Ten crucial unknowns in atmospheric chemistry in the cold

The Southern Ocean, wintertime cities, the upper troposphere, the Arctic and Antarctica, and alpine mountains are places where atmospheric chemistry impacts human health, air quality, climate, or geochemical cycles and that are characterized by low temperatures where ice or snow can be present. The atmospheric impact is evident from the role of polar biogenic sulphur emissions on aerosol formation, multiphase nitrogen and sulphur chemistry on wintertime haze, and industrial emissions in snow-covered areas on the ozone budget. The Cryosphere and ATmospheric CHemistry community (CATCH) addresses the environmental processes within these coupled cryosphere-atmosphere systems, and here we present open research questions specific to the cold environments, focusing on the unique interplay of chemistry and physics. These research needs call for interdisciplinary approaches to address atmospheric science in a warming climate with changing human impact in Earth's cold regions.