Institute of Chemical Processes Fundamentals of the CAS (ICPF), Czech Hydrometeorological Institute (CHMI), Global Change Research Institute of the CAS (GCRI), and the Research Center for Toxic Compounds in the Environment of Masaryk University (RECETOX, MU) are organizing an Open Day at the Czech National Atmospheric Observatory in Košetice on May 21, 2022, from 9:00 to 14:00. Inspections take place at hourly intervals (9:10, 10:10, 11:10, 12:10 and 13:10). The working team of the National Atmospheric Observatory Košetice (Czech Republic) will present meteorological measurements and observations, air quality monitoring, and activities within the ACTRIS-CZ research infrastructure, in which the Department of Chemistry and Aerosol Physics will present atmospheric aerosol measurements.