234
C. Weitkamp
Another example in which S02 distribution measurements with ARGOS played an important
role is the determination of the minimum safe distance between inhabited areas and naturalgas production facilities. In case of a blowout caused by a breaking pipe or valve people living
downwind of such a facility can be affected both by fire and by the toxic effects of gaseous sulfur
compounds contained in so-called sour-gas deposits. As results of model calculations differed by
a factor of 100 for relatively small variations in boundary conditions, a blowout was simulated
by the release and ignition of a large quantity of sour gas, and the resulting distribution of S02
and aerosols was measured with ARGOS.
A third example for S02 measurements with ARGOS is a campaign in the Erzgebirge, a mountain range a little more than 1000 m high extending from southwest to northeast and separating
Germany from the Czech Republic. Once densely forested, the area suffered from heavy damage
when mining and roasting of sulfidic ore began in the middle ages. In the early 19th century
silver and zinc mining was gradually given up, but after a short phase of recovery forest damage took over again and has gotten so severe today that bare rock appears in many places,
leaving authorities helpless as to how reforestation efforts should be started. Although it had
been clear that S02 was still the culprit, and that large quantities of the gas are released from
coal-fired power plants is the Czech Basin, the exact mechanism responsible fot the magnitude
of the effect had been unclear. Measurements made with ARGOS from several hilltops down
into the adjacent valleys and up into the troposphere revealed that exceptionally poor vertical
mixing occurs during long period of time and that the gas, instead of being mixed and diluted,
"creeps" through the valleys and up the slopes, strongly damaging the trees' leaves and needles.
Local sources, however, also playa role as was measured at Bad Schandau where the River
Elbe passes through a gorge in the mountains. Lidar measurements up the river (and upwind)
showed comparatively little S02, whereas massive S02 concentrations were observed downwind
of the town. An unexplained phenomenon was the occurrence of S02 in the valley on one side,
yet not in the one on the other side of a mountain top, indicating that such "microorographic"
effects deserve further investigation.
10.3.3 Combined Raman lidar: moisture, cirrus, Pinatubo dust
Because of the small Raman scattering cross sections, the use of the Raman Ii dar technique is
limited to gases with abundances of 10- 4 or more. Even so, high-power lasers, large receiver
telescopes and sensitive detectors along with efficient background-blocking devices are required.
Unlike DAS lidar, on the other hand, Raman lidar is not a doubly-differential technique and
thus much less sensitive to optical misalignment, small wavelength shifts, and all sources of
noise except photon statistics.
A schematic diagram of the GKSS combined Raman lidar setup is shown in Fig. 10.12. For
wavelength separation a grating or a filter polychromator (Fig. 10.13) can be used. In either
case sufficient suppression of the strong elastic backscatter signal in the Raman channels must
be guaranteed. Suppression factors required are 10 7 for the O2 and N2 channels and 10 10 or
better for the H20 and CO2 channels. Table 10.5 gives the most important technical data of
the system (Ansmann et aI., 1992).
The combined Raman lidar was used for many determinations of moisture profiles and profiles of
aerosol properties. Its appeal in cloud studies is the capability to simultaneously measure water
vapor, backscattering, and extinction. In the early days of the system numerous comparisons
were made with results of radiosonde ascents. Deviations at low and medium altitudes between
the lidar and radiosonde data could be explained by the fact that the Ii dar is Eulerian in
nature, whereas the radiosonde can be considered as "semi-Langrangian" as it travels with
the air mass that it measures - horizontally, but not vertically. At greater altitudes, however,
C. Weitkamp
Another example in which S02 distribution measurements with ARGOS played an important
role is the determination of the minimum safe distance between inhabited areas and naturalgas production facilities. In case of a blowout caused by a breaking pipe or valve people living
downwind of such a facility can be affected both by fire and by the toxic effects of gaseous sulfur
compounds contained in so-called sour-gas deposits. As results of model calculations differed by
a factor of 100 for relatively small variations in boundary conditions, a blowout was simulated
by the release and ignition of a large quantity of sour gas, and the resulting distribution of S02
and aerosols was measured with ARGOS.
A third example for S02 measurements with ARGOS is a campaign in the Erzgebirge, a mountain range a little more than 1000 m high extending from southwest to northeast and separating
Germany from the Czech Republic. Once densely forested, the area suffered from heavy damage
when mining and roasting of sulfidic ore began in the middle ages. In the early 19th century
silver and zinc mining was gradually given up, but after a short phase of recovery forest damage took over again and has gotten so severe today that bare rock appears in many places,
leaving authorities helpless as to how reforestation efforts should be started. Although it had
been clear that S02 was still the culprit, and that large quantities of the gas are released from
coal-fired power plants is the Czech Basin, the exact mechanism responsible fot the magnitude
of the effect had been unclear. Measurements made with ARGOS from several hilltops down
into the adjacent valleys and up into the troposphere revealed that exceptionally poor vertical
mixing occurs during long period of time and that the gas, instead of being mixed and diluted,
"creeps" through the valleys and up the slopes, strongly damaging the trees' leaves and needles.
Local sources, however, also playa role as was measured at Bad Schandau where the River
Elbe passes through a gorge in the mountains. Lidar measurements up the river (and upwind)
showed comparatively little S02, whereas massive S02 concentrations were observed downwind
of the town. An unexplained phenomenon was the occurrence of S02 in the valley on one side,
yet not in the one on the other side of a mountain top, indicating that such "microorographic"
effects deserve further investigation.
10.3.3 Combined Raman lidar: moisture, cirrus, Pinatubo dust
Because of the small Raman scattering cross sections, the use of the Raman Ii dar technique is
limited to gases with abundances of 10- 4 or more. Even so, high-power lasers, large receiver
telescopes and sensitive detectors along with efficient background-blocking devices are required.
Unlike DAS lidar, on the other hand, Raman lidar is not a doubly-differential technique and
thus much less sensitive to optical misalignment, small wavelength shifts, and all sources of
noise except photon statistics.
A schematic diagram of the GKSS combined Raman lidar setup is shown in Fig. 10.12. For
wavelength separation a grating or a filter polychromator (Fig. 10.13) can be used. In either
case sufficient suppression of the strong elastic backscatter signal in the Raman channels must
be guaranteed. Suppression factors required are 10 7 for the O2 and N2 channels and 10 10 or
better for the H20 and CO2 channels. Table 10.5 gives the most important technical data of
the system (Ansmann et aI., 1992).
The combined Raman lidar was used for many determinations of moisture profiles and profiles of
aerosol properties. Its appeal in cloud studies is the capability to simultaneously measure water
vapor, backscattering, and extinction. In the early days of the system numerous comparisons
were made with results of radiosonde ascents. Deviations at low and medium altitudes between
the lidar and radiosonde data could be explained by the fact that the Ii dar is Eulerian in
nature, whereas the radiosonde can be considered as "semi-Langrangian" as it travels with
the air mass that it measures - horizontally, but not vertically. At greater altitudes, however,