Current understanding suggests that the intrinsic properties of freshly nucleated particles critically determine their early growth behavior and potential to reach sizes where they can serve as cloud droplet formation seeds. In this contribution, we present our recent work of theoretically exploring the growth and hydration mechanisms of atmospheric clusters. Sulfuric acid and ammonia clusters will be focused, as they are established as the most prevalent drivers of NPF in many regions due to their strong binding properties and high atmospheric concentrations.

Better understanding of atmospheric sulphur chemistry is key to better modelling and predictions of new particle formation. This presentation will discuss the use of ab initio (DFT) calculations paired with metadynamics simulations to understand reactions in systems of SO₃, H₂O and atmospheric acids, in particular the branching fraction between production of sulphuric acid versus other organosulphates which have been recently measured experimentally. We also hypothesize the formation of new, hitherto unexamined complex organosulphates in the atmosphere.

In our study, we employ semi-empirical molecular dynamics (SEMD) at the GFN1-xTB level to investigate monomers sticking onto freshly nucleated particles, including combinations of sulfuric acid and various amines. Our results reveal that neglecting long-range interactions underestimates the number of collisions leading to sticking. By comparing SEMD and all-atom force field simulations, we find similar enhancement factors, although discrepancies appear at lower collision velocities. For systems with larger effective masses, where such velocities are more prevalent, we would expect the two methods to diverge.

Atmospheric aerosols contain diverse organic compounds, the majority of which remain uninvestigated. Quantifying their effect on aerosol processes depends on properties like the glass transition temperature (T_g). Molecular Dynamics (MD) simulations were carried out to predict T_g for various compounds, considering carbon and oxygen content, functional groups (-COOH > -OH > -CO), and molecular architecture (cyclic vs. linear). T_g increased with carbon number and was consistently higher in cyclic structures. A parameterization was developed based on the MD predictions and was evaluated against experimental measurements. A leave-one-out evaluation approach was utilized, providing insights into the contributions of various molecular features.

Aromatic hydrocarbons are prolific contributors to secondary organic aerosol (SOA). It is known that this contribution is multi-generational, where every oxidation step produces more reactive aromatics, each with successively larger contribution to SOA. However, our understanding of the underlying autoxidation mechanisms that produce low-volatility products is limited, preventing us from quantifying this SOA contribution. This work resolves this using quantum chemical calculations and targetted experiments. We show that a previously unknown geminal diol intermediate plays a key role in the autoxidation of multi-generation aromatic products. This mechanism will significantly reduce the current uncertainties in the contribution of aromatics to SOA.

I present an overview of new computational/theoretical results concerning branching points in the gas-phase oxidation chemistry leading from precursors (hydrocarbons and other VOCs emited to the air) to polytunctional low-volatility products relevant to aerosol formation.