Development of continuous monitoring of ambient carbon dioxide using near-infrared quartz-enhanced photoacoustic spectroscopy

We demonstrate continuous measurement of carbon dioxide (CO2) in the atmosphere using quartz-enhanced photoacoustic spectroscopy (QEPAS) at 2.008 μm with a fiber-coupled distributed-feedback laser. The QEPAS sensor is composed of a quartz tuning fork with a resonance frequency f0of 12.4 kHz. The photoacoustic second-harmonic (2f) signal resulting from CO2absorption was detected using a digital lock-in amplifier. The photoacoustic signal is proportional to the laser power. The limit of detection at an absorption line of 4980.13161 cm−1with a line strength of 3.87 × 10−22cm2molecule−1cm−1was 12 parts per million with an integration time of 10 s. The results of the continuous monitoring of ambient CO2using the QEPAS sensor corresponded to those obtained using a nondispersive IR sensor. The QEPAS sensor exhibited a suitable performance for field measurements of the CO2mixing ratio.