The Jiangmen Underground Neutrino Observatory (JUNO), a 20 kton multipurpose underground liquid scintillator detector, was proposed for the determination of the neutrino mass hierarchy as primary physics goal. The central detector will be submerged in a water Cherenkov detector to lower the background from the environment and cosmic muons. Radon is one of the primary background sources. Nitrogen will be used in several sub-systems, and a highly sensitive radon detector has to be developed to measure its radon concentration. A system has been developed based on 222Rn enrichment of activated carbon and 222Rn detection based on the electrostatic collection. This paper presents the detail of a μBq/m3 level 222Rn concentration measurement system and gives detailed information about how the adsorption coefficient was measured and how the temperature, flow rate, and 222Rn concentration affect the adsorption coefficient.