Demonstration of near-infrared open-path absorption spectroscopy for water vapor and temperature monitoring during structure-to-structure fire spread experiments.

With increasing threats from wildfires in wildland urban interface (WUI) areas, novel approaches for the characterization of fire dynamics and their interactions with built structures have accelerated. Here, we demonstrate an open-path, near-infrared laser absorption system for stand-off measurement of water vapor mole fraction and gas-phase temperature. The system was deployed along two optical paths during structure-to-structure fire spread experiments at the National Institute of Standards and Technology. Initial demonstration of the system was completed from March 2024 through May 2024 in the National Fire Research Laboratory, to assess whether open-path tunable diode laser absorption spectroscopy (TDLAS) can operate reliably under the challenging, smoke-laden, and rapidly evolving conditions of WUI-relevant fire exposures. Data were collected before ignition, during test burns, and throughout fire suppression. The system operates near 1393 nm with a typical signal-to-noise ratio exceeding 180. Results show increased temperature and water vapor mole fraction following fuel ignition, as well as variations related to fire-resistant eave vent activation times. These results highlight that open-path TDLAS is a viable nonperturbing measurement approach that can improve understanding of heat and gas flow for future large-scale fire studies, which can contribute to the enhancement of fire safety, building codes, and associated test methods.