with a DOAS technique, into a vertical column density
(VCD). Both approaches use an airmass factor (AMF). This page describes the
determination of the airmass factor using precalculate lookup tables,
made with an offline radiative transfer model.
The approach described here is applied to data from SCIAMACHY and
GOME2, with only minor differences necessary to take the differences
between the instruments (such as the viewing geometry) into account.
For OMI data the approach is rather similar, though there is no real
SCD available; for info and links see the section on the
SO_{2} column from OMI.
Determining the airmass factor
The value of the AMF depends on the length of the light path, the vertical
distribution of absorbing trace gas in the atmosphere, the reflectivity
(albedo) of the earth's surface, etc. The length of the light path depends
on the position of the Sun (expressed in the Solar Zenith Angle, SZA) and
the angle under which the satellite is looking at the atmosphere. For the
vertical distribution apriori information on the SO_{2} profile is used, based
on realistic concentrations.
If there are clouds in the atmosphere, things become more complicated.
Clouds namely reflect (and scatter) incoming sunlight and thus effectively
shield all that is going on below the clouds from the satellite's view.
Clearly, the satellite measurements provide an SCD which contains only
information on the atmosphere above the clouds.
To treat this situation, an "effective" AMF is computed, taking the
cloud fraction (which gives the percentage of the cloud cover) into account.
In the presence of clouds the VCD is clearly less accurate than the VCD
derived under clearsky conditions.
The AMF is precalculated with the radiative transfor model LIDORT in the
form of a lookup table with a set of entries: the time of the year, the
viewing geometry, the SZA, the surface albedo, the cloud fraction and cloud
top pressure, etc. Depending on the value of the SCD of SO_{2} (low or high), a
likely apriori SO_{2} profile is chosen and an AMF is interpolated from the
lookup table. The lookup table does not have an entry for the ozone
concentration, as the SO_{2} AMF depends only weakly on the ozone
concentration; an ozone concentration of 350 DU was assumed for the
lookup table.
In order to find the AMF the elevation of the SO_{2} plume (or of the cloud) must be
given, as the AMF is different for different SO_{2} plume heights. This is
shown in the following image for a typical viewing geometry. The lookup
table therefore also has an entry for the plume height; for the lookup
table it was assumed that the thickness of the plume is 1 km (this
value is not very critical).

Airmass factors (AMF) as function of Solar Zenith Angle (SZA) for an SO_{2}
plume of 1 km thickness and with low to moderate SO_{2} concentrations
(up to 10 DU). The viewing geometry is looking straight down to a surface
with albedo 0.05 at about sealevel.

In the practice of the SO_{2} retrieval, especially in an automatic
processing, there is no information available on the altitude of the
SO_{2} cloud. But the latter is not known from the retrieval
in an automated processing, so an assumption must be made. It has been
decided that the VCD is computed for three different assumed plume heights,
with in all cases an assumed plume thickness of 1 km:
 Low AMF: plume height assumed at
2.5 km for GOME2 2.5 km for OMI 2.0 km for SCIAMACHY
It represents passive degassing of volcanoes and anthropogenic acitivities.
 Middle AMF: plume height assumed at
6.0 km for GOME2 7.5 km for OMI 6.0 km SCIAMACHY
It represents moderate volcanic eruptions.
 High AMF: plume height assumed at
15.0 km for GOME2 17.0 km for OMI 14.0 km SCIAMACHY
It represents explosive eruptions.
To see more details bout what is done for SO_{2} column derived from OMI go to this link.
All three VCDs based on these assumptions are available in the data files. Because the use of the AMF corresponds to different plume heights, we have decided to present plots of the three type of AMF in the NRT and the archive services.
To find an estimate of the VCD for another plume height, one could lineary
interpolated between the three given values.
The Slant Column Densities (SCD) can also be plotted but these are not presented in SACS.
If you are interest by SCD you can contact us and do a request.
If the VCD is available, images of it are shown on the website and the
values are given in the data files. The data files also given an error
estimate, which simply is the SCD error estimate divided by the AMF;
possible errors in the AMF calculation are therefore not taken into account.
If no cloud cover information is available for a given ground
pixel, the AMF and VCD of that ground pixel cannot be computed. In that
case, the entries in the data files will get the "no data" value. Only
the AMF for the clearsky part will be given a value, as that can in
principle be computed always.

