Can Accretion Products Be Formed at the Interface of Freshly Nucleated Particles?

Area of Science:

• Atmospheric Chemistry
• Chemical Kinetics
• Particle Formation

Background:

• Gas-phase RO2 + R'O2 radical reactions form low-volatile ROOR' accretion products via triplet clusters.
• The occurrence and significance of these reactions at the interface of freshly nucleated particles (FNPs) remain unexplored.
• Organic shells on FNPs could potentially stabilize accretion products formed at the particle surface.

Purpose of the Study:

• To investigate the influence of FNP precursors (sulfuric acid, ammonia, dimethylamine) on RO2 + R'O2 reactions.
• To determine the feasibility and rates of accretion product formation at FNP model clusters.
• To assess the stability of accretion products formed at the FNP interface.

Main Methods:

• Quantum chemical calculations were employed to study the RO2 + R'O2 reaction mechanism.
• Systematic conformational sampling using ABCluster and CREST identified cluster structures.
• High-level theoretical methods (ωB97X-D, XMC-QDPT2, CASSCF) calculated energies, frequencies, and spin-orbit couplings.

Main Results:

• Intersystem crossing rates for ROOR' formation at FNP model clusters are comparable to gas-phase rates (10^6–10^9 s^-1).
• Intermediate clusters and ROOR' products exhibit strong interactions with FNP components, suppressing evaporation.
• The requirement of two RO2 radicals limits the overall formation rate, suggesting it's a rare event.

Conclusions:

• RO2 + R'O2 reactions can occur at FNP surfaces, forming stabilized accretion products.
• The formation pathway is likely limited by low surface concentrations of RO2 radicals.
• A hypothesized Langmuir-Hinshelwood mechanism for surface recombination is proposed but considered extremely rare.