Air-Mass Factor using Look-up Tables
There are two approaches taken to convert the SO2 slant column density
(SCD), retrieved with a DOAS technique, into a vertical column density
(VCD). Both approaches use an air-mass factor (AMF). This page describes the
determination of the air-mass factor using pre-calculate look-up tables,
made with an off-line radiative transfer model.
The approach described here is applied to data from SCIAMACHY and
GOME-2, 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
SO2 column from OMI.
Determining the air-mass 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 a-priori information on the SO2 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 clear-sky conditions.
The AMF is pre-calculated with the radiative transfor model LIDORT in the
form of a look-up 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 SO2 (low or high), a
likely a-priori SO2 profile is chosen and an AMF is interpolated from the
look-up table. The look-up table does not have an entry for the ozone
concentration, as the SO2 AMF depends only weakly on the ozone
concentration; an ozone concentration of 350 DU was assumed for the
In order to find the AMF the elevation of the SO2 plume (or of the cloud) must be
given, as the AMF is different for different SO2 plume heights. This is
shown in the following image for a typical viewing geometry. The look-up
table therefore also has an entry for the plume height; for the look-up
table it was assumed that the thickness of the plume is 1 km (this
value is not very critical).
Air-mass factors (AMF) as function of Solar Zenith Angle (SZA) for an SO2
plume of 1 km thickness and with low to moderate SO2 concentrations
(up to 10 DU). The viewing geometry is looking straight down to a surface
with albedo 0.05 at about sea-level.
In the practice of the SO2 retrieval, especially in an automatic
processing, there is no information available on the altitude of the
SO2 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 GOME-2
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 GOME-2
7.5 km for OMI
6.0 km SCIAMACHY
It represents moderate volcanic eruptions.
- High AMF: plume height assumed at
15.0 km for GOME-2
17.0 km for OMI
14.0 km SCIAMACHY
It represents explosive eruptions.
To see more details bout what is done for SO2 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 clear-sky part will be given a value, as that can in
principle be computed always.