Ab initio kinetics of the pyrene + OH reaction: a revisited study

Pyrene belongs to the highly toxic class of polycyclic aromatic hydrocarbons and has carcinogenic properties; thus, the kinetics of its OH-initiated oxidation is theoretically investigated in a wide range of conditions (T = 200-2000 K and P = 7.6-76,000 Torr). The T & P-dependent kinetic behaviors were studied within the stochastic RRKM-based master equation framework with the potential energy profile constructed at the ROCBS-QB3//M06-2X/aug-cc-pVTZ level. The computed total rate constants k_total are in good agreement with the laboratory values, thus helping resolve the discrepancy between the prior calculations and the measured values. The model reveals detailed mechanistic insights: (i) the OH-addition channels from the Cα and Cγ atoms of pyrene (to form the adducts 5-hydroxy-4,5-dihydropyren-4-yl (I1) and 1-hydroxy-1H-pyren-10a-yl (I2)) predominate under atmospheric conditions, while the direct H-abstraction pathways become dominating at T > 500 K; (ii) the U-shaped T-dependent behaviors of k_total and its slightly positive P-dependence at low T (e.g., T ≤ 500 K and P = 760 Torr) are due to the T-dependent mechanism shift. Additionally, pyrene should not be considered a persistent organic pollutant (POP) due to its short calculated atmospheric lifetime (∼4.1 hours toward OH), which is significantly shorter than those resulting from degradation by other abundant oxidants (i.e., Cl, NO₃, and O₃) present in the atmosphere. Moreover, pyren-4-ol (and pyrene-4,5-dione) and pyren-1-ol (and pyrene-1,2-dione) are suggested as the primary products of I1 and I2, respectively, when further oxidized by O₂/NO. The results reveal that both short and long-term pyrene exposure is highly toxic to aquatic organisms; while its main degradation products, namely, I1 and I2, show lower toxicity, they still pose a significant threat to marine organisms and the ecosystem.