CONTENTS 1. Introduction 4542 1.1. Historical Overview and Goals 4542 1.2. Background Basics of the Atmosphere 4543 2. The Exosphere 4545 3. Thermosphere 4546 3.1. Composition and Positive Ion Chemistry 4546 3.2. Ion−Molecule Reactions 4547 3.2.1. O 2 + Reactions 4547 3.2.2. O + Reactions 4548 3.2.3. N + Reactions 4550 3.2.4. N 2 + Reactions 4550 3.3. Airglow and Dissociative Recombination 4551 4. Mesosphere 4552 4.1. Positive Ion Composition and Chemistry 4552 4.2. Negative Ion Composition and Chemistry 4554 4.3. Mutual Neutralization 4555 4.4. Meteoric Smoke 4555 5. Stratosphere and Troposphere 4556 5.1. Acid−Base Chemistry 4556 5.2. Ion-Induced (Mediated) Nucleation 4558 6. Active Chemical Release Experiments 4559 6.1. Background and Historical Perspective 4559 6.2. Atmospheric Tracer Experiments 4559 6.3. Plasma Enhancement Releases − Samarium Chemi-ionization 4560 6.4. Plasma Depletion Experiments − Ionospheric Holes 4561 6.5. Plasma Depletion Experiments − Electron Holes 4562 7. Summary and Conclusions 4564 Author Information 4565 Corresponding Author 4565 Notes 4565 Biographies 4565 Acknowledgments 4566 References 4566