Unraveling the atmospheric oxidation mechanism and kinetics of naphthalene: Insights from theoretical exploration.

Naphthalene, the most abundant polycyclic aromatic hydrocarbon in the atmosphere, significantly influences OH consumption and secondary organic aerosol (SOA) formation. Naphthoquinone (NQ) is a significant contributor to ring-retaining SOA from naphthalene degradation, impacting the redox properties and toxicity of ambient particles. However, inconsistencies persist regarding concentrations of its isomers, 1,2-NQ and 1,4-NQ. In present work, our theoretical investigation into naphthalene's reaction with OH and subsequent oxygenation unveils their role in SOA formation. The reaction kinetics of initial OH and subsequent O oxidation was extensively studied using high-level quantum chemical methods (DLPNO-CCSD(T)/aug-ccpVQZ//M052x-D3/6-311++G(d,p)) combined with RRKM/master equation simulations. The reactions mainly proceed through electrophilic addition and abstraction from the aromatic ring. The total rate coefficient of naphthalene + OH at 300 K and 1 atm from our calculation (7.2 × 10 cm molecule s) agrees well with previous measurements (∼1 × 10 cm molecule s). The computed branching ratios facilitate accurate product yield determination. The largest yield of 1-hydroxynaphthalen-1-yl radical (add1) producing the major precursor of RO is computed to be 93.8 % in the ambient environment. Our calculated total rate coefficient (5.2 × 10 cm molecule s) for add1 + O closely matches that of limited experimental data (8.0 × 10 cm molecule s). Peroxy radicals (RO) generated from add1 + O include 4-cis/trans-(1-hydroxynaphthalen-1-yl)-peroxy radical (add1-4OO, 66.0 %/17.5 %), 2-cis/trans-(1-hydroxynaphthalen-1-yl)-peroxy radical (add1-2OO, 10.3 %/6.3 %). Regarding the debated predominance of 1,4-NQ (corresponding to the parent RO, i.e., add1-4OO) and 1,2-NQ (corresponding to the parent RO, i.e., add1-2OO) in the atmosphere, our findings substantiate the dominance of 1,4-NQ. This study also indicates potential weakening of 1,4-NQ's dominance due to competition from decomposition reactions of add1-4OO and add1-2OO. Precise reaction kinetics data are essential for characterizing SOA transformation derived from naphthalene and assessing their climatic impacts within modeling frameworks.