The ME4-NO2-E4 sensor is a constant-potential electrolytic NO2 sensor. NO2 and O2 undergo corresponding redox reactions at the working and counter electrodes, releasing charge to generate an electric current. The magnitude of this current is directly proportional to the NO2 concentration; therefore, the NO2 concentration can be determined by measuring the current. The sensor features a fourth electrode—an auxiliary electrode—used to compensate for the zero-point current, resulting in a device characterized by a strong signal level and a low zero-point current.