UC Riverside

Organic aerosols (OA) contribute to a substantial fraction of atmospheric fine particulate matter (PM2.5) that significantly impact the Earth’s radiative forcing, air quality, and human health. OA are formed from direct emissions or by secondary formation through oxidation of volatile organic compounds (VOC). Once formed in the atmosphere, OA can undergo various chemical and physical evolution throughout their lifetime on the order of ~ 10 days, constantly changing their compositions and properties. Thus, elucidating these changes is crucial for understanding their environmental impacts, and a comprehensive suite of state-of-art mass spectrometry techniques have been coupled to achieve an isomer-resolved picture of the OA compositions during the evolution. The dissertation focuses on examining (1) chemical aging at the gas-particle interface initiated by gas-phase oxidants (e.g., O3 and OH), and (2) evolution in the condensed-phase caused by ambient temperature and relative humidity (RH) change. The following projects have been studied to address underexplored scientific questions on OA chemical evolution and transformation, including: (I) Criegee intermediate dynamics during heterogeneous ozonolysis of endocyclic unsaturated OA proxies were examined to highlight the importance of particle-phase water and the rapid later-generation reactions that together govern the products. (II) Interfacial dimer formation during the heterogeneous OH-initiated oxidation of OA surrogates were demonstrated, elucidating new mechanisms of dimerization by organic radical (i.e., peroxy and alkoxy radicals) cross reactions at the gas-particle interface. (III) OA molecules with the same formula but different branching structures could affect the overall heterogeneous OH oxidation and control the oxidized OA composition via site-specific mechanisms, which results largely distinct functionalization, fragmentation, and oligomerization. (IV) Exploration of SOA from α-pinene ozonolysis in a smog chamber as the temperature and RH cycle determined that changing ambient conditions can dictate kinetics and the extent of gas-particle partitioning and impact diverse reversible and irreversible reactions between monomers and oligomers during the changes. (V) Influence of heating on the composition of OA was investigated because thermal desorption of OA is often applied to vaporize the particles for subsequent real-time mass spectrometry analysis. How thermal desorption leads to chemical decomposition of the OA constituents were studied.