Rapid field measurement of ventilation rate using a quartz-enhanced photoacoustic SF₆gas sensor

We reported the development of a quartz-enhanced photoacoustic sulfur hexafluoride (SF6) sensor for ventilation studies using a continuous-wave distributed-feedback quantum cascade laser (QCL) at 10.5 m. The SF6 sensor was developed by detecting the gas-absorption induced acoustic wave using a tiny quartz tuning fork that is enclosed in a gas cell with a sample volume of 2.5 ml. By locking the QCL wavelength at the absorption peak of SF6, we obtained a recording time interval of 0.4 s and a detection limit of 4.6 ppb. The sensor response time (t90) was found to be 2.8 s at a flow rate of 550 ml min-1. The sensor was then implemented in measuring air exchange rates in a laboratory room using the standard tracer gas concentration decay method. Our measurements are in good agreement with a commercial analyzer when studying two typical passive ventilation scenarios of infiltration and natural ventilation. Additionally, the developed photoacoustic gas sensor is fast enough to capture the transient variations of the SF6 concentration during natural ventilation. This study provides a promising method of studying transient contaminant transport that remains a major challenge in air quality research.