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The oxidation of nitrite and nitrous acid to *NO2 upon irradiation of dissolved Fe(III), ferric (hydr)oxides, and nitrate has previously been shown to enhance phenol nitration. This allowed the proposal of a new role for nitrite and nitrous acid in natural waters and atmospheric aerosols. This paper deals with the interaction between hydrogen peroxide, a key environmental factor in atmospheric oxidative chemistry, and nitrite/nitrous acid. The reaction between nitrous acid and hydrogen peroxide yields peroxynitrous acid, a powerful nitrating agent and an important intermediate in atmospheric chemistry. The kinetics of this reaction is compatible with a rate-determining step involving either H3O2+ and HNO2 or H2O2 and protonated nitrous acid. In the former case the rate constant between the two species would be 179.6 +/- 1.4 M(-1) s(-1), in the latter case it would be as high as (1.68 +/- 0.01) x 10(10) M(-1) s(-1) (diffusion-controlled reaction). Due to the more reasonable value of the rate constant, the reaction between H3O2+ and HNO2 seems more likely. In the presence of HNO2 + H2O2 the nitration of phenol is strongly enhanced when compared with HNO2 alone. The nitration rate of phenol in the presence of peroxynitrous acid decreases as pH increases, thus HOONO is a potential source of atmospheric nitroaromatic compounds in acidic water droplets. The mixture Fe(II) + H2O2 (Fenton reagent) can oxidize nitrite and nitrous acid to nitrogen dioxide, which results in phenol nitration. The nitration in the presence of Fe(II) + H2O2 + NO2-/HNO2 occurs more rapidly than the one with H2O2 + NO2-/HNO2 at pH 5, where little HNO2 is available to directly react with hydrogen peroxide. Both systems, however, are more effective than NO2-/HNO2 alone in producing nitrophenols from phenol. Another process leading to the oxidation of nitrite to nitrogen dioxide is the photo-Fenton one. It can be relevant at pH > or = 6, as nitrite does not react with H2O2 at room temperature. Under such conditions the source of Fe(II) is the photolysis of ferric (hydr)oxides (heterogeneous photo-Fenton reaction). In the presence of nitrite this reaction induces very effective nitrophenol formation from phenol.

This paper studies the transformation of phenol and of 3,5-dichlorophenol (3,5-DCP) upon UVA irradiation of anthraquinone-2-sulfonate (AQ2S). Light-excited AQ2S is able to oxidise phenol, 3,5-DCP, and AQ2S. Transformation reactions do not proceed at a significant extent in the absence of molecular oxygen, in which case recombination reactions of initially formed oxidised and reduced radical species (and/or radical ions) would yield back the initial substrates. AQ2S hydroxyderivatives are the main transformation intermediates, while the phenoxyl radicals arising upon oxidation of phenol and of 3,5-DCP react with the substrates to yield dihydroxybiphenyls and phenoxyphenols. Very small amounts of catechol and of 3,5-dichlorocatechol were observed, indicating a possible minor role of the hydroxyl radicals in the reactivity of the system. Interesting results from an environmental point of view are the formation of 2-hydroxydibenzofuran from phenol and of various tetrachlorinated dihydroxybiphenyls and phenoxyphenols from 3,5-DCP, suggesting that quinone photochemistry can be an important pathway for the formation of hazardous secondary pollutants in the environment.

The phototransformation of phenol in aqueous solution was studied with different quinoid compounds, which are usually detected on atmospheric particulate matter: 2-ethylanthraquinone (EtAQ), benzanthracene-7,12-dione (BAD), 5,12-naphthacenequinone (NQ), 9,10-anthraquinone (AQ), and 2,6-dihydroxyanthraquinone (DAQ). All the studied quinones were able to sensitise the phototransformation of phenol. Under blue-light irradiation the approximated, polychromatic quantum yields for phenol photodegradation were in the order AQ > BAD > EtAQ > NQ > DAQ. Quantum mechanical calculations showed that AQ and DAQ have a very different spin distribution in the triplet state (largely located on the carbonyl oxygen and delocalised over the aromatic ring, respectively) that could account for the difference in reactivity. The spin distribution of EtAQ is similar to that of AQ. Under simulated sunlight, EtAQ induced the highest rate of phenol degradation. Radiation-excited EtAQ would oxidise both ground-state EtAQ and phenol; a kinetic model that excludes the ˙OH radical and singlet oxygen as reactive species is supported by the experimental data. Quinones were also able to oxidise nitrite to nitrogen dioxide, thereby inducing phenol nitration. Such a process is a potential source of nitrogen dioxide and nitrophenols in the atmospheric aerosols.

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This paper gives an overview of the processes that can account for the occurrence of mononitroPAHs (1-nitronaphthalene -1NN-, 2-nitronaphthalene -2NN-, 9-nitroanthracene) in the Antarctic particulate matter. Long-range transport of these compounds from the continents to the Antarctica seems an unlikely possibility given the photolability of 1NN and 2NN. The alternative possibility is that nitration takes place in situ. The nitration of naphthalene is very likely to occur in the gas phase, because the parent compound is too volatile to be present in particulate matter, even at the low temperatures of the Antarctic summer near the coast. In contrast, the volatility of anthracene is sufficiently low under such conditions to allow this compound to be present in particles at a significant concentration. The field data on nitronaphthalene ratios, together with an evaluation of their removal rates, indicate that the gas-phase nitration of naphthalene is more likely to be initiated by• NO 3 + • NO 2 (yielding 1NN:2NN ≈ 2.5:1) than byNitronaphthalenes, less volatile than the parent compound, can then be found in particulate matter.In contrast, the occurrence of 9-nitroanthracene could be consistent with an electrophilic, condensed-phase nitration process.