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Nitrogen dioxide ( • NO 2 ) is produced in sunlit natural surface waters by the direct photolysis of nitrate, together with • OH, and upon the oxidation of nitrite by • OH itself. • NO 2 is mainly scavenged by dissolved organic matter, and here, it is shown that • NO 2 levels in sunlit surface waters are enhanced by high concentrations of nitrate and nitrite, and depressed by high values of the dissolved organic carbon. The dimer of nitrogen dioxide (N 2 O 4 ) is also formed in the pathway of • NO 2 hydrolysis, but with a very low concentration, i.e., several orders of magnitude below • NO 2 , and even below • OH. Therefore, at most, N 2 O 4 would only be involved in the transformation (nitration/nitrosation) of electron-poor compounds, which would not react with • NO 2 . Although it is known that nitrite oxidation by CO 3 • - in high-alkalinity surface waters gives a minor-to-negligible contribution to • NO 2 formation, it is shown here that NO 2 - oxidation by Br 2 • - can be a significant source of • NO 2 in saline waters (saltwater, brackish waters, seawater, and brines), which offsets the scavenging of • OH by bromide. As an example, the anti-oxidant tripeptide glutathione undergoes nitrosation by • NO 2 preferentially in saltwater, thanks to the inhibition of the degradation of glutathione itself by • OH, which is scavenged by bromide in saltwater. The enhancement of • NO 2 reactions in saltwater could explain the literature findings, that several phenolic nitroderivatives are formed in shallow (i.e., thoroughly sunlit) and brackish lagoons in the Rhône river delta (S. France), and that the laboratory irradiation of phenol-spiked seawater yields nitrophenols in a significant amount.

Product analyses of the NO3 radical-initiated oxidation of ortho-, meta-, and para-cresol have been performed in large-volume chamber systems at the University of Wuppertal (1080 L quartz glass reactor: QUAREC) and the European Photoreactor (EUPHORE), Valencia, Spain. The reaction of O3 with NO2 was used for the in situ generation of NO3 radicals in both QUAREC and EUPHORE. In the QUAREC experiments the gas-phase reaction of ortho-cresol isomer with NO3 yielded (11.5 ± 0.8) % 6-methyl-2-nitrophenol (6M2NP), (4.4 ± 0.3) % methyl-1,4-benzoquinone (MQUIN) and (77.2 ± 6.3) % HNO3. The reaction of NO3 radicals with meta-cresol yielded (21.2 ± 1.4) % 3-methyl-2-nitrophenol (3M2NP), (22.8 ± 1.8) % 3-methyl-4-nitrophenol (3M4NP), (23.5 ± 1.8) % 5-methyl-2-nitrophenol (5M2NP), (4.2 ± 0.7) % MQUIN and (72.3 ± 6.4) % HNO3. In the reaction of NO3 radicals with para-cresol, 4-methyl-2-nitrophenol (4M2NP) and HNO3 were identified as products with yields of (41.3 ± 3.7) % and (85.0 ± 10.2) %, respectively. In the EUPHORE chamber not all products were formed at levels above the detection limit, however, in cases where detection was possible similar product yields were observed. The product formation yields determined in both chambers are compared with available literature data and a gas-phase mechanism is proposed to explain the formation of the products observed from the reaction of NO3 and with cresol isomers.

Nitrite and nitrate are known to be involved in photochemical processes occurring in natural waters. In this study we have investigated the role played by these photosensitizers towards the transformation of xenobiotic organic matter in marine water, with the goal of assessing the typical transformation routes induced in seawater by irradiated nitrite/nitrate. For this purpose, phenol was chosen as model molecule. Phenol transformation was investigated under simulated solar radiation in the presence of nitrite (in the range of 1 × 10(-5)-1 × 10(-2)M) or nitrate ions, in pure water at pH 8, in artificial seawater (containing same dissolved salts as seawater but no organic matter), and in natural seawater. In all experiments, phenol degradation rate and formation of intermediates were assessed. As expected, phenol disappearance rate decreased with decreasing nitrite concentration and was slightly reduced by the presence of chloride. Other salts present in artificial seawater (e.g. HCO(3)(-), CO(3)(2-) and Br(-)) had a more marked effect on phenol transformation. Analysis of intermediates formed in the different matrices under study showed generation of hydroxyl-, nitro- and chloroderivatives of phenol, to a different extent depending on experimental conditions. 1,4-Benzoquinone prevailed in all cases, nitroderivatives were only formed with nitrite but were not detected in nitrate-spiked solutions. Competition was observed between halogenation and nitration of phenol, with variable outcome depending on nitrite concentration. The most likely reason is competition between nitrating and halogenating species for reaction with the phenoxyl radical. A kinetic model able to justify the occurrence of different intermediates under the adopted conditions is presented and discussed., (Copyright © 2012 Elsevier B.V. All rights reserved.)

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)

Background, Aim and Scope: Photochemical processes can decontaminate the aqueous environment from xenobiotics, but they also produce secondary pollutants. This paper presents field and laboratory evidence of the transformation of 4-chlorophenol (4CP) into 2-nitro-4-chlorophenol (2N4CP)., Materials and Methods: Field monitoring of 4CP and 2N4CP was carried out by solid phase extraction coupled with liquid chromatography-multiple reaction monitoring mass spectrometry. Laboratory irradiation experiments were carried out under a UV-Vis lamp, and the time evolution of the compounds of interest was followed by liquid chromatography., Purpose: The purpose of this study was elucidating the pathways leading to 2N4CP from 4CP in paddy field water., Results and Discussion: The field monitoring results suggest that 4CP can be transformed into 2N4CP in the paddy field water of the Rhône delta (Southern France). The laboratory study indicates that the transformation can take place via photonitration by (*)NO(2). The nitration process is inhibited by bicarbonate, possibly due to basification that favours the occurrence of the 4-chlorophenolate. The latter could consume (*)NO(2) without being nitrated. Photonitration in the presence of bicarbonate could account for the observed transformation in the field., Conclusions: Photonitration of 4CP to 2N4CP by (*)NO(2) could account for the observed interconversion of the two compounds in paddy fields. The results are of concern because 2N4CP is biorecalcitrant and toxic., Recommendations and Perspectives: Bicarbonate can modulate the photonitration of 4CP into 2N4CP, which can be very significant in bicarbonate-poor waters.

Contrary to common expectations, the hydroxyl scavengers, carbonate and bicarbonate, are able to enhance the phototransformation by nitrate of a number of substituted phenols. Carbonate and bicarbonate, in addition to modifying the solution pH, are also able to induce a considerable formation of the carbonate radicals upon nitrate photolysis. The higher availability of less-reactive species than the hydroxyl radical would contribute to substantially enhance the photodegradation of the phenols/phenolates that are sufficiently reactive toward the carbonate radical. This phenomenon has a potentially important impact on the fate of the relevant compounds in surface waters. In contrast, the degradation of compounds that are not sufficiently reactive toward CO(3)(-*) is inhibited by carbonate and bicarbonate because of the scavenging of *OH.

2,4-Dichlorophenol (2,4-DCP) and 2,6-dichlorophenol (2,6-DCP) undergo oxidation, nitrosation and nitration in the presence of nitrate under UV irradiation. Nitration is favoured under acidic conditions, most likely because HNO(2) is formed on nitrate photolysis. The most likely photonitration pathway is the reaction between radiation-excited dichlorophenols (DCP*) and HNO(2). HNO(2) is also able to nitrate DCP in the dark with elevated yields. Irradiation also causes DCP direct photolysis, which is more efficient for the dichlorophenolate anions. The photolysis of the dichlorophenols and that of the dichlorophenolates also produce different intermediates, by dechlorination in the former and ring contraction in the latter case.

Filtered lakewater samples, mainly collected in the province of Torino (Piedmont, NW Italy) were characterised from a spectrophotometric point of view. Spectral data were then used for the direct determination of nitrate by three-wavelength photometry, which should account for the spectral interference by dissolved organic matter (DOM), and the results compared with nitrate quantification by ion chromatography. The spectrophotometric method proved very suitable for nitrate measurement, with unity slope (micro +/- sigma = 0.99 +/- 0.03) of the correlation plot (spectral vs. ion chromatography data) up to 0.1 mM nitrate, and with r2 = 0.97 for 26 data points. Lakewater spectra were also used for the characterisation of DOM by means of the specific absorption at 285 and 254 nm (absorbance vs. NPOC, the latter to quantify the DOM amount), and the E2/E3 and E3/E4 indexes. The latter two make only use of radiation absorption data (250 vs. 365 and 300 vs. 400 nm). It could be concluded that lakewater DOM is mainly composed of autochthonous material (biologically produced aliphatic compounds and only a minor fraction of aromatic groups), with generally low molecular weight and degree of aromaticity. Some exceptions could be found in high-mountain lakes, but it should also be considered that NPOC measurement cannot be avoided if DOM origin is to be studied. From the absorption spectrum alone it is possible to get indication on the aromaticity degree of radiation-absorbing DOM, but most of the autochthonous DOM would escape spectrophotometric characterisation.

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.

This paper studies the pH effect on the photonitration of catechol, 1-naphthol, naphthalene, and benzene. The pH trend is influenced by the generation of HNO(2) and peroxynitrous acid (HOONO) upon nitrate photolysis. HNO(2) can be involved in a direct and an indirect nitration process. Direct nitration follows the pH distribution of HNO(2) (flexus around 3). Indirect nitration, possibly involving nitrosation+oxidation, would be highest around pH3. HOONO can be involved in electrophilic nitration, where the initial formation rate of the nitroderivatives is proportional to [H(+)], or take part in nitration directly, in which case a less important pH effect in photonitration is observed. The relative importance of the various nitration pathways for each substrate determines the resulting pH effect in photonitration upon nitrate photolysis.

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.

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 HNO(2) + H(2)O(2) in the dark (which produce HOONO) and in the presence of NO(2)(-)/HNO(2) 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 HNO(2) alone in the dark. An important feature of benzene nitration in the presence of NO(2)(-)/HNO(2) 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.

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 formation of nitrophenols was studied as a consequence of ultra violet (UV) irradiation of aqueous solutions of phenol and nitrate in the range of pH 1-12. The study was performed both in homogeneous phase and in the presence of water-suspended TiO2. The effects of pH, dissolved oxygen and 2-propanol as .OH scavenger have been evaluated. A reaction mechanism is proposed, based on the experimental results.

• Investigation of high-resolution temporal variations of nitrate in mountain pond. • Integration of in-situ UV–Vis spectrophotometric measurements with weekly samples. • Nitrate seasonal variations driven by snow-melt duration and temperature. • Summer storms and rain-on-snow events greatly influence nitrate dynamics. • Better understanding of biogeochemical processes occurring in tundra pond. High-resolution temporal measurements in remote, high-elevation surface waters are required to better understand the dynamics of nitrate (NO 3 −) in response to changes in meteoclimatic conditions. This study reports on the first use of a UV–Vis submersible spectrophotometric probe (UV–Vis probe) to measure the hourly concentration of nitrate nitrogen (NO 3 −-N) in a pond located at 2722 m a.s.l. in an alpine tundra area (NW Italian Alps), during two snow-free seasons (July–October) in 2014 and 2015. Weekly analyses of NO 3 −-N and stable isotopes of water (δ18O and δ2H), together with continuous meteorological, water temperature, and turbidity measurements, were performed over the same period. The integration of in-situ UV–Vis spectrophotometric measurements with weekly samples allowed depicting the role of summer precipitation, snow melt, and temperature (air and water) in influencing NO 3 − dynamics. Short-duration meteorological events (e.g., summer storms and rain-on-snow events) produced rapid variations of in-pond NO 3 − concentration, i.e., fivefold increase in 18 h, that would not be detectable using the traditional manual collection of discrete samples. The observed seasonal variability of NO 3 − concentration, negatively correlated with water temperature, highlighted the important role of in-pond biological processes leading to an enhanced N uptake and to the lowest NO 3 − concentration in the warmer periods. The occurrence of heavy rainfall events critically altered the expected seasonal NO 3 − trends, increasing the N supply to the pond. The comparison of N dynamics in two years characterised by extremely different meteoclimatic conditions allowed us to obtain insights on the potential effects of climate changes (e.g., high air temperature, heavy rainfalls, and rain-on-snow events) on sensitive aquatic ecosystems as high-elevation ponds. [ABSTRACT FROM AUTHOR]

Experiments are conducted to determine the effect of a cage of water molecules on the photolysis quantum yields of nitrate, FeOH2+, and H2O2. Results suggest that the quantum yields of nitrate and FeOH2+ are decreased by the recombination of photo-fragments (ꔷOH + ꔷNO2 and Fe2+ + ꔷOH, respectively) before they leave the surrounding cage of water molecules. However, no evidence is found for an enhanced quantum yield for H2O2. Therefore, the photolysis of nitrate and FeOH2+ could be enhanced if the cage of the solvent molecules is incomplete, as is the case at the air–water interface of atmospheric droplets. The photolysis rate constant distribution within nitrate, FeOH2+, and H2O2 aerosols is calculated by combining the expected quantum yield data in the bulk and at the interface with Mie theory calculations of light intensity. The photolysis rate constant of nitrate and FeOH2+ would be significantly higher at the surface than in the bulk if quantum yields are enhanced at the surface. In the case of H2O2, the photolysis rate constant would be enhanced by surface accumulation. The results concerning the expected rates of photolysis of these photoactive species are applied to the assessment of the reaction between benzene and ꔷOH in the presence of ꔷOH scavengers in an atmospherically relevant scenario. For a droplet of 1 μm radius, a large fraction of the total ꔷOH-benzene reaction (15% for H2O2, 20% for nitrate, and 35% for FeOH2+) would occur in the surface layer, which accounts for just 0.15% of the droplet volume. [ABSTRACT FROM AUTHOR]