Importance of Particle-Phase Reactions in the Growth of Newly Formed Particles

New particle formation (NPF) is a chemistry-driven process that results in the formation of secondary aerosols and is the main source of global cloud condensation nuclei. Currently, the majority of NPF parametrizations consider volatility-based gas-to-particle conversion of oxygenated organic molecules (OOMs), formed only in the gas phase, and assume thermodynamic equilibrium regardless of aerosol chemical composition or environmental conditions. Here, we performed a comprehensive chemical analysis of the OOMs produced from α-pinene ozonolysis in a fast-flow reactor to elucidate the role of gas- and particle-phase chemistry in the NPF processes. Gas- and particle-phase OOMs were measured with an iodide high-resolution time-of-flight chemical ionization mass spectrometer (HrTOF-CIMS) attached to the filter inlet for gas and aerosol (FIGAERO). Additionally, particle-phase OOMs were detected with off-line ultra-performance liquid chromatography-electrospray ionization (UPLC-ESI) high-resolution Orbitrap MS/MS analysis. Mass spectra of particle-phase OOMs detected with these two methods showed surprisingly similar features, despite entirely different sampling, ionization, and detection techniques. 100% of the OOMs detected with the UPLC-ESI Orbitrap mass spectrometer contained 2-8 isomers with different fragmentation ions. Volatility estimation methods based on elemental composition alone cannot account for chemical functionalities and molecular structures and thus do not differentiate isomers, despite the potential for large volatility differences. Our analysis shows that biogenic OOM dimers can also form directly within the particle phase, via either accretion or decomposition reactions. These particle-phase reactions can affect the chemical composition and volatilities of the OOMs, and, in turn, can affect the phase state and diffusivity of aerosols. Our observations strongly imply the importance of considering gas- and particle-phase chemistry in the growth of freshly formed particles.