<!--StartFragment--><!--StartFragment-->Laboratory Investigation of Particle Surface Organic Peroxy Radicals Chemistry<!--EndFragment-->

The proposed research aims to quantify how aerosol particle size, viscosity, and organic molecular structure influence NOx-free organic peroxy radical (RO2) chemistry, emphasizing the specific role of the particle surface. RO2 is critical in atmospheric chemistry, contributing to the production of secondary organic aerosols (SOA), that affect air quality, human health, and climate change. NOx-free RO2 chemistry is now recognized as increasingly important due to shifting to sustainable energy. Despite its significance, the fate of RO2, particularly at the particle surface, remains poorly understood. The challenge lies in the difficulty of distinguishing between surface and interior reactions within the submicron particles, as well as the instability of certain products. To overcome these challenges, we will employ a novel technique, Easy Ambient Sonic-Spray Ionization Mass Spectrometry (EASI-MS), with low-energy ionization to preserve labile compounds and separately detect the compositions of the particle surface and its interior. The RO2 reaction kinetics will be investigated in the flow tube with a movable inlet. Additionally, we will develop a multiphase kinetic model to account for processes of surface reaction, diffusion, and interior reaction inside the submicron particles. By integrating data from the EASI-MS measurement and the multiphase kinetic models, we will obtain a Surface-Associated Factor (SAF) to quantify how much the reactions on the particle surface contribute to the overall reactions in the particle. Quantum chemistry calculations will be conducted to illustrate energy differences on the particle surface and its interior. With this approach, we aim to investigate how the surface of aerosol particles affects the mechanisms and kinetics of NOx-free RO2 chemistry, characterized by the SAF that depends on the molecular structure and the surrounding microenvironment. Specifically, we aim to quantify the effect of particle size, particle viscosity, and molecular structure on NOx-free RO2 chemistry and derive a corresponding empirical value for SAF. The outcome of our research will not only pave a new framework for assessing surface and interior reactions for submicron particles but also provide a valuable parameter for future estimation of surface-associated reactions for RO2 chemistry that are key to the SOA. We believe our research will ultimately deepen our understanding of the surface reactions of aerosol and provide valuable insights into broader organic processes.