During 1983-2001, NASA conducted a collection of field campaigns as a part of the Global Tropospheric Experiment (GTE) to develop advanced instrumentation to measure critical atmospheric trace gases and quantify their sources, sinks, and distribution. Among those were the Chemical Instrumentation Test and Evaluation (CITE) missions, which had the overarching goal to test and evaluate the instruments developed for the GTE missions. To accomplish this objective, the CITE missions adopted the methodology of conducting intercomparisons of airborne measurements obtained for the same species by instruments utilizing fundamentally different detection principles (Hoell et al., 1990). The third phase of the CITE mission, CITE 3, occurred in the North and tropical Atlantic Ocean from August-September 1989. Its primary objective was to test and evaluate the capacity to collect reliable measurements of the following sulfur species: sulfur dioxide (SO2), dimethyl sulfide (DMS), carbonyl sulfide (COS), carbon disulfide (CS2), and hydrogen sulfide (H2S). The secondary objective of CITE 3 was to determine the abundance and distribution of major sulfur species over a wide range of atmospheric conditions, including altitude, solar flux levels, atmospheric mixing ratios, and surface source strengths of sulfur in a predominantly marine environment (Hoell et al., 1993).

CITE 3 utilized the NASA Electra research aircraft equipped with a suite of instruments for sulfur and ancillary measurements. The two main objectives were addressed through intercomparisons of airborne measurements obtained for the same species but utilizing fundamentally different detection principles in order to have multiple measurements of each of the sulfur species. In some instances, the same detection principle was used, but the method of sample collection and/or analysis procedures were different. A complete payload had the potential of five techniques for SO2, six techniques for DMS, and three techniques for H2S, CS2, and COS. However, due to operational problems or team decisions to focus on specific species during flights, not all measurement techniques were employed at all times (Hoell et al., 1993). There were three phases of field activity in this campaign: (1) intercomparison of sulfur calibration standards, (2) measurements in ambient air over the Northern Atlantic Ocean, and (3) measurements in ambient air over the tropical Atlantic Ocean. Cite 3 was based out of NASA Wallops Flight Facility in Virginia, USA and Natal, Brazil. Each base of operation provided a different environment for intercomparison. Air masses at NAS Wallops Flight Facility were influenced by anthropogenic emissions along the eastern United States compared to the relatively clean marine boundary layer over the ocean off the coast of Natal. As part of the Natal deployment, ozonesondes were launched from the Natal area to provide data on the general state of the atmosphere as well as serve as a frame of reference when compared to the seasonally averaged ozone data from this site.

seasonally averaged ozone data from this site. Sulfur gases and their reaction products play important roles in the chemistry of the global troposphere as well as the biogeochemical sulfur cycle. The sulfur database from CITE 3, and the results from both intercomparison studies and photochemical budget studies, significantly enhanced the ability to evaluate the confidence in the existing databases. Detailed description related to the motivation, implementation, and instrument payloads are available in the CITE 3 overview paper. A collection of the publications based on CITE 3 observations are available in the Journal of Geophysical Research special issue: Chemical Instrumentation Test and Evaluation (CITE 3).