François Bernard

Aerosol scattering coefficients and major chemical compositions of fine particles observed at a rural site in the central Pearl River Delta, south China

During November-December 2010 aerosol scattering coefficients were monitored using a single-waved (525 nm) Nephelometer at a regional monitoring station in the central Pearl River Delta region and 24-hr fine particle (PM2.5) samples were also collected during the period using quartz filters for the analysis of major chemical components including organic carbon (OC), elemental carbon (EC), sulfate, nitrate and ammonium. In average, these five components accounted for about 85% of PM2.5 mass and contributed 42% (OC), 19% (SO4(2-)), 12% (NO(3-)), 8.4% (NH(4+)) and 3.7% (EC), to PM2.5 mass. A relatively higher mass scattering efficiency of 5.3 m2/g was obtained for fine particles based on the linear regression between scattering coefficients and PM2.5 mass concentrations. Chemical extinction budget based on IMPROVE approach revealed that ammonium sulfate, particulate organic matter, ammonium nitrate and EC in average contributed about 32%, 28%, 20% and 6% to the light extinction coefficients, respectively.

Studies of the gas phase reactions of linalool, 6-methyl-5-hepten-2-ol and 3-methyl-1-penten-3-ol with O3 and OH radicals.

The reactions of three unsaturated alcohols (linalool, 6-methyl-5-hepten-2-ol, and 3-methyl-1-penten-3-ol) with ozone and OH radicals have been studied using simulation chambers at T ∼ 296 K and P ∼ 760 Torr. The rate coefficient values (in cm(3) molecule(-1) s(-1)) determined for the three compounds are linalool, k(O3) = (4.1 ± 1.0) × 10(-16) and k(OH) = (1.7 ± 0.3) × 10(-10); 6-methyl-5-hepten-2-ol, k(O3) = (3.8 ± 1.2) × 10(-16) and k(OH) = (1.0 ± 0.3) × 10(-10); and 3-methyl-1-penten-3-ol, k(O3) = (5.2 ± 0.6) × 10(-18) and k(OH) = (6.2 ± 1.8) × 10(-11). From the kinetic data it is estimated that, for the reaction of O(3) with linalool, attack at the R-CH═C(CH(3))(2) group represents around (93 ± 52)% (k(6-methyl-5-hepten-2-ol)/k(linalool)) of the overall reaction, with reaction at the R-CH═CH(2) group accounting for about (1.3 ± 0.5)% (k(3-methyl-1-penten-3-ol)/k(linalool)). In a similar manner it has been calculated that for the reaction of OH radicals with linalool, attack of the OH radical at the R-CH═C(CH(3))(2) group represents around (59 ± 18)% (k(6-methyl-5-hepten-2-ol)/k(linalool)) of the total reaction, while addition of OH to the R-CH═CH(2) group is estimated to be around (36 ± 6)% (k(3-methyl-1-penten-3-ol)/k(linalool)). Analysis of the products from the reaction of O(3) with linalool confirmed that addition to the R-CH═C(CH(3))(2) group is the predominant reaction pathway. The presence of formaldehyde and hydroxyacetone in the reaction products together with compelling evidence for the generation of OH radicals in the system indicates that the hydroperoxide channel is important in the loss of the biradical [(CH(3))(2)COO]* formed in the reaction of O(3) with linalool. Studies on the reactions of O(3) with the unsaturated alcohols showed that the yields of secondary organic aerosols (SOAs) are higher in the absence of OH scavengers compared to the yields in their presence. However, even under low-NO(X) concentrations, the reactions of OH radicals with 3-methyl-1-penten-3-ol and 6-methyl-5-hepten-2-ol will make only a minor contribution to SOA formation under atmospheric conditions. Relatively high yields of SOAs were observed in the reactions of OH with linalool, although the initial concentrations of reactants were quite high. The importance of linalool in the formation of SOAs in the atmosphere requires further investigation. The impact following releases of these unsaturated alcohols into the atmosphere are discussed.

Kinetics and Products of Gas-Phase Reactions of Ozone with Methyl Methacrylate, Methyl Acrylate, and Ethyl Acrylate

The kinetics and products of the gas-phase reactions of ozone with methyl methacrylate, methyl acrylate, and ethyl acrylate have been investigated at 760 Torr total pressure of air and 294 +/- 2 K. The rate coefficients obtained (in cm(3) molecule(-1) s(-1) units) were as follows: k(methyl methacrylate) = (6.7 +/- 0.9) x 10(-18), k(methyl acrylate) = (0.95 +/- 0.07) x 10(-18), and k(ethyl acrylate) = (1.3 +/- 0.1) x 10(-18). In addition to formaldehyde being observed as a product of the three reactions, the other major reaction products were methyl pyruvate from reaction of ozone with methyl methacrylate, methyl glyoxylate from reaction of ozone with methyl acrylate, and ethyl glyoxylate from reaction of ozone with ethyl acrylate. Possible reaction mechanisms leading to the observed products are presented and discussed.

A tunable diode laser absorption spectrometer for formaldehyde atmospheric measurements validated by simulation chamber instrumentation

A tunable diode laser absorption spectrometer (TDLAS) for formaldehyde atmospheric measurements has been set up and validated through comparison experiments with a Fourier transform infrared spectrometer (FT-IR) in a simulation chamber. Formaldehyde was generated in situ in the chamber from reaction of ethene with ozone. Three HCHO ro-vibrational line intensities (at 2909.71, 2912.09 and 2914.46 cm(-1)) possibly used by TDLAS were calibrated by FT-IR spectra simultaneously recorded in the 1600-3200 cm(-1) domain during ethene ozonolysis, enabling the on-line deduction of the varying concentration for HCHO in formation. The experimental line intensities values inferred confirmed the calculated ones from the updated HITRAN database. In addition, the feasibility of stratospheric in situ HCHO measurements using the 2912.09 cm(-1) line was demonstrated. The TDLAS performances were also assessed, leading to a 2sigma detection limit of 88 ppt in volume mixing ratio with a response time of 60 sec at 30 Torr and 294 K for 112 m optical path. As part of this work, the room-temperature rate constant of this reaction and the HCHO formation yield were found to be in excellent agreement with the compiled literature data.

Atmospheric Chemistry of Benzyl Alcohol: Kinetics and Mechanism of Reaction with OH Radicals

The atmospheric oxidation of benzyl alcohol has been investigated using smog chambers at ICARE, FORD, and EUPHORE. The rate coefficient for reaction with OH radicals was measured and an upper limit for the reaction with ozone was established; kOH = (2.8 ± 0.4) × 10(-11) at 297 ± 3 K (averaged value including results from Harrison and Wells) and kO(3) < 2 × 10(-19) cm(3) molecule(-1) s(-1) at 299 K. The products of the OH radical initiated oxidation of benzyl alcohol in the presence of NOX were studied. Benzaldehyde, originating from H-abstraction from the -CH(2)OH group, was identified using in situ FTIR spectroscopy, HPLC-UV/FID, and GC-PID and quantified in a yield of (24 ± 5) %. Ring retaining products originating from OH-addition to the aromatic ring such as o-hydroxybenzylalcohol and o-dihydroxybenzene as well as ring-cleavage products such as glyoxal were also identified and quantified with molar yields of (22 ± 2)%, (10 ± 3)%, and (2.7 ± 0.7)%, respectively. Formaldehyde was observed with a molar yield of (27 ± 10)%. The results are discussed with respect to previous studies and the atmospheric oxidation mechanism of benzyl alcohol.

UV photochemistry of carboxylic acids at the air‐sea boundary: A relevant source of glyoxal and other oxygenated VOC in the marine atmosphere

Photochemistry plays an important role in marine dissolved organic carbon (DOC) degradation, but the mechanisms that convert DOC into volatile organic compounds (VOCs) remain poorly understood. We irradiated carboxylic acids (C7–C9) on a simulated ocean surface with UV light (<320 nm) in a photochemical flow reactor and transferred the VOC products into a dark ozone reactor. Glyoxal was detected as a secondary product from heptanoic, octanoic, and nonanoic acid (NA) films, but not from octanol. Primary glyoxal emissions were not observed, nor was glyoxal formed in the absence of ozone. Addition of a photosensitizer had no noticeable effect. The concurrent detection of heptanal in the NA system suggests that the ozonolysis of 2-nonenal is the primary chemical mechanism that produces glyoxal. This source can potentially sustain tens of parts per trillion by volume (pptv) glyoxal over oceans, and helps to explain why glyoxal fluxes in marine air are directed from the atmosphere into the ocean.

Photosensitized Formation of Secondary Organic Aerosols above the Air/Water Interface

In this study, we evaluated photosensitized chemistry at the air–sea interface as a source of secondary organic aerosols (SOA). Our results show that, in addition to biogenic emissions, abiotic processes could also be important in the marine boundary layer. Photosensitized production of marine secondary organic aerosol was studied in a custom-built multiphase atmospheric simulation chamber. The experimental chamber contained water, humic acid (1–10 mg L–1) as a proxy for dissolved organic matter, and nonanoic acid (0.1–10 mM), a fatty acid proxy which formed an organic film at the air–water interface. Dark secondary reaction with ozone after illumination resulted in SOA particle concentrations in excess of 1000 cm–3, illustrating the production of unsaturated compounds by chemical reactions at the air–water interface. SOA numbers via photosensitization alone and in the absence of ozone did not exceed background levels. From these results, we derived a dependence of SOA numbers on nonanoic acid surface coverage and dissolved organic matter concentration. We present a discussion on the potential role of the air–sea interface in the production of atmospheric organic aerosol from photosensitized origins.

HCFC‐133a (CF3CH2Cl): OH rate coefficient, UV and infrared absorption spectra, and atmospheric implications

HCFC-133a (CF3CH2Cl), an ozone-depleting substance, is primarily removed from the atmosphere by gas-phase reaction with OH radicals and by UV photolysis. The rate coefficient, k, for the OH + HCFC-133a reaction was measured between 233 and 379 K and is given by k(T) = (9.32 ± 0.8) × 10−13 exp(−(1296 ± 28)/T), where k(296 K) was measured to be (1.10 ± 0.02) × 10−14 (cm3 molecule−1 s−1) (2σ precision uncertainty). The HCFC-133a UV absorption spectrum was measured between 184.95 and 240 nm at 213–323 K, and a spectrum parameterization is presented. The HCFC-133a atmospheric loss processes, lifetime, ozone depletion potential, and uncertainties were evaluated using a 2-D atmospheric model. The global annually averaged steady state lifetime and ozone depletion potential (ODP) were determined to be 4.45 (4.04–4.90) years and 0.017 (±0.001), respectively, where the ranges are based solely on the 2σ uncertainty in the kinetic and photochemical parameters. The infrared absorption spectrum of HCFC-133a was measured, and its global warming potential was determined to be 380 on the 100 year time horizon.

Reaction of NO2 with Selected Conjugated Alkenes

The gas phase reactions of selected alkenes (isoprene, myrcene, ocimene, and 1,3-cyclohexadiene) with NO2 under dark condition have been investigated at T ∼ 298 K and P ∼ 760 Torr of purified air. The kinetic studies were performed under pseudo-first-order conditions using a large excess of NO2 concentration to those of the alkenes. The rate coefficients (in 10(-19) cm(3) molecule(-1) s(-1)) obtained are 1.1 ± 0.2 for isoprene, 2.5 ± 0.3 for myrcene, 8.5 ± 1.2 for ocimene, and 15 ± 1 for 1,3-cyclohexadiene. Several products were identified by using in situ Fourier transform infrared (FT-IR) spectrometry, and acetone was found to be the major product from the reactions of NO2 with myrcene and ocimene, with a formation yield of 22 ± 3% and 26 ± 7%, respectively. The oxidation products from the reactions of NO2 with isoprene and 1,3-cyclohexadiene were found to be mainly nitro compounds identified by FT-IR spectroscopy. Reaction mechanisms were proposed to account for the products observed.

Temperature Dependent Rate Coefficients for the Gas-Phase Reaction of the OH Radical with Linear (L2, L3) and Cyclic (D3, D4) Permethylsiloxanes

Permethylsiloxanes are emitted into the atmosphere during production and use as personal care products, lubricants, and cleaning agents. The predominate atmospheric loss process for permethylsiloxanes is expected to be via gas-phase reaction with the OH radical. In this study, rate coefficients, k(T), for the OH radical gas-phase reaction with the two simplest linear and cyclic permethylsiloxanes were measured using a pulsed laser photolysis-laser induced fluorescence technique over the temperature range of 240-370 K and a relative rate method at 294 K: hexamethyldisiloxane ((CH3)3SiOSi(CH3)3, L2), k1; octamethyltrisiloxane ([(CH3)3SiO]2Si(CH3)2, L3), k2; hexamethylcyclotrisiloxane ([-Si(CH3)2O-]3, D3), k3; and octamethylcyclotetrasiloxane ([-Si(CH3)2O-]4, D4), k4. The obtained k(294 K) values and temperature-dependence expressions for the 240-370 K temperature range are (cm3 molecule-1 s-1, 2σ absolute uncertainties): k1(294 K) = (1.28 ± 0.08) × 10-12, k1( T) = (1.87 ± 0.18) × 10-11 exp(-(791 ± 27)/ T); k2(294 K) = (1.72 ± 0.10) × 10-12, k2( T) = 1.96 × 10-13 (T/298)4.34 exp(657/ T); k3(294 K) = (0.82 ± 0.05) × 10-12, k3( T) = (1.29 ± 0.19) × 10-11 exp(-(805 ± 43)/ T); and k4(294 K) = (1.12 ± 0.10) × 10-12, k4( T) = (1.80 ± 0.26) × 10-11 exp(-(816 ± 43)/ T). The cyclic molecules were found to be less reactive than the analogous linear molecule with the same number of -CH3 groups, while the linear and cyclic permethylsiloxane reactivity both increase with the increasing number of CH3- groups. The present results are compared with previous rate coefficient determinations where available. The permethylsiloxanes included in this study are atmospherically short-lived compounds with estimated atmospheric lifetimes of 11, 8, 17, and 13 days, respectively.