Oxidative aging of atmospheric organic aerosols (OA) substantially modifies their chemical compositions, physical properties, and hence the various environmental impacts. Here, we report observations of a previously unrecognized process leading to dimer formation during heterogeneous •OH-initiated oxidative aging of oxygenated OA. Isomer-resolved ion mobility mass spectrometry measurements and reaction-diffusion kinetic simulations are in good agreement, elucidating new mechanisms of dimerization by organic radical (i.e., peroxy and alkoxy radicals) cross reactions using glutaric acid as a surrogate oxygenated OA. These radical reactions are predicted to occur more prominently near the gas-particle interface following oxidation, especially in diffusion-limited viscous OA particles. Chemical structure analysis shows that esters dominate the detected dimers, followed by organic peroxides and ethers, highlighting the importance of acyl peroxy and acyloxy radicals. Simulations suggest that the reported dimer formation through the new interfacial mechanism could be appreciable under both laboratory and ambient conditions. Therefore, the dimers that are formed and enriched at the gas–particle interface are expected to play a crucial role in the effective reactivity, volatility, viscosity, and hygroscopicity of aged OA particles.

Atmospheric organic aerosols (OA) are complex mixtures of organic molecules that are usually highly functionalized through various oxidative processes. Understanding the volatilities and chemical compositions of OA is key to elucidating their environmental impacts. Thermal desorption coupling to mass spectrometry has been used as the main approach to examine both aspects of OA. In this work, we investigated the thermal desorption-induced chemical compositional change of OA from heterogeneous oxidation of glutaric acid and α-pinene ozonolysis. Using an ion mobility spectrometry mass spectrometer, coupled with total peroxide analysis and a mass transfer evaporation model, we determined diverse reactions in the particle phase during rapid heating under moderate desorption temperatures (less than 100 °C). These reactions include irreversible oligomer (e.g., esters and organic peroxides) decomposition into monomers and new oligomer formation from decarboxylation, CO elimination, decarbonylation, and dehydration. These chemical processes may effectively modify the volatility and chemical characteristics of the residual OA particles. Further, the monomeric products from thermal desorption could interfere with quantification of the original constituents without isomer separation. These findings could help reconcile the previously observed inconsistency of OA evaporation kinetics versus volatility distribution. Further, the results from this study could help interpret and constrain thermal desorption-based measurements of OA volatility and compositions.

Organic aerosols (OA) in the atmosphere are composed of molecules with highly diverse chemical structures and functionalities. The OH-initiated heterogeneous aging of these organic molecules could occur in multiple generations, exponentially increasing the complexity of the oxidation products. Furthermore, the detailed reaction mechanisms are likely different and site-specific as the OA's chemical structures vary. To systematically study these mechanisms, in this work, heterogeneous OH oxidation of five surrogate diacid OA with different positions and numbers of branched methyl groups was examined in a flow tube reactor. The oxidation products were characterized by an ion mobility mass spectrometer, which could resolve isomers and unambiguously identify individual products. Through detailed analysis of the molecular compositions of the oxidized OA, we suggest that the site-specific oxidation mechanisms largely modulate the oxidation products from the functionalization, fragmentation, and oligomerization pathways. Depending on the molecular structures and adjacent functional group(s), oxidation on primary carbons could play a key role, while that on secondary and tertiary carbons could be less important if hindered by the mesomeric effect. Our results also suggest that fragmentation products are most likely formed from later-generation reactions, while functionalization occurs only at certain carbon sites in later generations. Finally, oligomers are also largely affected by the molecular structures of the parent OA, evidenced by the distinct carbon oxidation state patterns and distributions across carbon numbers. This work provides new insights into the detailed reaction mechanisms from heterogeneous OH oxidation for diverse organic molecules.