Measurements Of Pollution In The Troposphere

Project Description
The MOPITT launched on the AM-1 platform of NASA's Earth
Observing System (EOS). The AM-1 satellite was placed in a
705km, sun-synchronous orbit with a 10:30am equator crossing
time. MOPITT will measure carbon monoxide and methane in the
troposphere over the entire globe for a period of five years.

Despite the fact that we all live in the troposphere, monitoring
of the tropospheric composition from space has lagged
considerably behind our monitoring of the upper regions of the
earth's atmosphere mainly because of the technical difficulty of
such measurements. The presence of the earth's surface provides
considerable interference to most measurement methodologies and
the presence of clouds further impedes the mission. Overcoming
these problems requires a very precise instrument with a very
high performance.

We want to monitor carbon monoxide and methane because they will
help us understand how the troposphere reacts to various
stimuli. These can range from natural phenomena such as the
growth of forests, through agricultural sources such as rice
paddies, to catastrophic events such as biomass burning. Most of
these sources can, and indeed are, being modified by human
activity on the planet.

Carbon monoxide is particularly interesting because of its
potential for showing us how chemicals are transported in the
troposphere as well as giving us information about chemical
reactions in the troposphere.

Measurements have already shown us the production of carbon
monoxide in biomass burning and its transport by atmospheric
circulation systems. This needs to be understood on a global
scale and incorporated into models of tropospheric transport.

Methane is a greenhouse gas and the major issue here is its
source strength. There are a large number of potential sources,
such as northern wetlands, ruminant animals, and natural gas
leakage. However the actual strength of the individual sources
is very poorly known. Since methane's greenhouse effect is far
stronger than that of the better known carbon dioxide, changes
in methane, although small in themselves, can potentially have a
significant effect on the overall climate system.

Measurements of these gases are made by intercepting the
infra-red radiation coming from the planet and then isolating
the required signals. MOPITT is a nadir sounding instrument
since this gives the maximal chance of avoiding cloud features,
but this implies that it can "see" the surface of the planet and
the desired signals must be seen against the background of the
surface radiation. The field-of-view of MOPITT is 22 x 22km and
it views four fields simultaneously by the use of a 4 x 1 array
of detector elements. The field of view is also continuously
scanned through a swath about 600km wide as the instrument moves
along the orbit increasing both the spatial coverage of the
instrument and the chance of finding gaps in the cloud coverage.

The MOPITT instrument makes use of the principle of correlation
spectroscopy whereby a cell of the gas to be measured is used as
an optical filter in the infra-red to measure the signal from
the same gas in the atmosphere. The amount of gas in the
instrument cell is modulated by varying either the pressure or
the length. In addition to the correlation technique MOPITT
makes use of mechanically cooled detectors and filters (at 100K)
to enhance the overall performance. The use of this cooling
technique, which relies on Stirling Cycle coolers supplied by
British Aerospace, is relatively new in satellite
instrumentation having been used on only two civilian satellite
instruments before. The use of mechanical cooling rather than
stored cryogen or radiative cooling permits a relatively large
amount of cooling - sufficient for both the detectors and the
filter systems - whilst still permitting a five year instrument

The MOPITT science team is international, having members from
Canada, the United States and the United Kingdom. The instrument
itself is being constructed by a consortium of Canadian
companies: COMDEV Atlantic of Moncton, BOMEM from Quebec City,
Hughes-Leitz from Midland and SED from Saskatoon. The instrument
is funded by the Space Science Division of the Canadian Space
Agency. The instrument will be tested in the University of

MOPITT completed its Preliminary Design Review in December 1993
and the Critical Design Review is scheduled for April
1995. Instrument delivery will be late in 1996 and the launch of
the AM-1 platform will be in mid- 1998. Discussions are under
way regarding a second copy of the instrument to be launched in
the 2003 time frame to permit the dataset of carbon monoxide and
methane to be extended to ten years to look for long term

For more information,
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[Summary provided by Univesity of Toronto]