UC Riverside

Wildfires have shown an increase in occurrence and severity in recent years. These biomass burning events release volatile organic compounds (VOCs) and particulate matter in the atmosphere which impact air quality, human health, and Earth’s radiative balance. The oxidation of these wildfire-emitted VOCs at nighttime by the nitrate radical (NO3) can lead to the formation of secondary organic aerosol (SOA) and light-absorbing brown carbon (BrC). Despite their significance, the understanding of NO3-initiated oxidation mechanisms of wildfire-emitted VOCs leading to SOA and BrC in previous studies have been very limited. The major SOA constituents from these reactions have remained a challenge to uncover largely due to limitations in analytical techniques used to uncover the gas- and particle-phase chemical composition. In the following projects, we use a suite of analytical methods to elucidate the major reactions leading to the formation of SOA and BrC from representative wildfire-emitted VOCs. In Chapter 2, we studied the NO3 oxidation of seven phenolic VOCs. From this study, we discovered many products that were previously undiscovered from this system and found nitrophenol products were very dominant adding to evidence that these are a major class of compounds responsible for significant light absorption in BrC. We also found evidence of diphenyl ether dimers forming from NO3 oxidation of each of the studied phenolic VOCs. In Chapter 3, we investigated the NO3 oxidation of limonene. From this project, we discovered that the primary nitrooxy peroxy radical formed from limonene can rapidly undergo autoxidation leading to highly oxidized organonitrates. We also identified the formation of several dinitrate compounds highlighting the importance of sequential oxidation for limonene. In Chapter 4, we investigated the NO3 oxidation of selected N-containing heterocyclic VOCs: pyrrole, 1-methylpyrrole (1-MP) and 2-methylpyrrole (2-MP). From the observed product distribution in the SOA from these systems, we concluded that the presence of an easily abstractable hydrogen in precursor structure regulates the mechanism of initial NO3 oxidation and has significant effect on light absorption of the SOA. Furthermore, we propose a novel gas-phase mechanism for the addition of three NO2 groups to the backbone of pyrrole.