Nucleation of pinic acid: Using matrix-isolation Fourier transform infrared spectroscopy to get a first glance at processes on the molecular level

New particle formation (NPF) is the predominant source of atmospheric aerosols globally in terms of particle number concentration [1]. NPF is a multi-step process, consisting of the nucleation of low-volatility vapors from the gas-phase, and subsequent growth of the initial molecular clusters through condensation. Nucleation is, in many environments, primarily driven by inorganic acids, such as sulfuric acid over land or iodic acid over the ocean. Organic acids, such as pinic acid, an oxidation product of the highly abundant alpha-pinene, are also contributing to the mass of fully grown particles. It is, however, not yet clear at what stage of NPF organic compounds become important and whether organic compounds can nucleate even without inorganic substances being present [2]. This knowledge gap arises partly from the fact that mass spectrometry provides only compositional data and lacks structural insights into potential bindings of initial-stage clusters.

Here, we show that matrix-isolation Fourier transform infrared spectroscopy (MI-FTIR) can be used to study the initial steps of nucleation. Monomers and dimers of pinic acid are studied in cryogenic argon matrices to characterize their bindings based on the vibrational spectrum. In such cryogenic matrices, the infrared spectra are greatly simplified compared to gas-phase FTIR measurements due to the suppression of the rotational bands, making small dimer bands clearly visible. In addition, we investigate the free energies and harmonic vibrational frequencies of pinic acid monomers and dimers using molecular dynamics simulations and quantum chemical calculations, aiming to assess how different molecular alignments during nucleation influence the IR spectrum. By comparing the experimental MI-FTIR data with these density functional theory (DFT) calculations, it is shown that the measured pinic acid dimer spectra best fit the calculated ones for dimers with only one OH-bridge having formed during dimerization. The existence of such singly bonded clusters, which are not predicted as the lowest free energy conformer by DFT, could be ideal for subsequent growth due to up to three unbound OH-groups available for further oligomerization. We also find that pinic acid forms dimers much more easily than other alpha-pinene oxidation products, such as pinonic acid.

This study on the nucleation of pinic acid shows that MI-FTIR is a versatile method to study the structure of precursor molecules of NPF and the very first stages of nucleation. The toolbox of MI-FTIR in conjunction with DFT calculations can readily be implemented for other organic NPF-precursors, such as MBTCA, and their nucleation, leading to valuable insights into the structure of initial clusters.

References:

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

[2]: Elm, J., et al.: Quantum Chemical Modeling of Organic Enhanced Atmospheric Nucleation: A Critical Review. WIREs Comp. Molec. Sc. 13, e1662 (2023).