Rate constants for the photolysis of the nitronaphthalenes and methylnitronaphthalenes

1-Nitronaphthalene (1NN) and 2-nitronaphthalene (2NN) are among the most abundant gas-phase nitro-polycyclic aromatic hydrocarbons (nitro-PAHs) found in ambient atmospheres, largely formed from the atmospheric reactions of naphthalene. The methylnitronaphthalenes (MNNs), of which there are 14 possible isomers, are gas-phase atmospheric transformation products of 1- and 2-methylnaphthalene. In this investigation, the photolysis rates of 1NN, 2NN, and 11 MNNs were determined indoors using black-lamp irradiation and outdoors using natural sunlight. The results of the photolysis experiments reveal that gas-phase photolysis is a major atmospheric degradation pathway for the NNs and MNNs. The nitro-PAHs examined here can be placed into three groups on the basis of their expected lifetimes due to ambient photolysis. 1M8NN and 2M1NN are predicted to have ambient lifetimes toward photolysis of ≤15 min; 1NN, 1M2NN, 1M4NN, 1M5NN, 2M4NN, 2M5NN, 2M8NN are predicted to have photolysis lifetimes ≤1 h; and 2NN, 2M6NN, 1M6NN and 1M3NN are predicted to have lifetimes on the order of 1–3 h. Thus, photolysis is expected to be the dominant atmospheric loss process for these volatile nitro-PAHs.