Potassium Radiative Emissions for Wildfire Detection: Developing a Path Toward Laser Heterodyne Radiometry

While optical techniques for fire detection have evolved from simple visual observation to more advanced hyperspectral imaging systems, there remains a need for compact, selective and field-deployable tools capable of quantifying and characterizing both intensity and spectral signatures of flames. This work introduces a novel approach to Fire Optical Measurements (FOM) that leverages spectral emissions from potassium, a key tracer of flaming biomass combustion.  We describe the development of a sensor platform operating in the near-infrared region to isolate potassium emission features (K-FOM) in both and simple “field-scale” demonstrations. By targeting radiative emission signatures specific to biomass burning, the system offers a promising method to differentiate wildfires from fossil fuel combustion, as well as to differentiate lower-temperature smoldering fires from intense crown fires.

The K-FOM system builds on our prior experience in developing Laser Heterodyne Radiometry (LHR) sensors used for greenhouse gas measurements via in solar occultation, employing a similar optical design.  In K-FOM, radiation from a potassium containing flame is collected (using a single-mode fiber or free-space collection optics). The collected radiation is mixed with light from a tunable diode laser operating near 770 nm.  The resulting radio frequency signal from the combined beam carries both broadband contributions from flame particulate and the sharp emission lines from excited potassium atoms.

This presentation focuses on testing this technique in a model, laboratory system. Conserved scalars are widely used in wildfire modeling to simplify complex thermochemistry by linking species concentrations or formation rates for species to a single, passively-advected quantity through a look-up library. Mixture fraction, which represents the local proportion of fuel mass (from the unburnt cold or gaseous fuel) relative to the total mixture mass, is a key conserved scalar used in wildfire models such as the NIST Fire Dynamics Simulator. To explore the validity of this approach to potassium fire chemistry, potassium chloride solutions are nebulized into a well-characterized laboratory flame system. By mapping  ground state potassium mixture fraction, using Tunable Diode Laser Absorption Spectroscopy (TDLAS), the spatial profile of potassium emission, and data from prior structural measurements and computations in this system, we refine detailed mechanisms for potassium fire chemistry and test mechanism reduction strategies.