Molecular Speciation of Organic Nitrogen Compounds Separated in Smoke Particles Emitted from Burning Western U.S. Wildland Fuels

Organic nitrogen (ON) compounds play a critical role in Earth’s biogeochemical cycles, air quality, and human health, but these compounds can be difficult to examine due to complex chemical characteristics (e.g., polarity, reactivity, and thermal stability). Biomass burning (BB) events from wildland forest fires (e.g., wildfires or prescribed burns), are an important source of ON compounds into the atmosphere. Wildfire seasons in the western U.S. are projected to start earlier and last longer, resulting in more frequent and intense wildfires in this region. Prescribed burning is being heavily encouraged in the western U.S. to reduce the fuel loading in wildland forest, thereby reducing the risk of severe wildfires. Characterizing smoke emissions specifically from western U.S. wildland forest fires is not thoroughly investigated, especially regarding ON compounds. Thus, to properly capture the impacts western U.S. wildland forest fire emissions can have on regional air quality and the environment, it is critical to have detailed chemical characterization of the ON compounds in the smoke particles. For my thesis research, I have applied high resolution instrumentation techniques to establish the molecular speciation and quantification of a wide range of organic nitrogen compounds from smoke relevant to western U.S. wildland forest fires. 

A sequential spot sampler (S3) was used throughout this thesis research to collect fresh smoke particles from biomass burning emissions. Samples were solvent extracted and analyzed offline using a high-performance liquid chromatography in tandem with an electrospray ionization ultra-high resolution orbitrap mass spectrometer (HPLC-OMS). Using the particle sampling mechanism (S3) and the offline instrumentation technique (HPLC-OMS), thousands of organic and organic nitrogen compounds were molecularly speciated and quantified in smoke from a variety of biomass burning sources relevant to western U.S. wildland forests.

This thesis presents results on the chemical characterization of organic nitrogen compounds in smoke from a variety of different sources. These sources include a prescribed burn campaign of western U.S. mixed conifer forests. Fresh smoke particles were collected during a four dayprescribed burn campaign of a first entry mixed conifer forest located in the Blodgett Forest Research Station (BFRS) in the Northern Sierra Nevada, California. The campaign was done in collaboration with University of California, Berkeley (UCB) and Riverside (UCR). A wide range of organic and organic nitrogen compounds were separated and molecularly speciated. The contribution each compound had toward overall smoke composition was estimated based on its ion intensity in the smoke samples, and ON compounds were found to make up to half of the overall smoke composition. Average emission factors (EF) were also established for separated organic and ON compounds across all burned forest stands. 

Dead and live fuels relevant to a western U.S. mixed conifer forests were collected from BFRS and burned in the lab. Thousands of organic and ON compounds were separated from the smoke samples. Additional statistical analysis was then performed using principal component analysis to establish if any organic or ON compounds may be unique to smoke emitted from specific biomass fuels (i.e., tracer compounds). Differences in the chemical composition of smoke particles were found between burning dead and live fuels in general. But also smoke particles from burning specific fuels such as tanoak showed to have potential organic and organic nitrogen tracer compounds. 

Emission factors of the organic and ON compounds separated from the laboratory burns of dead and live fuel were established and related to burning condition (quantified using modified combustion efficiency, MCE). Linear regression models that relate EF of ON compounds and MCE were quantified for each fuel and fuel mixture burned in the lab. Such regression models are beneficial toward predicting how emissions change at various burning conditions (e.g., smoldering or flaming combustion) across each biomass fuel. Results showed that EFs of ON compounds at various burning conditions were higher for live fuels compared to dead fuels. 

Overall, this thesis research expands on the ON compounds found and quantified in smoke emitted from fuels relevant to western U.S. mixed conifer wildland forest fires. Results can be applied towards better understanding the implications of smoke produced from prescribed burning and burning common biomass fuels can have on Earth’s biogeochemical cycles, regional air quality and the environment. Subsequently, this information can then be used by forest managers to better inform future forest management policies to best fit the needs of the environment.