Eric Villenave

Diurnal/nocturnal concentrations and sources of particulate-bound PAHs, OPAHs and NPAHs at traffic and suburban sites in the region of Paris (France).

Particulate concentrations of polycyclic aromatic compounds (PACs) including, 17 polycyclic aromatic hydrocarbons (PAHs), 9 oxygenated PAHs (OPAHs) and 18 nitrated PAHs (NPAHs) were determined at traffic and suburban sites located in the region of Paris. A 12 h sampling basis time resolution was applied in order to study their diurnal and nocturnal variations. Observed concentrations were about 10 times higher at the traffic site for all compounds and were higher during the night-time for both sites (except for NPAHs at the traffic site). No significant differences in PAH and OPAH profiles were observed at both sites whereas, for NPAHs, 1-nitropyrene (diesel source) was the most abundant at the traffic site and 2+3-nitrofluoranthene (secondary formed by gas-phase reaction) was predominant at the suburban site. The study of the specific ratio 2-nitrofluoranthene/1-nitropyrene (2-NFlt/1-NP) showed a local formation of NPAHs in gaseous phase at the suburban site. A detailed analysis showed that atmospheric humidity and rainfalls modified differently PAH and NPAH profiles, in comparison to OPAH. A difference of the scale variability of water solubility between, light (MW≤228 g mol(-1)) and heavy compounds (MW≥273 g mol(-1)), could explain these observations. The specific study of the relationships between PACs and other measured pollutants highlighted that particle resuspension could constitute a significant source of PM on the traffic site. Even if NPAH formation seemed clearly evident at the suburban site during periods characterised by high O(3) and NO(2) concentration levels, results showed also that the primary and/or secondary origins of OPAHs and NPAHs were strongly dependent on the sampling site and on sampling conditions. Finally, we conclude that higher time sampling resolutions would be helpful in investigating the atmospheric chemistry and behaviours of PACs in correlation with the local meteorological variations and the daily cycle of human activities.

Heterogeneous Reactivity of OH Radicals with Phenanthrene

A discharge flow technique was used to study the heterogeneous reaction of OH with phenanthrene adsorbed on the Pyrex surface of the reactor, in the presence of NO2 and in the absence of light. The reaction kinetics were investigated by measuring the total loss of OH, followed by laser-induced fluorescence (LIF). The large efficiency of the heterogeneous reaction and the change in the surface properties with exposure time were demonstrated. Oxidation products were also identified by gas chromatography coupled to mass spectrometry.

The UV absorption spectra of CH2Br and CH2BrO2 and the reaction kinetics of CH2BrO2 with itself and with HO2 at 298 K

Abstract The UV absorption spectra of the bromomethyl (CH 2 Br) and bromomethylperoxy (CH 2 BrO 2 ) radicals have been determined using the flash photolysis technique. CH 2 Br exhibits the typical absorption band of halomethyl radicals, peaking near 230 nm, and CH 2 BrO 2 exhibits the typical broad absorption of peroxy radicals, with a maximum at 240 nm. Rate coefficients were determined at 298 K for the self-reaction (CH 2 BrO 2 BrO 2 + CH 2 BrO 2 → 2 CH 2 BrO + O 2 (3), k 3 = (1.05 ± 0.4) × 10 −12 cm 3 molecule −1 s −1 ) and for the reaction with HO 2 (CH 2 BRO 2 + HO 2 → products (5), k 5 = (6.7 ± 3.8) × 10 −12 cm 3 molecule −1 s −1 ). As for fluorine and chlorine substitution, bromine substitution enhances the rate constant of the self-reaction, compared to that of the methylperoxy radical, whereas it does not significantly change the rate constant for the reaction with HO 2 .

A reinvestigation of the kinetics and the mechanism of the CH3C(O)O2+ HO2 reaction using both experimental and theoretical approaches

The kinetics and the mechanism of the reaction CH(3)C(O)O(2)+ HO(2) were reinvestigated at room temperature using two complementary approaches: one experimental, using flash photolysis/UV absorption technique and one theoretical, with quantum chemistry calculations performed using the density functional theory (DFT) method with the three-parameter hybrid functional B3LYP associated with the 6-31G(d,p) basis set. According to a recent paper reported by Hasson et al., [J. Phys. Chem., 2004, 108, 5979-5989] this reaction may proceed by three different channels: CH(3)C(O)O(2)+ HO(2)--> CH(3)C(O)OOH + O(2) (1a); CH(3)C(O)O(2)+ HO(2)--> CH(3)C(O)OH + O(3) (1b); CH(3)C(O)O(2)+ HO(2)--> CH(3)C(O)O + OH + O(2) (1c). In experiments, CH(3)C(O)O(2) and HO(2) radicals were generated using Cl-initiated oxidation of acetaldehyde and methanol, respectively, in the presence of oxygen. The addition of amounts of benzene in the system, forming hydroxycyclohexadienyl radicals in the presence of OH, allowed us to answer that channel (1c) is <10%. The rate constant k(1) of reaction (1) has been finally measured at (1.50 +/- 0.08) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K, after having considered the combination of all the possible values for the branching ratios k(1a)/k(1,)k(1b)/k(1,)k(1c)/k(1) and has been compared to previous measurements. The branching ratio k(1b)/k(1), determined by measuring ozone in situ, was found to be equal to (20 +/- 1)%, a value consistent with the previous values reported in the literature. DFT calculations show that channel (1c) is also of minor importance: it was deduced unambiguously that the formation of CH(3)C(O)OOH + O(2) (X (3)Sigma(-)(g)) is the dominant product channel, followed by the second channel (1b) leading to CH(3)C(O)OH and singlet O(3) and, much less importantly, channel (1c) which corresponds to OH formation. These conclusions give a reliable explanation of the experimental observations of this work. In conclusion, the present study demonstrates that the CH(3)C(O)O(2)+ HO(2) is still predominantly a radical chain termination reaction in the tropospheric ozone chain formation processes.

Kinetics and atmospheric implications of peroxy radical cross reactions involving the CH3C(O)O2 radical

The kinetics of the reactions of selected secondary and tertiary peroxy radicals (RO2) with CH3C(O)O2 have been investigated. Values of (1.0±0.3)×10−11, (1.1±0.3)×−11(1.0±0.5)×10−11 and (1.1±0.3)×10−11 (units of cm3 molecule−1 s−1, statistical errors 2σ) have been obtained at 298 K for the rate constants of the reactions of CH3C(O)O2 radicals with c-C6H11O2, sec-C10H21O2, sec-C12H25O2 and t-C4H9O2 radicals, respectively. A systematic propagation of error analysis has yielded overall (statistical + systematical) uncertainty factors of 1.6, 1.8, 1.9, and 1.5, respectively, for the above rate constant values. The present results, combined with the results previously reported for primary peroxy radicals, show that all cross reactions of CH3C(O)O2 are fast with rate constants of around 1.0×10−11 cm3 molecule−1 s−1, independent of the RO2 radical structure and of its self-reaction rate constant. This suggests that all acylperoxy radicals present the same high reactivity as CH3C(O)O2, and hence it is proposed to assign the above rate constant value to all cross reactions of acylperoxy radicals. These new rate constant values were implemented in the Regional Atmospheric Chemistry Mechanism [Stockwell et al., 1997] to estimate the importance of the cross reactions in the chemistry of acylperoxy radicals in the troposphere. In the case of a moderately polluted troposphere, under low NOx and high volatile organic compounds (VOC) concentrations, the cross reactions of acylperoxy radicals with organic peroxy radicals account for more than 20% of the acylperoxy loss reactions, and peroxyacyl nitrates concentrations decrease by more than 4%, compared with the previous estimates of Kirchner and Stockwell [1996].

Photochemical Aging of Secondary Organic Aerosols Generated from the Photooxidation of Polycyclic Aromatic Hydrocarbons in the Gas-Phase

Aging processes of secondary organic aerosol (SOA) may be a source of oxygenated organic aerosols; however, the chemical processes involved remain unclear. In this study, we investigate photochemical aging of SOA produced by the gas-phase oxidation of naphthalene by hydroxyl radicals and acenaphthylene by ozone. We monitored the SOA composition using a high-resolution time-of-flight aerosol mass spectrometer. We initiated SOA aging with UV photolysis alone and with OH radicals in the presence or absence of light and at different NOx levels. For naphthalene, the organic composition of the particulate phase seems to be dominated by highly oxidized compounds such as carboxylic acids, and aging data may be consistent with diffusion limitations. For acenaphthylene, the fate of oxidized products and the moderately oxidized aerosol seem to indicate that functionalization reactions might be the main aging process were initiated by the cumulative effect of light and OH radicals.

HETEROGENEOUS REACTIONS OF OH RADICALS WITH PARTICULATE-PYRENE AND 1-NITROPYRENE OF ATMOSPHERIC INTEREST

The heterogeneous reactivity of OH radicals with pyrene and 1-nitropyrene adsorbed on model particles has been investigated using a discharge flow reactor, in the presence of a large excess of NO 2 . Graphite was chosen as a simple model of carbonaceous particles, whereas silica was chosen as a representative surrogate of mineral atmospheric aerosol. The reaction kinetics was investigated by measuring the remaining pyrene and 1-nitropyrene adsorbed on particles after pressurized fluid extraction and gas chromatography/mass spectrometry analysis. Pseudo-first order rate constants were obtained from the fit of the experimental decay of particulate polycyclic compound concentrations versus reaction time. Rate constants were measured at room temperature for reactions of OH radicals with, respectively, pyrene, 1-nitropyrene, both adsorbed on silica, and 1-nitropyrene adsorbed on graphite, in the presence of NO 2 .

Kinetics and mechanism of the gas-phase reaction of the cyclohexadienyl radical c-C6H7 with O2

The kinetics and mechanism of the gas-phase reaction of the cyclohexadienyl radical c-C6H7 with O2 have been investigated using both experimental and theoretical approaches. The rate constant has been measured using conventional flash photolysis in the temperature range 302–456 K, 1 atm pressure. c-C6H7 radicals were produced by reacting Cl atoms with 1,4-cyclohexadiene. The rate expression is k1=(1.4±0.26)×10−13 exp[−(300±74) K/T] cm3 molecule−1 s−1 (2σ error bars). The reaction can proceed either by association, yielding a peroxy radical RO2 or by H-abstraction, yielding benzene+HO2, the two reaction channels involving two distinct transition states. In contrast to what is observed for the c-C6H6OH radical, no equilibrium with the peroxy radical could be characterised. The theoretical approach, involving both DFT and ab initio methods, was used to determine if the measured rate constant should be assigned to the association or to the H-abstraction channel. Comparison of experimental and theoretical results shows that H-abstraction must be the only significant reaction channel.

Atmospheric Heterogeneous Reactions of Benzo(a)pyrene

Abstract This experimental study deals with heterogeneous reactions of benzo(a)pyrene (BaP) with ozone, nitrogen dioxide and hydroxyl radicals. BaP was adsorbed on silica particles chosen here as a model of mineral atmospheric particles. Compound extractions were assisted by focused microwave and analyses were performed by gas chromatography coupled with mass spectroscopy in single ion monitoring mode. Pseudo-first order rate constants were obtained from the fit of experimental decays of particulate-BaP concentration versus reaction time. Second order rate constants were determined considering the different oxidant gaseous concentrations except for the case of hydroxyl radicals where only a pseudo-first order rate constant was proposed. Values obtained at room temperature are (2.1±0.5)×10−15 cm3 molecule−1 s−1 for (BaP + ozone), (5.8±1.4)×10−16 cm3 molecule−1 s−1 for (BaP + nitrogen dioxide) and (3.4±0.8)×10−2 s−1 for (BaP + OH) reactions. Products have only been investigated for the NO2 and the OH (in the presence of NOx) reactions. 1-, 3- and 6-nitrobenzo(a)pyrenes were detected as degradation products and quantified. Reaction rate constants for product formation are (3.7±0.9)×10−16 cm3 molecule−1 s−1 for 6-NBaP, (2.2±0.6)×10−17 cm3 molecule−1 s−1 for 1-NBaP and (5.3±1.3)×10−17 cm3 molecule−1 s−1 for 3-NBaP. 1-, 3- and 6-nitroBaP account respectively for approximately 5%, 12% and 83% of total nitrated species. If in the presence of only nitrogen dioxide, BaP was totally degraded within few minutes, only 20 to 25 % of the initial BaP led to nitrated compounds when reacting with OH (in the presence of NOx).

Experimental revaluation of the importance of the abstraction channel in the reactions of monoterpenes with OH radicals.

The primary oxidation steps of (γ-terpinene+OH) and (d-limonene+OH) reactions are investigated using two techniques: an excimer laser photolysis set-up coupled with UV absorption spectrometry performed at atmospheric pressure and a fast-flow reactor coupled to time of flight mass spectrometry at low pressure. OH radicals are generated either by photolysis of H(2)O(2) or via the reaction of H atoms with NO(2). The primary reaction of monoterpenes with hydroxyl radicals can proceed by two reaction pathways: OH-addition and H-abstraction. The branching ratios for these channels are measured at various pressures for (γ-terpinene+OH) and (d-limonene+OH) reactions and a discussion on the H-abstraction importance for reactions of monoterpenes with hydroxyl radicals is proposed. H-abstraction may contribute to (31±9)% and (34±8)% respectively, for γ-terpinene and d-limonene reactions with OH at atmospheric pressure and respectively to (28±6)% and (28±8)% at low pressure (between 0.5 and 2.8 torr). As already pointed out by the Leuven group of Peeters, H-abstraction may be a significant reaction pathway for the reactions of monoterpenes with hydroxyl radicals. Therefore, oxidation products resulting from the H-abstraction should not be neglected in the mechanisms describing the formation of secondary organic aerosols (SOA) from gas-phase reactions of monoterpenes+OH.