Monitoring atmospheric ⁸⁵Kr by atom counting

Radioactive ⁸⁵Kr is a major gaseous fission product emitted into the air by the nuclear fuel reprocessing industry. Measuring atmospheric ⁸⁵Kr has applications in environmental monitoring, atmospheric transport model validation and dating of environmental water samples, including groundwater, sea water and glacier ice. We present an ultra-sensitive method for fast analysis of atmospheric ⁸⁵Kr at 10^-5 parts per trillion level. This method is based on laser cooling and trapping and is capable of counting individual ⁸⁵Kr atoms. Measurements at the 3% precision level can be made on krypton extracted from 1L STP of air with a turnaround time of 1.5 h. Moreover, we have realized a system for continuous air sampling over days to weeks. Based on this atom-counting technology and a portable air sample integrator we have realized atmospheric ⁸⁵Kr baseline monitoring in Hefei, China, for over 20 months. The technological advances presented in this work lay the ground for a global atmospheric ⁸⁵Kr monitoring network.