The chemical amplification (PERCA) method has been widely used for measuring peroxy radical concentrations in the troposphere. The accuracy and sensitivity of the method is critically dependent on the chain length (CL)-that is, the number of radical amplification cycles. However, CL decreases strongly with higher relative humidity (RH). So far, there does not appear to be a method to overcome this impact. Here we report the development of a Nafion dryer based dual-channel PERCA instrument. The large diameter Nafion dryer efficiently removes water vapor in milliseconds and minimally affects the sample. The low losses of peroxy radicals on the Nafion membrane make it an attractive tool for raising the CL, and thereby the measurement accuracy and sensitivity of PERCA systems. The reported instrument demonstrates this promising and simple method to minimize water vapor interference.