The Atmospheric Trace Molecule Spectroscopy(ATMOS) Experiment

Introduction

Over the last three or four decades scientists have gained a keener understanding of the Earth's atmosphere. During this time the atmosphere has been recognized as a medium comprised of many regions distinguished by temperature and chemical composition. Moreover, the chemical composition of these regions is constantly evolving in response to natural changes as well as changes brought about by human activity. So there is a need to study the atmosphere extensively to establish the nature and the extent of changes in chemical composition of atmospheric regions. One of the many experiments in this ongoing study is the Atmospheric Trace Spectroscopy(ATMOS) Experiment.

The Atmospheric Trace Molecule Spectroscopy(ATMOS) Experiment is designed to make global measurements of the composition of the trophosphere, stratosphere, and mesosphere. The spectrometer simultaneously measures the concentrations of gases that are involved in the complex chemical and radiative interactions occuring at altitudes between 10 and 150 km(6 to 93 mi). With the instrument's ability to detect these trace gases in concentrations of lower than 1 part per billion, its data are used to make critical tests of theroretical models that describe the physics and chemistry of the stratosphere.

Trace gas molecules absorb specific wavelengths of incoming solar radiation. By determining which wavelengths are absorbed by the atmosphere, ATMOS scientists can develop a more detailed picture of the molecular composition of the atmosphere,thereby providing a baseline of data against which to detect future changes in atmospheric composition. These data(with information collected from similar experiments carried on balloons, rockets, satellites, and aircraft) will provide a reference source for atmospheric scientists in the future.

The ATMOS Instrument

The ATMOS sensor instrumentis a state-of-the-art version of a 19th century invention by Nobel-laureate physicist Albert Michelson. This device splits incoming light into two beams and varies the distance one beam travels with respect to the other. The beams are then combined. The interference property of light causes the formation of patterns that reveal frequency and intensity characteristics of the incoming light. These measurements, in turn, help to identify the composition of the atmospheric region through which the light sample has passed.

The components of the ATMOS instrument include a suntracker,a telescope, an interferometer,an infra-red detector, and a data processor. The suntracker keeps the instrument's field-of-view on the sun,while the telescope collects light to be processed by the interferometer. Light from the interferometer is focused onto an infra-red detector. The detector's output is then amplified, filtered, digitized, and recorded by an on-board data recorder. With this system it is possible to study radiation in wavelengths from 2.5 to 16 micrometers.

Mission Results

The ATMOS instrument was first flown on the Spacelab 3 Shuttle mission in 1985. On that first flight 40 different gases were detected in an altitude range from 0 to 150 km. ATMOS made the first simultaneous inventory of important trace gases, including molecules involved in ozone photochemistry. The instrument was also able to detect and measure zonawind patterns in the mesosphere and the troposphere.

ATMOS flew again in 1992 as part of the ATLAS-1 project flight. ATMOS provided information concerning the recent eruption of Mount Pinatubo and it's effect on the atmosphere. The Pinatubo eruption, the largest volcanic eruption this century, ejected large amounts of crusatl matter and sulfur-based aerosol gases into the stratosphere at heights coinciding with the ozone layer. The ATMOS instrument also detected changes in the atmosphere predicted by scientists after the 1985 flight. A dramatic increase in CFC gases over the time period between flights was detected, and the role of thes gases in the overall chemistry of the atmosphere was more clearly defined.

ATMOS has helped prove that CFC gases contribute to the ozone depletion process As a result, agreements have been made to phase out the manufacture and use of the gases in industrialized countries. However there is still a need to examine changes in the chemical makeup of the atmosphere. Atmos will be an important tool in the efforts to underatand those changes.

For further information, please contact Dr. Curtis Rinsland, Atmospheric Sciences Division, NASA Langley Research Center, Hampton, VA.

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