Ppb-level low-cost miniaturized simultaneous nitrogen dioxide and sulfur dioxide sensor based on differential Helmholtz photoacoustic spectroscopy

Sulfur dioxide (SO 2 ) and nitrogen dioxide (NO 2 ) are among the most common air pollutants. To address the issues of high cost and complex configuration in current ppb-level gas sensors, we propose a low-cost simplified dual-gas detection system based on Helmholtz photoacoustic spectroscopy, incorporating a couple of commercial bare light emitting diodes (LEDs) with a minimized differential Helmholtz resonator. The proposed sensor uses two bare-LEDs with central wavelengths of 273 and 452 nm to align with absorptions of SO 2 and NO 2 without secondary optical coupling. Simultaneous detection of the two gases was achieved with frequency-division multiplexing. To reduce background noise, an optimized differential Helmholtz photoacoustic resonant cell matching the properties of LEDs has been designed and fabricated through 3D printing. In experiments, limits of detection of SO 2 and NO 2 reached 330 and 180 ppb (for 1 s averaging), respectively, and were found to be capable for further reduction down to 95 and 31 ppb by increasing the integration time at 180 s and 440 s respectively. Moreover, the impact of the gas interference on the simultaneous multiple gas measurement has been analyzed. It has been found that SO 2 has negligible effect on NO 2 measurements, while the impact of NO 2 on detection of SO 2 does not exceed 5%. The T90 response time of this sensor is approximately 12 s. This proposed gas sensing solution can be considered as highly attractive for simplified, low-cost and sensitive detections of air pollutants.