GTE is primarily an aircraft-based program supplemented by ground-based measurements and satellite observations. The space shuttle observations of tropospheric carbon monoxide distribution by the MAPS instrument, for example, were important in the planning of expeditions over tropical rain forests (the GTE ABLE-2 mission), and the satellite observations of enhanced concentrations of ozone over the tropical Atlantic ocean were the focus of a major international study in Brazil and southern Africa (the GTE TRACE-A mission). Landsat surface images have also facilitated the extrapolation of regional arctic-tundra measurements into global-scale conclusions (the ABLE-3 mission), and weather data returned by environmental satellites have guided flight planning for research aircraft.
Global observations from space will ultimately be the technique of choice for mapping the distributions of reactive, spatially variable chemical species. Aircraft experiments will in the future provide "ground truth" for satellite measurements and explore in detail the processes responsible for the observed distributions. GTE has already demonstrated the value of both approaches, although space observations are at present very sparse. In the future, instruments aboard polar-orbiting satellites of the Earth Observing System (EOS) will greatly expand the range of measurements available from space. Supported by the complementary techniques and measurements pioneered by GTE, a new generation of satellite instruments can gather the data required for a definitive understanding of global tropospheric change.
Understanding the chemical processes in the troposphere on a global scale is critical if we are to predict, and potentially ameliorate, harmful man-made changes to the global environment. Guidance for the GTE field studies has come from ad hoc planning meetings, initiated in July 1984, and convened by the Manager of the Tropospheric Chemistry Program. Emerging from this first planning meeting was a recommended long range plan for a series of field studies to be conducted through the NASA Tropospheric Chemistry Program, and coordinated with other national and international studies. The recommended field measurements included studies of the tropical forest, Arctic tundra, instrument intercomparisons, global distributions, and long-range transport. Since the initial 1984 meeting, four other planning meetings have been convened to review and update this long range plan. The recommendations from these meetings have resulted in field studies of the Amazon Rain Forest and the northern latitude tundra (e.g. Atmospheric Boundary Layer Experiments, ABLE-2 and ABLE-3), evaluation of instrumentation for measurements of odd nitrogen and sulfur species (e.g. Chemical Instrument Test and Evaluation, CITE -1, -2 and -3), measurements over the tropical Atlantic (e.g. Transport and Atmospheric Chemistry near the Equator- Atlantic, TRACE-A), and a series of studies over the Pacific ocean(e.g. Pacific Exploratory Mission, PEM-West phases A and B and PEM-Tropics).
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