UC Riverside

This dissertation investigates the formation of secondary brown carbon (BrC) from nighttime oxidation of heterocyclic volatile organic compounds (VOCs) such as furans and pyrroles under diverse environmental conditions. This dissertation focuses on clarifying the effects of different environmental factors such as nitrate radical (NO3) levels, pre-existing particles, and relative humidity (RH) on the optical properties of secondary BrC and the chemical composition of secondary organic aerosols (SOAs) derived from the heterocyclic VOCs. NO3 levels exhibit insignificantly influence on the overall composition of furan-derived SOA, while pre-existing particles show considerable effects on its chemical composition. The mass absorption coefficients (MAC) revealed the divergent impacts of NO3 levels and pre-existing particles on BrC light absorption, suggesting NO3 facilitates the generation of light-absorbing compounds whereas pre-existing particles may facilitate gas-to-particle partitioning of non-absorbing products. The effects of RH on secondary BrC formation were investigated through furan and pyrrole oxidation, revealing substantial effects on particulate size distribution dynamics. The pyrrole oxidation products exhibit higher potential in producing ultrafine particles via nucleation. The increase of RH leads to increased mass fractions of oxygenated compounds in both SOAs, indicating the enhanced gas-phase and/or multiphase oxidation processes in humid environments. Interestingly, higher RH leads to decreased MAC, contrasting to the trend observed in the secondary BrC derived from homocyclic precursors. This divergence is owing to the formation of non-absorbing high-molecular-weight oxygenated compounds and the reduced mass fractions of molecular chromophores. Furthermore, this dissertation highlights the crucial role of carbonyl chromophores in the light absorption of secondary BrC derived from a variety of heterocyclic VOCs. In particular, N-containing carbonyl chromophores (e.g., imides and amides) derived from pyrroles are potentially important chromophores in secondary BrC. The contribution of chromophores on the overall BrC light absorption may vary significantly across wavelengths, underscoring their distinct significance at different wavelength ranges. Overall, this dissertation contributes to the fundamental insights of secondary BrC formation in the complicated atmospheric environments, especially those within the wildfire smoke. All of these findings will be helpful for improving the evaluation of BrC's radiative forcing and its impact on climate change.