Tiffany Charbouillot

Photochemistry of 1-nitronaphthalene: a potential source of singlet oxygen and radical species in atmospheric waters.

1-Nitronaphthalene (1NN) was used as a model of nitro-PAHs to investigate photosensitized reactions in aqueous solution in the presence of oxygen and halides. Laser flash photolysis (LFP) was employed to investigate electron transfer between halide anions and the triplet state of 1NN, leading to the formation of dihalogen radical anions (X(2)(*-)) in solution. The experiments were performed in the absence or presence of oxygen, showing a bimolecular quenching rate constant for the triplet state of 1NN ((3)1NN) by oxygen of (1.95 +/- 0.05) x 10(9) M(-1) s(-1). The decay of (3)1NN was observed to strongly depend on the pH of the medium. At pH > 2, (3)1NN decayed with a pseudofirst order rate constant close to 6.0 x 10(5) s(-1). The rate constant was markedly enhanced at lower pHs, reaching 2.0 x 10(6) s(-1) at pH approximately 0.1, which suggests formation of the protonated triplet state ((3)1NNH(+)) at low pHs. Furthermore, we showed that the photoreactions of (3)1NN in the presence of oxygen are potential sources of HO(2)(*), (1)O(2), and possibly (*)OH in aqueous media. In Milli-Q water (pH approximately 6.5) and in the presence of halide anions, the quenching rate constant of (3)1NN was evaluated to be (2.9 +/- 0.4) x 10(4) M(-1) s(-1) for chloride, (7.5 +/- 0.2) x 10(8) M(-1) s(-1) for bromide, and (1.1 +/- 0.1) x 10(10) M(-1) s(-1) for iodide. Also in this case a pH-dependent reactivity was evidenced, and the quenching rate constant was (7.7 +/- 1.2) x 10(5) M(-1) s(-1) with chloride at pH 1.1.

Sensitive determination of glyoxal, methylglyoxal and hydroxyacetaldehyde in environmental water samples by using dansylacetamidooxyamine derivatization and liquid chromatography/fluorescence

In this study we improved the dansylacetamidooxyamine (DNSAOA)-LC-fluorescence method for the determination of aqueous-phase glyoxal (GL), methylglyoxal (MG) and hydroxyacetaldehyde (HA). As derivatization of dicarbonyls can potentially lead to complex mixtures, a thorough study of the reaction patterns of GL and MG with DNSAOA was carried out. Derivatization of GL and MG was shown to follow the kinetics of successive reactions, yielding predominantly doubly derivatized compounds. We verified that the bis-DNSAOA structure of these adducts exerted only minor influence on their fluorescence properties. Contrary to observations made with formaldehyde, derivatization of GL, MG and, to a lesser extent of HA, was shown to be faster in acidic (H(2)SO(4)) medium with a maximum of efficiency for acid concentrations of ca. 2.5 mM. Concomitant separation of GL, MG, HA and of single carbonyls was achieved within 20 min by using C(18) chromatography and a gradient of CH(3)CN in water. Detection limits of 0.27, 0.17 and 0.12 nM were determined for GL, MG and HA, respectively. Consequently, low sample volumes are sufficient and, unlike numerous published methods, neither preconcentration nor large injection volumes are necessary to monitor trace-level samples. The method shows relative measurement uncertainties better than ±15% at the 95% level of confidence and good dynamic ranges (R(2)>0.99) from 0.01 to 1.5 μM for all carbonyls. GL, MG and HA were identified for the first time in polar snow samples, but also in saline frost flowers for which unexpected levels of 0.1-0.6 μM were measured. Concentrations in the 0.02-2.3 μM range were also measured in cloud water. In most samples, a predominance of HA over GL and MG was observed.

Photochemical and photosensitised reactions involving 1-nitronaphthalene and nitrite in aqueous solution

The excited triplet state of 1-nitronaphthalene, 1NN, ((3)1NN) is able to oxidise nitrite to ˙NO(2), with a second-order rate constant that varies from (3.56 ± 0.11) × 10(8) M(-1) s(-1) (μ±σ) at pH 2.0 to (3.36 ± 0.28) × 10(9) M(-1) s(-1) at pH 6.5. The polychromatic quantum yield of ˙NO(2) photogeneration by 1NN in neutral solution is Φ(˙NO(2))(1NN)≥ (5.7 ± 1.5) × 10(7)× [NO(2)(-)]/{(3.4 ± 0.3) × 10(9)× [NO(2)(-)] + 6.0 × 10(5)} in the wavelength interval of 300-440 nm. Irradiated 1NN is also able to produce ˙OH, with a polychromatic quantum yield Φ(˙OH)(1NN) = (3.42 ± 0.42) × 10(-4). In the presence of 1NN and NO(2)(-)/HNO(2) under irradiation, excited 1NN (probably its triplet state) would react with ˙NO(2) to yield two dinitronaphthalene isomers, 15DNN and 18DNN. The photonitration of 1NN is maximum around pH 3.5. At higher pH the formation rate of ˙NO(2) by photolysis of NO(2)(-)/HNO(2) would be lower, because the photolysis of nitrite is less efficient than that of HNO(2). At lower pH, the reaction between (3)1NN and ˙NO(2) is probably replaced by other processes (involving e.g.(3)1NN-H(+)) that do not yield the dinitronaphthalenes.

Atmospheric Aqueous-Phase Photoreactivity: Correlation Between the Hydroxyl Radical Photoformation and Pesticide Degradation Rate in Atmospherically Relevant Waters

In the present study, we investigated the correlation between the hydroxyl radical formation rate (R˙OH) and the degradation of a pesticide (mesotrione) in synthetic cloud water solutions and in two real atmospheric cloud waters collected at the top of puy de Dôme station (France). Using terephthalic acid as the hydroxyl radical chemical probe, we established the linear correlation between the photogenerated hydroxyl radical under polychromatic wavelengths and the pesticide degradation rate: (m s−1) = (1.61 ± 0.15) × 10−1(m s−1). Moreover, the formation rate of hydroxyl radical in two natural cloud waters was estimated considering H2O2 and NO3− and the difference between the predicted values and those experimentally obtained could be attributed to the presence of other photochemical sources: iron‐complexes and total organic matter. The organic constituents could play a dual role of sources and scavengers of photoformed hydroxyl radicals in the aqueous phase.