Combustion exhaust measurements of nitric oxide with an ultraviolet diode-laser-based absorption sensor

A diode-laser-based sensor has been developed for ultraviolet absorption measurements of the nitric oxide (NO) molecule. The sensor is based on the sum-frequency mixing (SFM) of the output of a tunable, 395-nm external-cavity diode laser and a 532-nm diode-pumped, frequency-doubled Nd:YAG laser in a beta-barium borate crystal. The SFM process generates 325 +/- 75 nW of ultraviolet radiation at 226.8 nm, corresponding to the (v' = 0, v" = 0) band of the A2Sigma+-chi2II electronic transition of NO. Results from initial laboratory experiments in a gas cell are briefly discussed, followed by results from field demonstrations of the sensor for measurements in the exhaust streams of a gas turbine engine and a well-stirred reactor. It is demonstrated that the sensor is capable of fully resolving the absorption spectrum and accurately measuring the NO concentration in actual combustion environments. Absorption is clearly visible in the gas turbine exhaust even for the lowest concentrations of 9 parts per million (ppm) for idle conditions and for a path length of 0.51 m. The sensitivity of the current system is estimated at 0.23%, which corresponds to a detection limit of 0.8 ppm in 1 m for 1000 K gas. The estimated uncertainty in the absolute concentrations that we obtained using the sensor is 10%.