Photoacoustic measurement of N_2O concentrations in ambient air with a pulsed optical parametric oscillator

摘要

Photoacoustic spectroscopy, in combination with a pulsed grazing-incidence optical parametric oscillator (GIOPO), was used for sensitive detection of nitrous oxide (N_2O) in ambient air. The ν_1 + ν_3 combination band of N_2O was excited with the idler beam of the GIOPO in the 2.76 μm-2.91 μm spectral region, where CO_2 and water-absorption lines are also present. Three chemical filters filled with KOH, CaCl_2, and P_2O_5 were used to reduce the CO_2 and water concentrations to the level of several parts per billion (10~9) by volume (ppbv). Photoacoustic spectra containing several absorption lines were recorded and the concentration was determined by an integral evaluation method and a fast Fourier transform evaluation method. The photoacoustic signal was calibrated by a standard mixture of 50.6 parts per million by volume (ppmv) of N_2O in synthetic air. Values of 311 ± 5 ppbv, 314 ±5 ppbv, and 316 ±5 ppbv were found for three ambient samples collected at nearby roads.