Humidity enhanced NO photoacoustic sensor with a 4.53 μm quantum cascade laser and Kalman filter.

A high-sensitivity NO photoacoustic sensor using a 4.53 μm quantum cascade laser was developed. Sharply enhancement of photoacoustic signal of NO with the increasing of humidity was investigated experimentally. Finally, 2.3 % water vapor was added to the analyzed sample to improve the vibrational-translational (V-T) relaxation rate of NO molecule transition, and therefore enhance the NO photoacoustic signal. High performance with a minimum detection limit of 28 ppbv in 1 s and a measurement precision of 34 ppbv have been achieved, respectively. Kalman adaptive filtering was used to remove the shot-to-shot variability related to the real-time noise in the measurement data and further improve the measurement precision. Without sacrificing the time resolution of the system, the Kalman adaptive filtering improves the measurement precision of the system by 2.3 times. The ability of the NO photoacoustic sensor was demonstrated by continuous measurement of atmospheric NO concentration for a period of 7 h.