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The NASA Langley Research Center's Airborne UV DIAL system has been flown on many tropospheric and stratospheric field campaigns since 1980. The current system, in its tropospheric configuration, transmits four beams in both nadir and zenith directions making measurements of O3 and aerosols from near the surface to above the tropopause. This system has been flown on the NASA DC-8 during the Global Tropospheric Experiments (GTE) to provide real-time remote sensing of tropopause heights, stratospheric intrusions, cloud tops/bases, boundary layer heights, and, in general, the large-scale variations of O3 and aerosol distributions across the troposphere. In addition, post-mission analyses of relative amounts of ozone O3 and aerosols as compared to discriminator profiles of each, permits an airmass characterization scheme including stratospherically influenced air, plumes with either high O3 or high aerosols or both, continental outflows, convective transport, and clean marine air. During the missions the DIAL instrument is routinely used to identify altitudes of O3 and/or aerosol layers so that the aircraft can then be directed to fly subsequent legs at specified heights to allow for in situ sampling of key species. In post-mission analyses the DIAL measurements provide the broader context for the in situ measurements of other trace species and, when coupled with meteorological and trajectory analyses, play a vital role in the interpretation of the dynamics and photochemistry of the troposphere. To date, NASA's GTE program has conducted three exploratory missions over the Pacific Ocean. PEM West-A (September-October 1991) and PEM West-B (February-March 1994) were focused in the western Pacific of the northern hemisphere to investigate the budget of O3 and its precursors and to assess both the natural and anthropogenic impact of key atmospheric trace species in this region. PEM West-A was staged in late summer/early fall when the airflow is dominated by easterly across the Pacific. The second phase, PEM West-B, was staged during late winter when the airflow is characterized by maximum outflow from the Asian continent. PEM Tropics-A (September-October 1994) involved two NASA aircraft in a study of the chemistry of the troposphere over the central and southern Pacific with a focus on the tropics in the southern hemisphere. The intent was to study one of the few remaining clean air regions of the world to better understand the role of nitrogen oxides in the O3 budget. A follow-up to this study, PEM Tropics-B, is in the process of being planned for February-- April 1999 in a contrasting meteorological regime. Selected O3 and aerosol distributions are illustrated from each of these field campaigns showing some of the key features in each experiment as observed by the airborne UV DIAL system.
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