CONTENTS 1. Introduction and Overview of the Field 4260 2. Experimental Methods 4262 2.1. Chambers for Cloud and Aerosol Studies 4262 2.1.1. Cloud Chamber Studies 4262 2.1.2. Aqueous Aerosol Chamber Studies 4263 2.2. Analytical Techniques 4263 2.2.1. Transfer-MS and ESI-MS 4263 2.2.2. High-Resolution Mass Spectrometry (HRMS) 4263 2.2.3. Other MS-Based Studies 4264 2.2.4. NMR 4264 2.2.5. Droplet Evaporation Techniques 4264 2.2.6. Kinetics 4265 3. A Comparison of Aqueous Aerosol, Fog, and Cloud Chemistry 4266 3.1. Overview of Conditions 4266 3.1.1. Occurrence of the Tropospheric Aqueous Phase: RH, ALW, and Clouds on a Global Scale 4266 3.2. Aqueous-Phase Transfer 4267 3.3. pH Effects 4268 3.3.1. Acid−Base Equilibria of Acids and Diacids 4268 3.3.2. Dehydration Reactions of Reaction Intermediates: Alkyl Radical Reformation 4268 3.3.3. Organic Accretion Reactions 4269 3.4. Ionic Strength Effects and Treatment of Nonideal Solutions 4269 3.4.1. Radical Reactions 4269 3.4.2. Nonradical Reactions 4270 3.4.3. Salting-in and Salting-out 4270 3.4.4. Treatment of Nonideality in ALW Chemistry 4270 4. Photochemistry 4271 4.1. Inorganic Bulk Photolysis and Radical Sources 4.1.1. Hydrogen Peroxide Photolysis 4.1.2. Nitrite Photolysis 4.1.3. Photolysis of Chlorine-Containing Species 4.1.4. Peroxomonosulfate Photolysis 4.1.