Due to a combination of climate change and fire suppression, wildfires are becoming larger and more frequent in the western United States. Biomass burning is the largest global source of carbonaceous particulate matter (PM), and the second largest global source of volatile organic compounds (VOCs) in the atmosphere. Many organic compounds emitted from wildfires have human health impacts, and their atmospheric transformations produce hazardous secondary pollutants. However, there is a lack of comprehensive molecular level characterization of organic compounds in wildfire smoke, due to the chemical complexity and wildfires’ relatively episodic nature. In this work, PM and VOCs in the wildfire smoke from the point of emission to the ambient atmosphere where exposure occurs were examined. The goal is to characterize the compositions of wildfire emissions and aged wildfire smoke, their impact on urban air quality, and how indoor environments are affected by outdoor intrusion of the wildfire particulate matter.

Chapter 1 introduces the background and motivation for this work. It provides a review of current knowledge and identifies knowledge gaps in the atmospheric chemistry of wildfire particulate matter and volatile organic compounds. To elucidate the air quality impact of wildfire smoke, we should first determine the compounds emitted from wildfires. In Chapter 2, as a part of the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign, we had the opportunity to collect fresh western US wildfire smoke samples of gas and particles very close to the fires. The samples were analyzed using two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC×GC ToF-MS) instruments. The modified combustion efficiency (MCE), an indicator of the completeness of combustion, was found to be a good predictor of carbonaceous particulate matter emissions, except elemental carbon. The molecular level analysis reveals high emissions of monoterpenes, diterpenoids and resin acids, probably because of the distillation driven by heat from the fires. The speciated measurements also help to confirm that evaporation of semi-volatile particle phase organic compounds took place in smoke plumes.

We also examined the organic composition of wildfire smoke on the UC Berkeley campus, when smoke from the October 2017 Northern California wildfires (~55-65 km upwind) reached the San Francisco Bay Area. Unlike the FIREX-AQ study which focuses on wildfires in the remote atmosphere, we had a great opportunity to study the interactions between urban pollution and wildfire smoke. In Chapter 3, the composition of volatile organic compounds in smoke plumes measured by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) is reported. By correlation analysis with known primary and secondary tracers, compounds consumed or formed in the daytime aging of wildfire smoke were identified. It is also demonstrated that the wildfire VOCs added to the organic reactivity in the urban atmosphere and raised the level of ozone in the San Francisco Bay Area. In Chapter 4, the composition of organic particulate matter speciated using the GC×GC HR-ToF-MS is reported. Sugar and sugar derivatives were found to be the most abundant GC-measurable compounds in wildfire plumes, followed by mono-carboxylic acids, aromatic compounds, other oxygenated compounds, and terpenoids. By tracking 572 compounds’ time series using hierarchical analysis, a unique daytime aging factor which consists of oxygenated aromatic compounds and multifunctional acids was identified.

The wildfire particulate matter can also affect the indoor air quality. Previous characterizations of exposure to wildfire smoke particles were based mainly on outdoor concentrations of PM2.5 (particles with aerodynamical diameters < 2.5 µm). Since people mainly shelter indoors during smoke events, the infiltration of wildfire PM2.5 into buildings determines exposure. Chapter 5 presents analysis of infiltration of wildfire PM2.5 into more than 1,400 buildings in California using data from the PurpleAir sensor network. Our study reveals that infiltration of PM2.5 during wildfire days was substantially reduced compared with non-fire days, due to people’s behavioral changes. These results improve understanding of exposure to wildfire particles and facilitate informing the public about effective ways to reduce their exposure.

A summary of the major findings of this work, and recommendations for future research, are presented in the final chapter of this dissertation.