Fast formation of aerosol precursors in polycyclic aromatic hydrocarbon oxidation: Evidence for ozone-assisted chemistry

The formation of highly oxygenated organic molecules (HOM) during the OH-initiated oxidation of naphthalene remains poorly understood, despite experimental evidence for efficient aerosol precursor formation (Molteni et al., 2018; Garmash et al., 2020). As the simplest polycyclic aromatic hydrocarbon (PAH) and a major anthropogenic volatile organic compound in urban atmospheres, naphthalene is ubiquitous and readily oxidized under ambient conditions. However, current molecular-level descriptions of its oxidation predict autoxidation rates that are too slow to explain the observed HOM abundances, indicating missing or overlooked chemical pathways (Zhang et al., 2012; Shiroudi et al., 2015; Lannuque et al., 2024).

Ozone is one of the most abundant atmospheric oxidants, yet it is generally assumed to play a negligible role in the gas-phase oxidation of PAHs and their contribution to secondary organic aerosol (SOA) formation. Here, we show that this assumption does not hold for naphthalene oxidation, and that ozone can strongly influence the early stages of its autoxidation chemistry.

We investigated the hydroxyl radical (OH)–initiated oxidation of naphthalene using a flow reactor coupled to a nitrate-based chemical ionization mass spectrometer (NO₃⁻-CIMS), with reaction times ranging from 0.7 to 1.8 s. The presence of ozone led to a pronounced enhancement in product signal intensities, particularly for monomeric species (C₁₀H₉O_5-10). At the shortest reaction time (0.7 s), a distinct suite of oxygenated monomers was observed only in the presence of ozone, indicating rapid ozone-assisted chemistry. Experiments using isotopically labelled ozone (¹⁸O₃) demonstrate that ozone directly influences the early stages of OH-initiated naphthalene oxidation. High-level quantum chemical calculations support mechanistic pathways in which ozone alters the fate of key radical intermediates, enabling efficient HOM formation. Moreover, the experiments on OH initiated oxidation of 1-naphthol, 2-naphthol, biphenyl, and anthracene show that this behavior is strongly structure-dependent, highlighting the broader relevance of ozone-assisted chemistry for PAHs.

Finally, global modeling indicates that ozone-driven pathways can increase anthropogenic SOA formation from naphthalene by up to 7% on a global scale. Together, these results reveal an unrecognized role of ozone in PAH oxidation and provide a mechanistic framework that helps to resolve the discrepancies between laboratory observations and current molecular-level understanding of PAH derived SOA formation.

 

References:

Molteni, U. et al (2018) Atmos. Chem. Phys. 18, 1909-1921.

Garmash, O. et al (2020) Atmos. Chem. Phys. 20, 515-537.

Zhang, Z. et al (2012) Phys. Chem. Chem. Phys. 14, 2645 - 2650.

Shiroudi, A. et al (2015) Phys. Chem. Chem. Phys. 17, 13719-13732.

Lannuque, A. et al (2024) Atmos. Chem. Phys. 24, 8589–8606.