A passive equilibrium probe, sonic-assisted free ion recorder (SAFIR), was recently developed to monitor free Cu ion activity ({Cu(2+)}) in aquatic environments. The SAFIR is user-friendly and can be deployed on-site to record the ambient {Cu(2+)} in an accurate and fast manner. It consists of a chemically modified polystyrene disk (Primaria disk) containing metal ion-binding functional groups, coupled with a sonic horn in a flow-through cell. The calibration of the disk was successfully conducted in metal-ion buffered seawater solutions (pH 8.2, salinity = 32) with {Cu(2+)} ranging from 10(-9.0) to 10(-15.0) mol/L. The Cu adsorbed on the surface of the Primaria disk (Cu(ads)) correlated linearly with the ambient {Cu(2+)} through a Frendlich equilibrium adsorption model. The adsorption process was kinetically enhanced by the introduction of ultrasound, allowing equilibrium between the SAFIR and the ambient {Cu(2+)} in 10 min. The SAFIR was deployed in several coastal regions, and the {Cu(2+)} recorded by SAFIR were in good agreement with the results obtained from corresponding electrochemical analysis.