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2.1. TROPOSPHERIC RESEARCH GROUP 23
2.1.5 Ground based MAX-DOAS measurements
Participating scientists Ossama Ibrahim, Thomas Wagner, Ulrich Platt
Abstract The Multi-axis DOAS technique uses scattered sunlight to obtain information about the
trace gases in the atmosphere by pointing a telescope to different elevation angles and thus receiving
light from different directions. Using Multi-axis DOAS instrument on a stable or movable ground
based platform can also be used to validate DOAS measurements done by satellite instruments.
Figure 2.5: Latitudinal distribution of Stratospheric NO2 VCDs in comparison with SCIAMACHY
results as an example of satellite data validation using ground based MAX-DOAS. The morning
measurements were taken between 5-7 AM O’clock morning and the afternoon ones are between 5-7
PM O’clock afternoon and SCIAMACHY data were collected at 10 AM local equator time.
Background The measurement of trace gases
(identification and quantification) is important for
the investigation of the chemical and physical processes
in the atmosphere. The development of the
Multi Axis DOAS was an important milestone on
the way of DOAS technique evolution. The ability
to direct telescopes to different elevation angles
(multi-axis) allows to emphasize absorption
paths from lowermost atmospheric layers onto the
stratosphere. Measurements from the ground can
provide information on the vertical profile of the
absorbing trace gases under investigation.
Funding IMPRS fellowship
Methods and results Measurements using
Multi-axis DOAS (Differential Optical Absorption
Spectroscopy) on board of a ship platform
(Polarstern research vessel of Alfred-Wegner Institute
für Polar- und Meeresforschung) were carried
out successfully in a latitudinal range between
52 ◦ N and 72 ◦ S every year in the last four years
and gave the possibility to measure several trace
gases like NO2, O3 and BrO in the stratosphere
and/or in the marine boundary layer (MBL). The
measurements showed elevated tropospheric NO2
values in the English channel area and the existence
of the tropospheric BrO in the MBL westerly
from Africa in the latitudinal range between
10 ◦ N and 30 ◦ N, which cannot be seen by satellite
instruments because of their higher detection
limit in mid-latitudes. Also a latitudinal cross sectional
distribution of the stratospheric NO2, O3
and BrO were compared to those obtained from
satellite measurements from GOME and SCIA-
MACHY (see figure 2.5). Ground based measurements
from a stable platform in Cabauw in Holland
were performed to retrieve information about
the tropospheric values of trace gases like NO2
and compare them to those obtained from satellite
OMI. Arranging the viewing geometry using
three different telescopes in three viewing planes
can provide some data on the variation of the NO2
concentrations within an OMI pixel. For information
about the satellites see satellite group report.
Outlook/Future work More investigation of
trace gas concentrations in the troposphere and
stratosphere using ground based measurements
from different platforms comparing with results
from satellite measurements for validation of especially
the newer generations SCIAMACHY and
OMI is going to be performed in the near future.
The Multi-axis DOAS will be used on a moving
platform (car) in urban areas to make estimation
of the in and out flux of trace gases from cities besides
measurements for single plumes from powerplants
or industrial activities. This is a promising
application which has already started.