Role of Water Molecule in the Gas-Phase Formation Process of Nitrated Polycyclic Aromatic Hydrocarbons in the Atmosphere: A Computational Study

Nitro-PAHs are globally worrisome air pollutants because their high direct-acting mutagenicity and carcinogenicity. A mechanistic understanding of their formation is of crucial importance for successful prevention of their atmospheric pollution. Here, the formation of nitro-PAHs arising from the OH-initiated and NO₃-initiated atmospheric reactions of PAHs was investigated by using quantum chemical calculations. It is widely assumed that OH or NO₃ radicals attack on the C atoms of the aromatic rings in the PAH molecule, followed by the addition of NO₂ to the OH–PAH or NO₃–PAH adducts at the ortho position and the loss of water or nitric acid to form nitro-PAHs. However, calculations show that the direct loss of water from the OH–NO₂–PAH adducts via the unimolecular decomposition is energetically unfavorable. This study reveals for the first time that water molecule plays an important catalytic effect on the loss of water from the OH–NO₂–PAH adducts and promotes the formation of nitro-PAHs. In addition, the introduction of water unwraps new formation pathway through the addition of NO₂ to the OH–PAH or NO₃–PAH adduct at the para position. The individual and overall rate constants for the addition reactions of PAHs with OH and NO₃ radicals were deduced by using the Rice–Ramsperger–Kassel–Marcus (RRKM) theory.