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Irradiated nitrophenols can produce nitrite and nitrous acid (HONO) in bulk aqueous solutions and in viscous aqueous films, simulating the conditions of a high-solute-strength aqueous aerosol, with comparable quantum yields in solution and viscous films (10 -5 -10 -4 in the case of 4-nitrophenol) and overall reaction yields up to 0.3 in solution. The process is particularly important for the para-nitrophenols, possibly because their less sterically hindered nitro groups can be released more easily as nitrite and HONO. The nitrophenols giving the highest photoproduction rates of nitrite and HONO (most notably, 4-nitrophenol and 2-methyl-4-nitrophenol) could significantly contribute to the occurrence of nitrite in aqueous phases in contact with the atmosphere. Interestingly, dew-water evaporation has shown potential to contribute to the gas-phase HONO levels during the morning, which accounts for the possible importance of the studied process.

Light-induced nitration pathways of phenols are important processes for the transformation of pesticide-derived secondary pollutants into toxic derivatives in surface waters and for the formation of phytotoxic compounds in the atmosphere. Moreover, phenols can be used as ˙NO(2) probes in irradiated aqueous solutions. This paper shows that the nitration of 4-chlorophenol (4CP) into 2-nitro-4-chlorophenol (NCP) in the presence of irradiated nitrate and nitrite in aqueous solution involves the radical ˙NO(2). The experimental data allow exclusion of an alternative nitration pathway by ˙OH + ˙NO(2). Quantum mechanical calculations suggest that the nitration of both phenol and 4CP involves, as a first pathway, the abstraction of the phenolic hydrogen by ˙NO(2), which yields HNO(2) and the corresponding phenoxy radical. Reaction of phenoxyl with another ˙NO(2) follows to finally produce the corresponding nitrated phenol. Such a pathway also correctly predicts that 4CP undergoes nitration more easily than phenol, because the ring Cl atom increases the acidity of the phenolic hydrogen of 4CP. This favours the H-abstraction process to give the corresponding phenoxy radical. In contrast, an alternative nitration pathway that involves ˙NO(2) addition to the ring followed by H-abstraction by oxygen (or by ˙NO(2) or ˙OH) is energetically unfavoured and erroneously predicts faster nitration for phenol than for 4CP., (This journal is © The Royal Society of Chemistry and Owner Societies 2012)

Anthraquinone-2-sulphonate (AQ2S) under UVA irradiation is able to oxidise nitrite to (·)NO(2) and to induce the nitration of phenol. The process involves the very fast reactions of the excited triplet state (3)AQ2S(*) and its 520-nm absorbing exciplex with water, at different time scales (ns and μs, respectively). Quinones are ubiquitous components of coloured dissolved organic matter (CDOM) in surface waters and AQ2S was adopted here as a proxy of CDOM. Using a recently developed model of surface-water photochemistry, we found that the oxidation of nitrite to (·)NO(2) by (3)CDOM(*) could be an important (·)NO(2) source in water bodies with high [NO(2)(-)] to [NO(3)(-)] ratio, for elevated values of column depth and NPOC., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

The effect of selected organic and inorganic compounds, present in snow and cloudwater was studied. Photolysis of solutions of nitrate to nitrite was carried out in the laboratory using a UVB light source. The photolysis and other reactions were then modelled. It is shown that formate, formaldehyde, methanesulphonate, and chloride to a lesser extent, can increase the initial formation rate of nitrite. The effect, particularly significant for formate and formaldehyde, is unlikely to be caused by scavenging of hydroxyl radicals. The experimental data obtained in this work suggest that possible causes are the reduction of nitrogen dioxide and nitrate by radical species formed on photooxidation of the organic compounds. Hydroxyl scavenging by organic and inorganic compounds would not affect the initial formation rate of nitrite, but would protect it from oxidation, therefore, increasing the concentration values reached at long irradiation times. The described processes can be relevant to cloudwater and the quasi-liquid layer on the surface of ice and snow, considering that in the polar regions irradiated snow layers are important sources of nitrous acid to the atmosphere. Formate and (at a lesser extent) formaldehyde are the compounds that play the major role in the described processes of nitrite/nitrous acid photoformation by initial rate enhancement and hydroxyl scavenging.

Based on literature data of sunlight spectrum, photolysis quantum yields, and absorption spectra, the relative role of nitrite and nitrate as *OH sources in surface waters was assessed, and its dependence on the season and the depth of the water column studied. In the majority of surface water samples (river, lake and seawater) nitrite is expected to play a more important role as *OH source compared to nitrate, in spite of the usually lower [NO2(-)] values. Interestingly, under the hypothesis of a constant ratio of the concentrations of nitrate and nitrite (to be corrected later on for the actual concentration ratio in a given sample), the relative role of nitrite compared to nitrate would be minimum in summer, at noon, in the surface layer of natural waters. Any decrease in the sunlight intensity that can be experienced in the natural environment (different season than summer, water column absorption, time of the day other than the solar noon), with its associated influence on the sunlight spectrum, would increase the relative role of nitrite compared to nitrate.

Based on available literature data of [NO2-], steady-state [*OH], and *OH generation rate upon nitrate photolysis in environmental aqueous samples under sunlight, the steady-state [*NO2], could be calculated. Interestingly, one to two orders of magnitude more *NO2 would be formed in photochemical processes in atmospheric water droplets compared to transfer from the gas phase. The relative importance of nitrite oxidation compared to nitrate photolysis as an *NO2 source would be higher in atmospheric than in surface waters. The calculated levels of *NO2 could lead to substantial transformation of phenol into nitrophenols in both atmospheric and surface waters.

The nitration of naphthalene was studied in aqueous solution to gain insight into the processes leading to the nitration of aromatic compounds in atmospheric hydrometeors. Reactants used were nitric acid, nitrogen dioxide and peroxynitrous acid in the dark, nitrate, and nitrite/nitrous acid under illumination. Naphthalene nitration can lead to two possible isomers, 1- and 2-nitronaphthalene. The former nitrocompound preferentially forms upon electrophilic processes and in the presence of nitrogen dioxide. Electrophilic nitration of naphthalene takes place in the presence of concentrated nitric acid, but nitration with nitric acid and oxidants (charge-transfer nitration) occurs under much milder conditions than with nitric acid alone. Charge-transfer nitration may have some environmental significance in particular cases, e.g. in acidic aerosols in the presence of HNO3 and oxidants. Nitrogen dioxide is thought to have a role in PAH nitration in the Antarctic particulate matter. In previous papers we have found that nitration induced by peroxynitrous acid, HOONO, can follow two pathways, the former electrophilic (leading for instance to the formation of nitrophenols from phenol) and the latter probably involving HOONO itself (accounting for the formation of nitrobenzene from benzene). In the case of naphthalene and HOONO the electrophilic pathway mainly leads to 1-nitronaphthalene, while the other one preferentially yields 2-nitronaphthalene. The nitration of naphthalene in the presence of nitrite/nitrous acid under irradiation leads to both nitroisomers in similar ratios, and the process is not inhibited by hydroxyl scavengers. This excludes nitrogen dioxide as reactive species for nitration and marks a difference with phenol photonitration and a similarity with the behavior of benzene under comparable conditions. Nitrite photochemistry (and nitrite-induced photonitration as well) is expected to be relevant in fog and cloudwater in polluted areas. An important difference with the gas-phase nitration is that the radicals OH and NO3 are unlikely to play a relevant role in the nitration of naphthalene in aqueous solution.

An interesting aspect of the chemistry of nitrite is the possibility for this compound to interact with other environmental factors and many oxidising species, which results in the oxidation of nitrite to nitrogen dioxide. This is a potentially interesting process that can lead to the formation of nitroaromatic compounds in the environment. In previous papers we have shown that nitrite can interact with dissolved Fe(III) and nitrate under irradiation, Fenton and heterogeneous photo-Fenton reagents, and semiconductor oxides such as TiO2, alpha-Fe2O3, and beta-FeOOH under irradiation. This paper reports on the interaction between nitrite/nitrous acid and the Mn(III,IV) (hydr)oxides beta-MnO2 and gamma-MnOOH, both in neutral solution under irradiation and in acidic conditions in the dark. beta-MnO2 and gamma-MnOOH originate from the oxidation of Mn(II) and play a key role in the redox cycling of manganese in the environment. These Mn(III,IV) (hydr)oxides show some photocatalytic activity, and they can act as thermal oxidants at acidic pH. The photoinduced oxidation of nitrite and the thermal oxidation of nitrous acid by Mn(III,IV) (hydr)oxides yield nitrogen dioxide and lead to the formation of nitrophenols in the presence of phenol. These processes can take place at the water-sediment or water-colloid interface in natural waters and on the surface of atmospheric particulate. Furthermore, the phenol/gamma-MnOOH/HNO2 system in dark acidic solution is an interesting model due to the formation of phenoxyl radical upon phenol monoelectronic oxidation by gamma-MnOOH. The kinetics of nitrophenol generation under such conditions indicates that phenol nitration is unlikely to take place upon reaction between phenoxyl and *NO2 and suggests a solution to a literature debate on the subject.

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 work describes the nitration of aromatics upon near-UV photolysis of nitrate and nitrite in aqueous solution and upon photocatalytic oxidation of nitrite in TiO2 suspensions. Phenol is used in this work as a model aromatic molecule and as a probe for *NO2/N2O4. The photoinduced nitration of phenol in aqueous systems occurs upon the reaction between phenol and *NO2 or N2O4, and is enhanced by the photocatalytic oxidation of nitrite to *NO2 by TiO2. Aromatic photonitration in the liquid phase can play a relevant role in the formation of nitroaromatics in natural waters and atmospheric hydrometeors, thus being a potential pathway for the condensed-phase nitration of aromatics. Furthermore, the photoinduced oxidation of nitrite to nitrogen dioxide suggests a completely new role for nitrite in natural waters and atmospheric aerosols.

The role of nitrite as an environmental factor has been widely recognized. Nitrite is a relevant source of *OH in the atmosphere, both in the gas phase via photolysis of gaseous HNO2 and in atmospheric hydrometeors by photolysis of NO2-. In aqueous systems, *OH production through nitrite photolysis can be negligible due to the competition for light absorption by dissolved Fe(III), colloidal iron oxides, and nitrate. These photoexcited oxidants interact with NO2- and HNO2 to form *NO2, either directly or via formation of *OH. As a consequence, nitrite and nitrous acid may act as *NO2 rather than *OH sources. The radical *NO2 is involved in the nitration of many aromatic compounds, of which phenol is a model in this work. Kinetic measurements using 2-propanol as *OH scavenger show that the direct production of *OH by aqueous Fe(III) species decreases as pH increases. At slightly acidic and neutral pH values, oxidation of nitrite occurs by direct electron transfer to photoexcited Fe(III)aq species or colloidal iron oxides, in addition to the *OH-mediated oxidation of NO2-. The reported findings suggest a completely new role of nitrite in aquatic environments.

2-Methyl-4-chlorophenol (MCP) is the main transformation intermediate of the herbicide MCPA in surface waters and it is more toxic than its parent compound. MCP is produced from MCPA by both direct photolysis and•OH reaction. The latter process has higher yield of MCP from MCPA: 0.5 vs. 0.3 for the direct photolysis. Our model results show that the formation rate of MCP would be higher in waters that contain low organic matter and high nitrate and nitrite. Such conditions are favourable to MCPA direct photolysis and•OH-induced transformation, which are both inhibited by organic matter, while•OH formation is enhanced by nitrate and nitrite. Good agreement is obtained between model predictions and field data of MCPA transformation in the Rhône river delta (Southern France). The field data also suggest that MCP undergoes slightly faster transformation than MCPA in that environment. [ABSTRACT FROM PUBLISHER]

The excited triplet state of 1-nitronaphthalene, 1NN, (31NN) is able to oxidise nitrite to NO2, with a second-order rate constant that varies from (3.56 ± 0.11) × 108M−1s−1(μ ± σ) at pH 2.0 to (3.36 ± 0.28) × 109M−1s−1at pH 6.5. The polychromatic quantum yield of NO2photogeneration by 1NN in neutral solution is ΦNO21NN≥ (5.7 ± 1.5) × 107× [NO2−]/{(3.4 ± 0.3) × 109× [NO2−] + 6.0 × 105} in the wavelength interval of 300–440 nm. Irradiated 1NN is also able to produce OH, with a polychromatic quantum yield ΦOH1NN= (3.42 ± 0.42) × 10−4. In the presence of 1NN and NO2−/HNO2under irradiation, excited 1NN (probably its triplet state) would react with NO2to yield two dinitronaphthalene isomers, 15DNN and 18DNN. The photonitration of 1NN is maximum around pH 3.5. At higher pH the formation rate of NO2by photolysis of NO2−/HNO2would be lower, because the photolysis of nitrite is less efficient than that of HNO2. At lower pH, the reaction between 31NN and NO2is probably replaced by other processes (involving e.g.31NN-H+) that do not yield the dinitronaphthalenes. [ABSTRACT FROM AUTHOR]

Field studies have shown that the powerful phytotoxic agent 2,4-dinitrophenol is very likely to form in the atmospheric aqueous phase upon nitration of 2-nitrophenol or 4-nitrophenol. However, until now, the nitration pathway and the relative importance of the two mononitrophenols as sources of 2,4-dinitrophenol were not known. The present study shows that 2,4-dinitrophenol formation from mononitrophenols can take place upon photolysis and photooxidation of nitrite/nitrous acid (NO2-/HONO) and that nitrogen dioxide plays a key role in the process. A possible pathway might be the reaction between light- excited mononitrophenols (both 2- and 4-isomers) and nitrogen dioxide, in the presence of oxygen. As an alternative, nitration might involve ...NO3 + ...NO2. Possible sources of nitrogen dioxide in the atmospheric aqueous phase are dissolution from the gas phase and oxidation of NO2-. In the latter case, however, it is necessary that NO2- oxidation is faster than the oxidation of mononitrophenols. This would happen, for instance, in the presence of hematite under irradiation. Radiation absorption and scattering by hematite would also inhibit the direct photolysis of nitrophenols. The formation rate and the yield of 2,4- dinitrophenol are slightly higher when starting from 2-nitrophenol than those from 4-nitrophenol, but they are compensated by the higher concentration of 4-nitrophenol in the atmospheric aqueous phase. [ABSTRACT FROM AUTHOR]

This paper studies the nitration and hydroxylation of benzene in the presence of nitrite/nitrous acid in aqueous solution, both in the dark upon addition of hydrogen peroxide and under 360 nm irradiation. In both cases the detected transformation intermediates were phenol (P), nitrobenzene (NB), 2-nitrophenol (2NP) and 4-nitrophenol (4NP). P and NB directly form from benzene, and the initial formation rate of P is at least an order of magnitude higher than that of NB. In our experiments nitrophenols arise from P nitration, as can be inferred by their time evolution and isomer ratio (2NP:4NP=60:40, 3NP below detection limit). Nitrophenols may also form upon hydroxylation of NB, but in a different ratio (2NP:3NP:4NP=45:30:25). The detection of 3NP is thus a marker for the hydroxylation of NB, since this isomer is not formed in P nitration processes. The formation rates of P and NB increase with decreasing pH, both in the presence of HNO2 + H2O2 in the dark (which produce HOONO) and in the presence of NO2-/HNO2 under irradiation. In the former case the pH dependence reflects the formation rate of HOONO. In the case of the irradiation experiments the pH effect can be accounted for by the higher molar absorbivity and photolysis quantum yield of nitrous acid when compared with nitrite. Interestingly, benzene does not react with HNO2 alone in the dark. An important feature of benzene nitration in the presence of NO2-/HNO2 under irradiation is that the process is not inhibited by the addition of hydroxyl scavengers, differently from the case of phenol nitration. This finding indicates that nitrite irradiation might lead to the nitration of certain aromatic compounds in natural waters even in the presence of natural hydroxyl scavenging agents, which are usually thought to limit the environmental role of many photochemical processes. [Copyright &y& Elsevier]

Nitrophenols have been detected in some Antarctic lakes, the water of which is basic and rich in nitrate, nitrite and other nutrients. Nitrate or nitrite photolysis could be a possible reaction to explain the presence of these compounds. This work presents evidence for the formation of 2-nitrophenol (2NP), 4-nitrophenol (4NP) and 4-nitrosophenol (4NOP) upon UV irradiation of phenol and nitrite in aerated basic solutions. The pH dependence of the 2NP initial formation rate is different from those of 4NP and 4NOP. The dependence of the first mainly reflects the phenol/phenolate equilibrium, with phenol yielding 2NP at a higher rate than phenolate. In the case of 4NOP, the initial formation rate vs pH has a maximum at pH 9.5. The pH dependence of 4NOP formation rate suggests that three pathways are likely to operate: nitrosation of undissociated phenol by N 2 O 3 , prevailing at pH<8.7, nitrosation of phenolate by N 2 O 3 , prevailing in the pH interval 8.7-10.8, and reaction between phenoxyl radical and • NO, prevailing at pH>10.8. Phenol nitrosation by N 2 O 3 is favoured when phenol is negatively charged (phenolate), but it is also disfavoured at alkaline pH values, owing to the depletion of N 2 O 3 (the nitrosating agent) by basic hydrolysis. Differently from 2NP, the initial formation rate vs pH of 4NP is very similar to that of 4NOP, suggesting that 4NP may originate from the oxidation of 4NOP. Moreover, while in neutral and acidic solutions the formation rate of 2NP is slightly higher than that of 4NP, in the pH interval 8-12 the formation of 4NP is much more rapid than that of 2NP. This indicates that the pH of natural waters influences the ratio of nitroisomers. [ABSTRACT FROM AUTHOR]

Nitrite photochemical transformation is an important sink for nitrite and a source of nitric oxide, which affects the air–water partitioning of NO. It is shown here that aliphatic and aromatic OH scavengers alter the photochemical fate of nitrogen, in addition to enhancing nitrite photodegradation. In aerated solution, the addition of phenol as scavenger induced a significant formation of gas-phase nitrogen species from irradiated nitrite, differently from nitrite alone or in the presence of 2-propanol. Moreover, phenol strongly favoured the photochemical production of gas-phase nitric oxide in aerated solution: an upper limit of about 20% could be established for the nitrite fraction that may be transformed into gas-phase NO under these conditions. The photochemical production of NO was considerably higher in the absence of oxygen, most likely because the scavenging of nitric oxide by superoxide is an important NO sink in aerated systems. These results suggest that the concentration of dissolved oxygen and the nature of the OH scavenger(s) may considerably affect the phototransformation of nitrite into gas-phase nitrogen compounds, and particularly into nitric oxide, in aqueous solution and at water-air interfaces. [ABSTRACT FROM AUTHOR]