Mid-infrared photoacoustic spectroscopy for atmospheric NO_22 measurements

摘要

A photoacoustic (PA) sensor for spectroscopic measurements of NO_2-N_2 at ambient pressure and temperature is demonstrated. The PA sensor is pumped resonantly by a nanosecond pulsed single-mode mid-infrared (MIR) optical parametric oscillator (OPO). Spectroscopic measurements of NO_2-N_2 in the 3.25 μm to 3.55 /mi wavelength region with a resolution bandwidth of 5 cm"1 and with a single shot detection limit of 1.6 ppmV (μmol/mol) is demonstrated. The measurements were conducted with a constant flow rate of 300 ml/min, thus demonstrating the suitability of the gas sensor for real time trace gas measurements. The acquired spectra is compared with data from the Hitran database and good agreement is found. An Allan deviation analysis shows that the detection limit at optimum integration time for the PAS sensor is 14 ppbV (nmol/mol) at 170 seconds of integration time, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 3.3×10~(-7) W cm~(-1) Hz~(-1/2).