Radon gas is the largest source of public exposure to naturally occurring radioactivity, and concentration maps based on atmospheric measurements facilitate compliance with national implementation of the Council Directive 2013/59/Euratom. A number of different metrology projects dealing with the traceability of radon measurements for different reasons and in different conditions have been funded. As a health risk in dwellings and workplaces (MetroRADON, EMPIR), as an atmospheric tracer in a climate change mitigation context (traceRadon, EMPIR), networked in big buildings or future cities (RadonNET, EPM), and at coastal or remote oceanic island sites in a climate change monitoring context (NuClim, EURATOM). Calibrations with small uncertainties that are traceable to the Système international d’unités (SI) (The International System of Units (SI), 2019) are needed for low 222Rn activity concentrations under difficult conditions. Starting with different classical calibration solutions, we then propose and explain a new in-situ operando calibration technique (the pulse calibration method), and provide two examples of these calibration techniques for research grade outdoor air activity concentration detectors.