Human activities have greatly impacted the Earth system so much so that it has ushered in a new epoch - the Anthropocene. The consequent changes in the oceans, terrestrial regions and biosphere have highlighted important societal issues, such as climate change, ocean acidification, air quality degradation and ozone layer depletion. The central component leading to changes are the processes that alter the composition of the atmosphere. This Faraday Discussion focuses on emerging issues, such as interactions between anthropogenic and biogenic emissions, new mechanisms for atmospheric chemistry, the impacts of climate on air quality and new instrumental tools and platforms for atmospheric chemistry. It brings together a global network of experimentalists, field scientists, theoreticians, chemists, physicists and environmental scientists working at the forefront of these emerging issues, providing a forum for cross-disciplinary exchange and discussion of ideas on the processes that control the composition of the atmosphere.