Quantifying Downwind Deposition of Wildfire-Emitted Particles to Ecosystems

Pyrogenic airborne particle deposition downwind of active wildfires has traditionally been examined primarily as near-term hazards of fire spotting by firebrands or long-range transport of smoke particles (<10 µm). However, wildfires also emit substantial quantities of intermediate-sized airborne particles (10-2000 µm) that carry nutrients and contaminants affecting ecosystems downwind of the fire perimeter. The production, transport, and deposition of these intermediate-sized particles remain understudied. Here we develop a physics-based modeling framework for particle generation at fire lines, lofting by fire-driven convection, transport by prevailing winds, and subsequent ballistic settling. The framework enables characterization of this largely overlooked wildfire deposition footprint. Sensible heat flux from the fire feeds a convective plume capable of lofting particles to heights governed by fire intensity, particle size, shape, and density. Once aloft, particles are carried by ambient winds and ultimately ballistically deposited. The model performance was assessed using a unique dataset of particle deposition measured 3-40 km downwind of the fire front during the 2021 Caldor Fire. Supplemental observations of fire behavior, fuel properties, and meteorological conditions serve as inputs for model evaluation. The framework relies on various assumptions and constraints regarding unknown variables, including the mass fraction of emitted particles (5-7%), particle density (150-300 kg/m³), and drag coefficient formulation (fixed versus size-dependent), whose values were selected based on existing literature and physical plausibility. Over a 16-day sampling period, measured particle deposition ranged from 0.35 to 11.1 g/m². The largest deposition values (9.12-11.10 g/m²) occurred at collection sites closest to the fire (4-8 km), with progressively lower deposition (0.58-2.62 g/m²) observed at distant sites (10-40 km). When extrapolated to the landscape scale, a deposition rate of 10 g/m² over 1 km² corresponds to approximately 10 metric tons of pyrogenic material delivered to ecosystems for two weeks, an amount comparable to inputs from volcanic ashfall events. Within the modeling framework, simulations assuming a particle density of 300 kg/m³ and a pyrogenic emission fraction of 7% most closely matched field observations (RMSE < 1.8 g/m²; modest positive bias 0.8 g/m²; R > 0.90; p > 0.2). This configuration successfully reproduced both the magnitude and spatial gradients of observed pyrogenic mass deposition, demonstrating the framework’s potential to predict and quantify downwind delivery of wildfire-emitted particulate material to ecosystems.