Measurements Of Pollution

In The Troposphere

The MOPITT instrument is scheduled for launch on the AM-1 platform of NASA's Earth Observing System (EOS). The AM-1 satellite will be 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.

MOPITT Instrument

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 life.

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 Toronto.

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 effects.

MOPITT science team

MOPITT is funded by the
Canadian Space Agency

MOPITT Prime Contractor
COM DEV Atlantic of Moncton, NB

Prof. James R. Drummond
Department of Physics
University of Toronto
60 St. George Street,
Toronto, Ontario,
CANADA M5S 1A7
(jim@atmosp.physics.utoronto.ca)