Book excerpt: Among the chemical and physical processes involved in the transformation of pollutants between their sources and their ultimate deposition, those associated with clouds, aerosols and precipitation must be rated as the most difficult both to study and to understand. This book presents a variety of recent advances in this field, including the properties and composition of aerosol particles, chemical transformation and scavenging processes, the relationship between liquid-phase chemistry and cloud micro-physics, entrainment, evaporation and deposition, trends in high Alpine pollution, transport processes, and developments in instrumentation. This book is Volume 5 in the ten-volume series on Transport and Chemical Transformation of Pollutants in the Troposphere.