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Nitro-PAHs,the globally concerned air pollutants,have been found to account for over 50%of the total direct-acting mutagenicity and the total carcinogenicity activity of ambient air.The specific congeners observed in ambient air revealed that the secondary formation makes a significant contribution to the abundance profiles of nitro-PAHs.A mechanistic understanding of the transformation of PAHs and the formation of nitro-PAHs in the atmosphere is of crucial importance for successful prevention of their atmospheric pollution.The formation of nitro-PAHs arising from the OH-initiated and NO3-initiated atmospheric reactions of PAHs was investigated by using quantum chemical calculation.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,our 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 also deduced by using the Rice-Ramsperger-Kassel-Marcus theory.