Atmospheric chemistry: from the gas phase to the multiphase system

The chemistry of the atmosphere, with a focus on its transition from gas to particle phases, is described. Emphasis is placed on how our understanding of this chemistry has evolved over time, reaching the current point, where every atmospheric compartment can be modeled, helping to tackle the emergent issues of air pollution and climate change. The main reactions occurring in the different compartments and contributing to the final observed aerosol compositions are presented. The different possible pathways of compounds, from their emission to their presence in aerosol particles, passing through an interface to reach a bulk aqueous phase, are pedagogically described. The goal is to provide the reader with the opportunity to understand key aspects of this important field of chemistry and to offer insights into how it has evolved over the years, also touching on new and still little-explored facets, such as surface reactions.