Atmospheric Fate of n-Propyl Nitrate: Unraveling ^•OH-Initiated Oxidation Pathways, Kinetics, and Subsequent Degradation Mechanisms

N-propyl nitrate (NPN, CH₃CH₂CH₂ONO₂), a prevalent atmospheric alkyl nitrate (RONO₂), is a key component of secondary organic aerosol (SOA) and a critical NO_x reservoir. To support atmospheric pollution control, its ^•OH-initiated degradation mechanism, kinetics, atmospheric lifetime, and subsequent reactions were systematically investigated using density functional theory (DFT) and multistructural canonical variational transition state theory with small curvature tunneling (MS-CVT/SCT). Conformational searches identified the most stable conformers of NPN and the transition states for α-, β-, and γ-C–H H-abstraction, with α- and β-C–H abstractions as dominant pathways. Rate constants and branching ratios were calculated with multistructural torsional (MS-T) anharmonicity correction. The total rate constants show minimal variation (±2%) at 200–263 K, with an average value of ∼7.03 × 10^–13 cm³ molecule^–1 s^–1; at 263–1000 K, they agree well with experimental values and display weak positive temperature dependence, with a calculated value of 7.59 × 10^–13 cm³ molecule^–1 s^–1 at 298 K. The atmospheric lifetime of NPN is 1.02–18.56 days at 217–298 K. Given the scarcity of experimental data on ^•OH-initiated oxidation of NPN, the comprehensive kinetic and mechanistic results presented herein provide valuable supplementary data for atmospheric chemistry databases and offer guidance for future experimental investigations.