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PART II ASSESSMENTS OF CURRENT UNDERSTANDING Part I of this report, "A Plan For Action," presents a coordinated national blueprint for scientific investigations of biogeochemical cycles in the global troposphere. Part II, "Assess- ments of Current Understanding," presents much of the background information from which the proposed Global Tropospheric Chemistry Program described in Part I was developed. Brief authored reviews are presented in Part II, and they evaluate both the present knowledge and gaps in the understanding of tropospheric chemical cycles and processes. Four review papers discuss the sources, transport, transformation, and removal oftrace species in the troposphere. These are followed by a review of the role of modeling in understanding tropospheric chemical processes. A general review of tropospheric biogeochemical cycles is given followed by individ- ual reviews of the water, ozone, nitrogen, sulfur, carbon, halogen, trace element, and aerosol particle cycles. Instrumentation development needs for use of mass-balance techniques is reviewed, and an instrument and platform survey completes the main part of Part II. Brief bibliographies are given at the end of each review paper. These bibliographies are not intended to be comprehensive but simply to serve as representative reference lists from which further infor- mation can be obtained for each topic. Appendix A presents a brief review of current tropospheric chemistry research in the United States. In recognition of the future importance of spaceborne remote sensing techniques in tropospheric chemistry research, a brief review of this subject is presented in Appendix B. A matrix approach was used by our panel to systematically evaluate present knowledge of the primary species in several of the major elemental cycles in the troposphere. These matrices are presented in Appendix C.

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