CONTENTS 1. Background 4219 1.1. Why Condensed Matter Matters: Types and Significance of Condensed Matter and Surfaces Encountered in the Atmospheric Environment 4219 1.2. Principles of Atmospheric Photochemistry 4221 1.2.1. Classification of Different Types of Photochemical Processes 4221 1.2.2. Free Radicals and UV Radiation as the Primary Drivers of Chemistry in the Atmosphere 4221 1.2.3. Photosensitized Processes and Photochemistry Driven by Visible Radiation 4222 1.2.4. Rates and Yields of Photochemical Reactions 4222 1.3. Gas Phase versus Condensed Phases 4223 1.3.1. The Importance of "Matrix Effects" 4223 1.3.2. Diffusion and Transport Limitations 4224 1.3.3. Influence of the Physical State and Viscosity 4224 1.3.4. Special Properties of Interfacial Regions between Different Phases 4225 2. Photophysical Properties of Observed Atmospheric Particles and Interfaces 4225 2.1. Primary Chromophores 4225 2.1.1. Mineral Dust 4225 2.1.2. Inorganic Anions 4226 2.1.3. Hydrogen Peroxide 4227 2.1.4.

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