Inline multi-harmonic calibration method for open-path atmospheric ammonia measurements

We demonstrate a real-time inline calibration method for an open-path ammonia sensor using a quantum cascade laser (QCL) at 9.06 μm. Ethylene (C₂H₄) has an absorption feature partially offset from the ammonia absorption feature, and the ethylene signal serves as a reference signal for ammonia concentration in real time. Spectroscopic parameters of ammonia and ethylene are measured and compared with the HITRAN database to ensure the accuracy of the calibration. Multiple harmonic wavelength modulation spectroscopy (WMS) signals are used to separate the ambient ammonia and reference ethylene absorption signals. The ammonia signal is detected with the second harmonic (2f), while it is calibrated simultaneously with a high-harmonic (6-12f) signal of ethylene. The interference of ambient ammonia absorption on the ethylene reference signal is shown to be negligible when using ultra high-harmonics (≥6f). This in situ calibration method yields a field precision of 3 % and accuracy of 20 % for open-path atmospheric ammonia measurements.