A. Chakir

Heterogeneous ozonolysis of folpet and dimethomorph: a kinetic and mechanistic study.

The research study discussed in this paper concerns determination of the kinetic rate constants for heterogeneous degradation of the pesticides folpet and dimethomorph by ozone at room temperature. The study also involves identification of the degradation products of the analyzed compound in the condensed phase by solid-phase microextraction/gas chromatography/mass spectrometry (SPME/GC/MS). The heterogeneous O3 reactivity of the identified degradation product of dimethomorph, 4-chlorophenyl 3,4-dimethoxyphenyl methanone (CPMPM), is also evaluated experimentally. The obtained results show that the rate constant values of the analytes are (1.7 ± 0.5) × 10(-19), (2.1 ± 0.8) × 10(-19), (2.6 ± 0.2) × 10(-20), and (2.7 ± 0.2) × 10(-20) cm(3)·molecule(-1)·s(-1) for (Z)-dimethomorph, (E)-dimethomorph, folpet, and CPMPM, respectively. Such values implicate heterogeneous ozone lifetimes that vary from a few days to several months, meaning that, depending on their reactivity with respect to other atmospheric oxidants, these compounds might be relatively persistent and may be transported to regions far from their point of application.

First Experimental Determination of the Absolute Gas-Phase Rate Coefficient for the Reaction of OH with 4-Hydroxy-2-Butanone (4H2B) at 294 K by Vapor Pressure Measurements of 4H2B

The reaction of the OH radicals with 4-hydroxy-2-butanone was investigated in the gas phase using an absolute rate method at room temperature and over the pressure range 10-330 Torr in He and air as diluent gases. The rate coefficients were measured using pulsed laser photolysis (PLP) of H(2)O(2) to produce OH and laser induced fluorescence (LIF) to measure the OH temporal profile. An average value of (4.8 ± 1.2) × 10(-12) cm(3) molecule(-1) s(-1) was obtained. The OH quantum yield following the 266 nm pulsed laser photolysis of 4-hydroxy-2-butanone was measured for the first time and found to be about 0.3%. The investigated kinetic study required accurate measurements of the vapor pressure of 4-hydroxy-2-butanone, which was measured using a static apparatus. The vapor pressure was found to range from 0.056 to 7.11 Torr between 254 and 323 K. This work provides the first absolute rate coefficients for the reaction of 4-hydroxy-2-butanone with OH and the first experimental saturated vapor pressures of the studied compound below 311 K. The obtained results are compared to those of the literature and the effects of the experimental conditions on the reactivity are examined. The calculated tropospheric lifetime obtained in this work suggests that once emitted into the atmosphere, 4H2B may contribute to the photochemical pollution in a local or regional scale.

An experimental and theoretical study of the kinetics of the reaction between 3-hydroxy-3-methyl-2-butanone and OH radicals

Absolute experimental and theoretical rate constants are determined for the first time for the reaction of 3-hydroxy-3-methyl-2-butanone (3H3M2B) with OH radicals as a function of temperature. Experimental studies were carried out over the temperature range of 277 to 353 K and the pressure range of 5 to 80 Torr, by using a cryogenically cooled cell coupled to the PLP-LIF technique. OH radicals were generated for the first time from the photodissociation of the reactant 3H3M2B at 266 nm and the OH formation yield in 3H3M2B photolysis at 266 nm was measured under our experimental conditions. In addition, the reaction of 3H3M2B with OH radicals was studied theoretically by using the Density Functional Theory (DFT) method under three hydrogen abstraction pathways. According to these calculations, H-atom abstraction occurs more favourably from the methyl group adjacent to the hydroxyl group with a small barrier height. The calculated theoretical rate constants are in good agreement with the experimental data over the temperature range of 278 to 1000 K. No significant temperature dependence can be observed although a very slight effect was observed within the error bars.

Pesticide residue levels in peppers cultivated in Souss Masa valley (Morocco) after multiple applications of azoxystrobin and chlorothalonil

Residue levels of azoxystrobin and chlorothalonil were determined in peppers grown in an experimental greenhouse. These two pesticides were selected on the basis of previous excesses of 26 and 24%, respectively, found in peppers samples cultivated in 2008 in eastern Morocco. The measurements were made over a 7 week period in which up to three successive treatments with azoxystrobin and a 4 week period in which up to three successive treatments with chlorothalonil were carried out. In all cases, plants were sprayed separately with azoxystrobin and chlorothalonil with application rates of active ingredients of 50 and 200 cc hl−1, respectively. Sampling was carried out at 0, 2, 4, 7, 12, 15 and 22 days for azoxystrobin and 0, 1, 3, 7, 8 and 10 days for chlorothalonil. Residue levels of azoxystrobin and chlorothalonil were determined by liquid–liquid extraction (LLE) and gas chromatography with electron-capture detector (GC-ECD). During the study, residue levels in the plantation ranged between 1.14 and 0.02 mg kg−1 for azoxystrobin and between 0.55 and 0.04 mg kg−1 for chlorothalonil. The application of an intensive washing process to the pepper samples did not lead to a significant reduction in the residue levels of either pesticide. Likewise, significant differences were not found between the residue levels in the ‘edible’ and ‘inedible’ parts of the peppers.

Study of a benzoylperoxy radical in the gas phase: ultraviolet spectrum and C6H5C(O)O2 + HO2 reaction between 295 and 357 K.

This work reports the ultraviolet absorption spectrum and the kinetic determinations of the reactions 2C(6)H(5)C(O)O(2) → products (I) and C(6)H(5)C(O)O(2) + HO(2) → C(6)H(5)C(O)O(2)H + O(2) (IIa), → C(6)H(5)C(O)OH + O(3) (IIb), → C(6)H(5)C(O)O + OH + O(2) (IIc). Experiments were performed using a laser photolysis technique coupled with UV-visible absorption detection over the pressure range of 80-120 Torr and the temperature range of 293-357 K. The UV spectrum was determined relative to the known cross section of the ethylperoxy radical C(2)H(5)O(2) at 250 nm. Kinetic data were obtained by simulating the temporal behavior of the UV absorption at 245-260 nm. At room temperature, the rate constant value of reaction I (cm(3)·molecule(-1)·s(-1)) was found to be k(I) = (1.5 ± 0.6) × 10(-11). The Arrhenius expression for reaction II is (cm(3)·molecule(-1)·s(-1)) k(II)(T) = (1.10 ± 0.20) × 10(-11) exp(364 ± 200/T). The branching ratios β(O3) and β(OH), respectively, of reactions IIb and IIc are evaluated at different temperatures; β(O3) increases from 0.15 ± 0.05 at room temperature to 0.40 ± 0.05 at 357 K, whereas β(OH) remains constant at 0.20 ± 0.05. To confirm the mechanism of reaction II, a theoretical study was performed at the B3LYP/6-311++G(2d,pd) level of theory followed by CBS-QB3 energy calculations.

Atmospheric Reaction of Cl with 4-Hydroxy-2-pentanone (4H2P): A Theoretical Study

The kinetics and the mechanism of the reaction of 4-hydroxy-2-pentanone (4H2P) with Cl atom were investigated using quantum theoretical calculations. Density functional theory, CBS-QB3, and G3B3 methods are used to explore the reaction pathways. Rice-Ramsperger-Kassel-Marcus theory is employed to obtain rate constants of the reaction at atmospheric pressure and the temperature range 278-400 K. This study provides the first theoretical and kinetic determination of Cl rate constant for reactions with 4H2P over a large temperature range. The obtained rate constant 1.47 × 10-10 cm3 molecule-1 s-1 at 298 K is in reasonable agreement with those obtained for C4-C5 hydroxyketones both theoretically and experimentally. The results regarding the structure-reactivity relationship and the atmospheric implications are discussed.