Theoretical Mapping of the Gas-Phase Ozonolysis of α-Pinene: Formation of First-Generation Products under Different Atmospheric Conditions

Abstract

Ozonolysis of α-pinene is a significant and well-established source of atmospheric secondary organic aerosol (SOA), which plays a pivotal role in climate, air quality, and human health. The products of α-pinene ozonolysis measured experimentally are typically characterized by only their molecular formulas, while their structures and formation mechanisms often remain unclear. In this work, we theoretically map the oxidation pathways, structures, and formation time scales of the major first-generation products formed from α-pinene ozonolysis by calculating the H-shift and bond-scission reaction rate coefficients of the peroxy (RO₂) and alkoxy (RO) radicals that arise under atmospheric conditions with different RO₂ bimolecular reaction rates (k_bi): polluted (k_bi > 0.2 s^–1), moderate (0.2 s^–1 > k_bi > 0.01 s^–1), and pristine (k_bi ≈ 0.01 s^–1). In polluted environments, almost no RO₂ unimolecular reactions are of importance and ozonolysis leads to nitrates and small fragmented products. By contrast, in moderate to pristine atmospheres, C₁₀ highly oxygenated organic molecules (HOMs) with up to 12 oxygen atoms can form from either purely unimolecular or a combination of unimolecular and bimolecular reactions. Our results suggest that explicit chemical mechanisms of α-pinene ozonolysis used ubiquitously in the literature require significant revision in their treatment of unimolecular-isomerization and stereoisomer-specific reactions.