Photochemistry and Partitioning of Semivolatile Nitro-PAH in the Atmosphere

Ab Initio Study of Decay Dynamics of 1-Nitronaphthalene Initiated from the S2(ππ* + nNOπ*) State

Irradiation of nitro-PAHs in solution at ambient conditions leads to formation of its lowest excited triplet, dissociation intermediates nitrogen oxide (NO^•) and aryloxy radical (Ar-O^•). Experimental and theoretical studies demonstrated that Franck-Condon excited singlet state S_FC(ππ*) to a receiver, higher-energy triplet state T_n(nπ*) controlled the ultrafast population of the triplet state and, hence, the slight fluorescence yield of nitronaphthalenes. However, the detailed information about the curve-crossings of potential energy surfaces and the major channels for forming T₁ species and Ar-O^• radical were unclear. Here, by using the CASSCF//CASPT2 method, an efficient decay channel is revealed: S_2-FC-1NN → S_2-MIN-1NN or S₂T_3-MIN-1NN → T_3-MIN-1NN or T₃T_2-MIN-1NN→ T_2-MIN-1NN or T₂T_1-MIN-1NN → T_1-MIN-1NN. This explains the high yield of T_1-1NN species and minor yield of Ar-O^• and NO^• radicals. The calculation results suggest the bifurcation processes take place predominantly after the internal conversion to the T_1-1NN state via T₂T_1-MIN-1NN, one leads to T_1-MIN-1NN, while the other to T_1-MIN-ISO to produce Ar-O^• and NO^• radicals.

Nitrated polycyclic aromatic hydrocarbons (nitro-PAHs) in the environment - A review

Nitrated polycyclic aromatic hydrocarbons (nitro-PAHs) are derivatives of PAHs with at least one nitro-functional group (-NO₂) on the aromatic ring. The toxic effects of several nitro-PAHs are more pronounced than those of PAHs. Some nitro-PAHs are classified as possible or probable human carcinogens by the International Agency for Research on Cancer. Nitro-PAHs are released into the environment from combustion of carbonaceous materials (e.g. fossil fuels, biomass, waste) and post-emission transformation of PAHs. Most studies on nitro-PAHs are about air (gas-phase and particulate matter), therefore less is known about the occurrence, concentrations, transport and fate of nitro-PAHs in soils, aquatic environment and biota. Studies on partition and exchange of nitro-PAHs between adjacent environmental compartments are also sparse. The concentrations of nitro-PAHs cannot easily be predicted from the intensity of anthropogenic activity or easily related to those of PAHs. This is because anthropogenic source strengths of nitro-PAHs are different from those of PAHs, and also nitro-PAHs have additional sources (formed by photochemical conversion of PAHs). The fate and transport of nitro-PAHs could be considerably different from their related PAHs because of their higher molecular weights and considerably different sorption mechanisms. Hence, specific knowledge on nitro-PAHs is required. Regulations on nitro-PAHs are also lacking. We present an extensive review of published literature on the sources, formation, physico-chemical properties, methods of determination, occurrence, concentration, transport, fate, (eco)toxicological and adverse health effects of nitro-PAHs. We also make suggestions and recommendations about data needs, and future research directions on nitro-PAHs. It is expected that this review will stimulate scientific discussion and provide the basis for further research and regulations on nitro-PAHs.

Relationship of polycyclic aromatic hydrocarbons with oxy(quinone) and nitro derivatives during air mass transport

Airborne concentrations of Polycyclic Aromatic Hydrocarbons (PAH), quinone and nitro derivatives have been measured at three sites on the coast of Saudi Arabia to the north of the city of Jeddah. The PAH show a general reduction in concentrations from northwest to southeast, consistent with a source from a petrochemical works to the northwest of the sampling sites. In comparison, the concentrations of quinones show little variation between the sampling sites consistent with these being predominantly longer lived secondary pollutants formed from PAH oxidation. The nitro-PAH show a gradient in concentrations similar to but smaller than that for the PAH suggesting a balance between atmospheric formation and removal by photolysis. The 2-nitrofluoranthene:1-nitropyrene ratio increases from north to south, consistent with atmospheric chemical formation of the former compound, while the ratio of 2-nitrofluoranthene:2-nitropyrene is consistent with hydroxyl radical as the dominant reactant. An investigation of the changes in PAH congener ratios during air mass transport along the Red Sea coast shows consistency with reaction with a relatively low concentration of hydroxyl radical only for the day with the highest concentrations. It is concluded that while PAH degradation is occurring by chemical reaction, emissions from other locations along the air mass trajectory are most probably also leading to changes in congener ratios.

Rapid formation of secondary organic aerosol from the photolysis of 1-nitronaphthalene: role of naphthoxy radical self-reaction

The chemical composition of secondary organic aerosol (SOA) formed from the photolysis of 1-nitronaphthalene in a series of simulation chamber experiments has been investigated using an aerosol time-of-flight mass spectrometer (ATOFMS). The resulting SOA is characterized by the presence of a dimer (286 Da) proposed to be formed through the self-reaction of naphthoxy radicals along with the expected product nitronaphthol. The molecular formulas of the SOA products were confirmed by collecting filter samples and analyzing the extracts using ultrahigh resolution mass spectrometry. Further evidence for the radical self-reaction mechanism was obtained by photolyzing 1-nitronaphthalene in the presence of excess nitrobenzene, where it was shown that the resulting SOA contained a product consistent with the cross-reaction of phenoxy and naphthoxy radicals. The naphthoxy dimer was formed from the photolysis of 1-nitronaphthalene under a variety of different experimental conditions including the presence of excess butyl ether as an OH scavenger and the presence of ambient air and particles. However, formation of the dimer was suppressed when 1-nitronaphthalene was photolyzed in the presence of excess NO and nitronaphthol was instead found to be the dominant particle-phase product indicating that the yield of the dimer is dependent upon the concentration of pre-existing NO(x). The results of this work suggest that photolysis of 1-nitronaphthalene represents a previously unidentified pathway to SOA formation in the troposphere. Analogous mechanisms may also be important for other nitrated polycyclic aromatic hydrocarbons.

Photocatalytic oxidation of dinitronaphthalenes: theory and experiment

A combination of photocatalytic oxidation experiments and quantum mechanical calculations was used in order to describe the mechanism and the nature of the photocatalytic oxidation reactions of dinitronaphthalane isomers and interprete their reactivities within the framework of the Density Functional Theory (DFT). The photocatalytic oxidation reactions of three dinitronaphthalene isomers, 1,3-dinitronaphthalene, 1,5-dinitronaphthalene and 1,8-dinitronaphthalene in the presence of TiO(2) Degussa P-25 grade were investigated experimentally. The reactions were carried out in a Solarbox photoreactor equipped with a Xenon lamp. The removal of the individual substrates was followed by means of a gas chromatographic method. Nonpurgable organic carbon contents of the samples were determined by means of the catalytic oxidation method using Total Organic Carbon analyzer. With the intention of determining the best reactivity descriptors to explain the differences in the photocatalytic oxidation rates in terms of the molecular properties, geometry optimizations of the compounds were performed with the Density Functional Theory DFT at B3LYP/6-31G( *) level. In order to take the effect of adsorption on the oxidation rate, a cluster Ti(9)O(18) cut from the anatase bulk structure was modeled. The binding energies for the compounds were calculated by using the double-zeta basis set. Global hardness, softness, Fukui functions, local hardness-softness and local softness differences were calculated. The results show that the reactions investigated are orbital-controlled and electrophilic in nature. Local DFT descriptors reflect the reactivities of the dinitronaphthalene isomers better than the global ones, due to the differences in their adsorptive capacities.