Concluding remarks: Atmospheric chemistry in cold environments.

Atmospheric chemistry in cold environments refers to key chemical processes occurring in Earth's atmosphere in locations relevant for society including the polar areas, the free and upper troposphere, and the stratosphere. Atmospheric chemistry in these areas is relevant for local air quality, ecosystem health, regional and global climate. This comprised excellent coverage of these areas in terms of longitude and latitude, altitude and temperature. It also featured a broad coverage of disciplines between physical, analytical and theoretical chemistry and also the related fields covering aspects of biology, health, meteorology, social sciences and even including policy and economic aspects. A core aspect of the discussions was rooted in interfacing the related diverse competences. Because traditional atmospheric chemistry has evolved around knowledge of mechanisms and kinetics of chemical reactions first in the gas phase and later including condensed phases of aerosol particles and ground surfaces centering around room temperature, the speciality of relevance in this was the recent progress in better understanding the evolution of multiphase chemistry at low temperatures, where many relevant properties such as solubility and volatility change dramatically. This was embedded in discussions of the results and challenges of the most recent measurements from a range of campaigns and long-term observations at research stations. The discussion evolved around the chemical cycles of important trace constituents, the formation and evolution of particulate matter under cold conditions, the link between cloud glaciation and air-mass characteristics, air-quality in cold urban environments, biosphere-atmosphere interactions in a warming Arctic, but also the role of interfacial chemistry and reactivity as they are involved in multiphase chemistry processes. Future threats for the cold part of our atmosphere come from increasing human activities in both polar regions with their impacts on ecosystems, air quality and broader scale atmospheric composition as well as from discussions of geoengineering solar radiation modification by stratospheric aerosol injection.