Open-path anti-pollution multi-pass cell-based TDLAS sensor for the online measurement of atmospheric H₂O and CO₂ fluxes

Abstract

We report an open-path and anti-pollution multi-pass cell based tunable diode laser absorption spectroscopy (TDLAS) sensor, which was designed for online measurement of atmospheric H₂O and CO₂ fluxes. It is mainly composed of two plano-convex mirrors coated on a convex surface, which makes it different from traditional multi-pass cells. This design does not allow a direct contact between the coating layer of the lens and air, thereby realizing the anti-pollution effect of the coating layer. Two DFB lasers operating at 1392 nm and 2004 nm were employed to target H₂O and CO₂ absorption lines, respectively. Allan analysis of variance indicated that detection limits of H₂O and CO₂ were 5.98 ppm and 0.68 ppm, respectively, at an average time of 0.1 s. The sensor performance was demonstrated by measuring CO₂ and H₂O flux emissions at Jiangdu Agricultural Monitoring Station in Jiangsu Province. The results were compared with those obtained using the commercial instrument LI-7500, which is based on non-dispersive infrared technology. The developed gas analysis instrument exhibited good consistency with commercial instruments, and its accuracy was comparable; thus, it has strong application prospects for flux measurements in any ecosystem.