UC Berkeley

Wildfires and wildland–urban interface (WUI) fires are increasingly frequent and destructive events that threaten ecosystems, infrastructure, and public health across California and other fire-prone regions. While the chemistry of biomass burning has been studied extensively, comparatively little is known about how oxygen availability and fuel composition jointly control the emissions and toxicity of WUI fires, where natural and synthetic materials burn together. The lack of controlled data under different oxygen conditions has made it difficult to accurately describe combustion behavior in emission inventories and exposure models. This dissertation addresses that knowledge gap by developing and applying a novel experimental framework to quantify gaseous and particulate emissions from representative wildland and WUI fuels under well-defined oxygen-limited conditions. A custom linear tube-heater combustion system was designed to generate steady, repeatable burning across a range of external heat fluxes (25–50 kW m-2) and oxygen concentrations (0–21%). The apparatus integrates a moving ceramic heater, variable oxidizer control, and a dual-sampling design that enables simultaneous gas-phase and particulate measurements. Gas species were analyzed in real time using Fourier-transform infrared (FTIR) spectroscopy, while particulates were collected by a cascade impactor or filter cassette. Three major fuel categories were examined. (1) For chaparral samples, higher moisture promoted smoldering and greater emissions of incomplete combustion products, while dry fuels burned more efficiently with higher modified combustion efficiency (MCE) and CO2 output. Smoldering combustion produced roughly one order of magnitude higher particulate emissions than flaming, and emission variability reflected both species and moisture effects. (2) For ponderosa pine needles, decreasing oxygen concentration from ambient to sub-ambient levels had a stronger effect on flaming emissions than on smoldering hydrocarbons, as MCE declined sharply and toxic gases and particulate matter increased under oxygen limitation. (3) For WUI structural materials, combustion produced much higher levels of halogenated and nitrogenated species (HCl, HF, HCN), indicating greater toxicity than natural fuels. A toxicity-based evaluation framework was developed in this study and linked measured emissions with health-relevant exposure limits. This dissertation contributes an integrated experimental and analytical foundation for emission quantification under controlled oxygen limitation. The resulting data and analysis framework enhance understanding of fire emission processes, improve air-quality and climate modeling, and support exposure and health-risk assessments for communities and firefighters.