The CITE projects were designed to validate the instuments developed for the GTE measurements through rigorous intercomparisons under actual field conditions. The three CITE projects completed to date have established the credibility (or, in some cases, the limitations) of powerful new techniques for atmospheric-chemistry measurements; calibrated these new techniques through comparisons with older, poven approaches; and provided important new data on trace-gas concentrations in the clean-air resions that served as test sites. (You may wish to review the complexity of tropospheric chemistry to help in understanding CITE-1 and CITE-2.)
The CITE-1 project constedof one ground-based experiment (at Wallops Island, Virginia, in 1983) and two expeditions employing the NASA Convair-990 aircraft (over California and the central Pacific Ocean in autumn 1983, and over California and the southwestern United States in spring 1984). These investigations tested instruments designed to measure CO, NO, and OH-all of which play critical roles in atmospheric photochemistry. The instruments were: for CO, a new laser differential absorption system, together with two proven gas chromatographs; for NO, an experimental laser-induced fluorescence (LIF) system and two chemiluminescence (CL) systems; and for OH, two LIF systems (one lidar based, one in situ) and a radiochemical tracer technique. (See figure 1.)
The two CO techniques were in good agreement; the laser technique is more costly to deploy but offers shorter response time and real-time data read-out. The NO test showed that bot the LIF and CL techniques have the parts-per-trillion sensitivity needed for measurements in clean-air regions, as well as desirably wide dynamic range. The OH experiments, by contrast, revealed that none of the detectors tested had the sensitivity required for OH measurements throughout the golbal troposphere, although the results have helped to point the way to future technique development.
The CITE-2 project was conducted abord the NASA Electra aircraft over california and the eastern Pacific Ocean in 1986. It compared instruments that measured components of the nitrogen oxide photochemical cycle, which strongly influences ozone and OH concentrations in the troposphere. The largest field project ever mounted to study this key cycle, CITE-2 obtained new scientific data on nitrogen dioxide (NO2), nitric acid (HNO3), and peroxyacetyl nitrate (PAN) within both clean (Pacific) and polluted (continental) air. Ancillary instruments recorded abundances of other tropospheric chemical sspecies to test models of tropospheric photochemistry.
CITE-2 investigated four instruments for NO2 measurements, three methods for HNO3 detection, and two instruments for PAN studies. Intercomparisions yielded generally concordant results for sufficiently high concentrations of these species. (See figure 2.) But the CITE-2 measurements also confirmed, within a variety of tropospheric environments, earlier indications that too few nitrogen species are included in current photochemistry models to account for the total abundances of reactive nitrogen compounds in the troposphere. Explanation of this "nitrogen deficit" will provide a focus for atmospheric chemistry research over the next sereral years-research that will be greatly aided by the advanced instruments tha CITE-2 has helped to develop.
The CITE-3 project, and Electra aircraft expedition over the vicinity of Wallops Island and ares of the topical Atlantic Ocean off the coast of Brazil (completed in September 1989), was devoted to a study of the sulfur cycle in the global atmosphere. Sulfur compounds are important products and hence indicators of biological activity, particulary over productive ocean areas. Biogenic sulfur compounds are oxidized in the atmosphere to yield sulfur dioxide, and ultimately sulfuric acid droplets and solid sulfate aerosols. CITE-3 was designed to evaluate current measurement instrumentation-mostly based upon classical chemical analysis-and guide the development of next-generation techniques, as well as to provide a new data base on sulfur chemistry for the marine environment in contact with the global troposphere.
The task of developing accurate, practicable instruments for global tropospheric chemistry measurements is a difficult one. Careful attention to detail, an objective and systematic approach, and some sacrifice of immediate scientific return to the establishment of reliable, long-term research capabilities are all required. However, this activity is vital to the health of tropospheric chemistry and to prospects for understanding the relationship between tropospheric chemical change and global environmental change. Continued instrumentation development and validation, such as pioneered by the CITE projects, will remain essential for many years to come.
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