Quartz-Enhanced Photoacoustic Spectroscopy for Environmental Monitoring

Air pollution is a critical global issue, contributing to over 4.2 million deaths annually due to stroke, heart disease, lung cancer, and chronic respiratory diseases. It also poses significant economic and social challenges, including increased healthcare costs and reduced productivity. The need for real-time, high-resolution air quality monitoring is essential to minimize public exposure, particularly for vulnerable populations. However, existing ambient pollutant detectors are bulky and impractical for widespread deployment, and current electrochemical sensors lack the stability and sensitivity required for regulatory compliance.

In this context, we report the results obtained within the European Project PASSEPARTOUT in advancing the development of miniature, hyperspectral optical sensors based on Quartz Enhanced Photoacoustic Spectroscopy (QEPAS). QEPAS is trace gas optical detection technique that exploits the photoacoustic effect occurring in a gas sample when a modulated, resonant light is absorbed by the target analytes. A weakly damped propagating acoustic (pressure) wave with wavelengths in the centimeter range is generated in the proximity of the exciting light beam. In QEPAS, these sound waves are detected by a spectrophone, composed of a quartz-tuning fork (QTF) transducer and a pair of millimeter-size resonator tubes, aligned on both sides of the QTF.

Eight different air pollutants, namely CH₄, NO₂, CO₂, N₂O, CO, NO, SO₂ and NH₃ have been detected with the same acoustic detection module (containing the spectrophone) and interchangeable laser sources, to prove the modularity of the technique as well as the adaptability to different lasers. Each gas species was detected with an ultimate detection limit well below their typical natural abundance in air even with a signal integration time as low as 0.1 s.

Two significant advancements have also been achieved. The first involves the development of a portable, field-deployable QEPAS sensor that eliminates the need for free-space optical components, thereby increasing the mechanical stability and robustness of the system. By integrating a single-mode fiber to guide the laser beam into the spectrophone, the sensor achieves enhanced flexibility and ease of deployment. The second innovation consists of a custom-designed three-wavelength laser module, combining three quantum cascade lasers (QCLs) into a single collimated beam, further extends the sensor’s capabilities for multi-gas detection with a single sensor architecture.

These advancements pave the way for the deployment of mobile, high-precision air quality monitoring systems that are scalable, adaptable, and capable of providing real-time data for regulatory compliance and public health protection.