Freezing atmospheric organic nucleation: A matrix isolation study

New particle formation (NPF) is the largest source of atmospheric aerosols with respect to their number and has a large impact on the global climate and human health. During this process, low-volatility vapors form stable molecular clusters, which subsequently grow through the condensation of additional molecules. Inorganic acids such as sulfuric acid or iodic acid are often the main drivers of clustering. While organic molecules contribute significantly to the growth processes, it remains unclear at what stage they start to contribute to NPF, i.e., the exact clustering routes of organic or organic-inorganic mixtures are unknown. This is in part due to the fact that current state-of-the-art mass spectroscopic methods only provide compositional information and not information on the actual structure of the molecular clusters or the functionalization of the growing nanoparticles. However, ultimately, the interaction between functional groups defines the properties of the molecular clusters [1].

Here, we use matrix isolation Fourier transform infrared spectroscopy (MI-FTIR) as a new tool to investigate the formation of molecular clusters. It was previously demonstrated [2,3] that MI-FTIR is a reliable tool for studying small molecular clusters' structure. In the present work, we investigate organic precursor vapors, their oxidation products, and newly formed clusters as stabilized in inert noble-gas matrices. Due to cryogenic temperatures, rotational transitions are suppressed, making the identification of the cluster constituents and the molecular structure of the cluster easier compared to conventional gas-phase FTIR.

The current study focuses on NPF involving α-pinene, a monoterpene that is recognized to be rapidly convertible to extremely low volatile organic compounds (ELVOC). Spectra of α-pinene and first-order oxidation products from α-pinene, such as pinic and pinonic acid, isolated in noble gas matrices are presented. The infrared absorption bands are compared to calculations based on density functional theory (DFT), as specific bands can be associated with the presence of multimers in the matrix. This gives insights into potential cluster formation pathways and can be used to benchmark the most widely used DFT approaches with experimental data. Altogether, we demonstrate that our MI-FTIR setup provides a new approach to NPF studies, complementing mass spectrometry-based measurements.

References:

[1]: Stolzenburg, D. et al. Atmospheric nanoparticle growth. Rev. Mod. Phys. 95, 045002 (2023).

[2]: Köck, E.-M. et al. Alpha-Carbonic Acid Revisited: Carbonic Acid Monomethyl Ester as a Solid and its Conformational Isomerism in the Gas Phase Chem. Eur. J. 26, 285 (2020).

[3]: Dinu, D. F. et al. Increase of Radiative Forcing through Midinfrared Absorption by Stable CO 2 Dimers? J. Phys. Chem. A 126, 2966–2975 (2022).