Role of ozone in enhancing the formation of aerosol precursors in the OH-initiated oxidation of naphthalene

The high yield of condensable vapors from OH-initiated oxidation of naphthalene raises intriguing questions about the role of ozone in this process. As the simplest polycyclic aromatic hydrocarbon (PAH), naphthalene is a significant component of anthropogenic volatile organic compounds (AVOCs) in urban atmospheres, characterized by its high reactivity and prevalence among PAHs. Emitted primarily through the incomplete combustion of fossil fuels and biomass, naphthalene plays a pivotal role in atmospheric chemistry under ambient conditions. While recent studies (Zhang et al., 2012; Garmash et al., 2020) highlight the substantial contribution of naphthalene to secondary organic aerosol (SOA) formation, these are in conflict with our current molecular level understanding of the oxidation process. In the atmosphere, naphthalene is quickly oxidized by the addition of an OH radical to the aromatic ring, forming a carbon-centered radical (Shiroudi et al., 2015, Gnanaprakasam et al., 2017). This subsequently reacts with O₂ to generate peroxy radicals, which undergo autoxidation, resulting in the formation of low-volatility products containing multiple oxygen atoms i.e., highly oxygenated organic molecules (HOM) which contribute to SOA formation. However, molecular level studies indicate autoxidation rates that are much too slow to explain the observed HOM in the oxidation of naphthalene. Also, previous experiments that measured HOM yields from OH-initiated oxidation of naphthalene (Molteni et al., 2018, Garmash et al., 2020) did not investigate the effect of ozone. We think that ozone is the missing piece in resolving the discrepancy between our current molecular level understanding of naphthalene oxidation and measurements.

In this study, laboratory experiments were conducted to investigate the oxidation of naphthalene by hydroxyl (OH) radicals using a flow reactor coupled with a nitrate-based chemical ionization mass spectrometer (NO₃⁻-CIMS). The influence of ozone on the reaction products was systematically explored. Results revealed a significant enhancement in product intensities, particularly monomers (C₁₀H₉O₅_-₁₀), in the presence of ozone. The reaction time was varied from 2.1 – 0.7 seconds. At a reaction time of 0.7 seconds, the addition of ozone led to the formation of a series of monomeric products that were absent in the ozone-free environment. Complementary high-level quantum chemical calculations provided mechanistic insights into the role of ozone in product formation. To further elucidate product formation pathways, experiments were also conducted for the OH-initiated oxidation of naphthalene derivatives such as 1-naphthol and 2-naphthol. A significant effect of ozone was observed in the oxidation of 1-naphthol, whereas no prominent change was noted in the case of 2-naphthol. These findings indicate that the oxidation of naphthalene proceeds rapidly enough to compete with other bimolecular reactions such as RO2 + RO2/HO2/NO, and the presence of ozone is crucial for the formation of HOM and consequently has a pronounced effect on the SOA formation.

References:

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

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

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

Gnanaprakasam, M. et al (2017) Theor. Chem. Acc. 136, 131.

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