Ph. Mirabel

Mass transfer at the air/water interface: Mass accommodation coefficients of SO2, HNO3, NO2 and NH3

We present here experimental determinations of mass accommodation coefficients β using a low pressure tube reactor in which monodispersed droplets, generated by a vibrating orifice, are brought into contact with known amounts of trace gases. The uptake of the gases and the accommodation coefficient are determined by chemical analysis of the aqueous phase.We report in this article measurements of βexp=(6.0±0.8)×10−2 at 298 K and with a total pressure of 38 Torr for SO2, (5.0±1.0)×10−2 at 297 K and total pressure of 52 Torr for HNO3, (1.5±0.6)×10−3 at 298 K and total pressure of 50 Torr for NO2, (2.4±1.0)×10−2 at 290 K and total pressure of 70 Torr for NH3.These values are corrected for mass transport limitations in the gas phase leading to β=(1.3±0.1)×10−1 (298 K) for SO2, (1.1±0.1)×10−1 (298 K) for HNO3, (9.7±0.9)×10−2 (290 K) for NH3, (1.5±0.8)×10−3 (298 K) for NO2 but this last value should not be considered as the true value of β for NO2 because of possible chemical interferences.Results are discussed in terms of experimental conditions which determine the presence of limitations on the mass transport rates of gaseous species into an aqueous phase, which permits the correction of the experimental values.

Experimental determination of HONO mass accommodation coefficients using two different techniques

The mass accommodation coefficient αHONO of gaseous nitrous acid on water surfaces has been determined in a cooperation between the Universities of Strasbourg and Bonn. The droplet train technique (Strasbourg) yielded 0.04<αHONO<0.09 for an estimated surface temperature of 245 K, while the liquid jet technique (Bonn) yielded 0.03<αHONO<0.15 for a surface temperature of 297 K. The uncertainty ranges allow for experimental scatter and estimated uncertainties in diffusion coefficients. The same HONO source and analytical equipment were used for both experiments, which were run in parallel. The results indicate that the exchange rate of HONO between atmospheric water droplets and interstitial air is not inhibited by interfacial resistance.

Adsorption of acetic acid on ice: experiments and molecular dynamics simulations.

Adsorption study of acetic acid on ice surfaces was performed by combining experimental and theoretical approaches. The experiments were conducted between 193 and 223 K using a coated wall flow tube coupled to a mass spectrometric detection. Under our experimental conditions, acetic acid was mainly dimerized in the gas phase. The surface coverage increases with decreasing temperature and with increasing concentrations of acetic acid dimers. The obtained experimental surface coverages were fitted according to the BET theory in order to determine the enthalpy of adsorption deltaH(ads) and the mololayer capacity N(M(dimers)) of the acetic acid dimers on ice: deltaH(ads) = (-33.5 +/- 4.2) kJ mol(-1), N(M(dimers)) = (l1.27 +/- 0.25) x 10(14) dimers cm(-2). The adsorption characteristics of acetic acid on an ideal ice I(n)(0001) surface were also studied by means of classical molecular dynamics simulations in the same temperature range. The monolayer capacity, the configurations of the molecules in their adsorption sites, and the corresponding adsorption energies have been determined for both acetic acid monomers and dimers, and compared to the corresponding data obtained from the experiments. In addition, the theoretical results show that the interaction with the ice surface could be strong enough to break the acetic acid dimers that exist in the gas phase and leads to the stabilization of acetic acid monomers on ice.

Emission of ions and charged soot particles by aircraft engines

Abstract. In this article, a model which examines the formation and evolution of chemiions in an aircraft engine is proposed. This model which includes chemiionisation, electron thermo-emission, electron attachment to soot particles and to neutral molecules, electron-ion and ion-ion recombination, ion-soot interaction, allows the determination of the ion concentration at the exit of the combustor and at the nozzle exit of the engine. It also allows the determination of the charge of the soot particles. For the engine considered, the upper limit for the ion emission index EIi is of the order of (2-5) x1016 ions/kg-fuel if ion-soot interactions are ignored and the introduction of ion-soot interactions lead about to a 50% reduction. The results also show that most of the soot particles are either positively or negatively charged, the remaining neutral particles representing approximately 20% of the total particles. A comparison of the model results with the available ground-based experimental data obtained on the ATTAS research aircraft engines during the SULFUR experiments (Schumann, 2002) shows an excellent agreement.

Solubility of polyvalent cations in fogwater at an urban site in Strasbourg (France)

Abstract The concentrations in the soluble and total (soluble + insoluble) fractions of Mg, Ca, Fe, Mn, Zn, Al, Cd and Pb have been analysed by “inductively coupled plasma (ICP)” in 14 fog events collected in 1992 at an urban site in France (Strasbourg). For each fog event, two droplet size categories (2–6 μm and 5–8 μm) have been collected separately. For the analysis of the polyvalent cations in the soluble and total fractions, an analytical procedure using ICP and filtration on cellulose/PVC filters has been developed. The study of the solubility of some polyvalent cations has shown that two of the most important factors controlling the partitioning between the soluble and insoluble fraction are the nature of the particles and the pH of the fogwater. The influence of pH depended on the element. The solubility of Pb, Cd, Al, Fe, Mg, and Ca were pH dependent whereas, Zn and Mn solubility varied but no relationship with pH existed, ranging between 25 and 100% and 10 and 100%, respectively. On the other hand, Mg, Pb and Ca were predominantly present in the soluble phase, whereas Al was prevalent in the insoluble fraction. In the case of Cd and Fe., the presence in the soluble or insoluble phase depended largely on the fogwater pH.

Adsorption studies of acetone and 2,3-butanedione on ice surfaces between 193 and 223 K

Adsorption studies of acetone and of 2,3-butanedione on ice surfaces were performed using a new vertical coated wall flow tube coupled to a mass spectrometric detection. Adsorption of acetone on ice was found to be totally reversible for ice temperatures ranging from 193 to 223 K and for gas phase acetone concentrations varying between 5.4 × 1010 and 6.4 × 1013 molecules cm−3. Adsorption of 2,3-butanedione was also reversible at 213 and 223 K but partially irreversible at 193 and 203 K when its concentrations were larger than 1 × 1013 molecules cm−3. It was shown that, at 203 K, the surface coverage increases when the ice surface contains large and dense cracks but is independent of the presence of cracks at 223 K. The surface coverage also increases with decreasing temperature and with increasing acetone or 2,3-butanedione concentrations. The obtained experimental surface coverages were fitted according to the Langmuir and BET theories in order to determine the enthalpy of adsorption ΔHads and the monolayer capacity NM. The following values of NM were derived (in units of 1014 molecule cm−2): NM = 1.3 ± 0.2 for acetone and NM = 1.2 ± 0.5 for 2,3-butanedione. The corresponding enthalpies of adsorption are (in kJ mol−1): −49 ± 7 for acetone and −59 ± 8 for 2,3-butanedione. The results are discussed and compared with previous determinations for acetone. Finally, the obtained results are used to estimate the partitioning of acetone between the ice and gas phases in clouds of the upper troposphere.

Uptake Measurements of Acetaldehyde on Solid Ice Surfaces and on Solid/Liquid Supercooled Mixtures Doped with HNO3 in the Temperature Range 203-253 K

Uptake of acetaldehyde on ice surfaces has been investigated over the temperature range 203-253 K using a coated wall flow tube coupled to a mass spectrometric detection. The experiments were conducted on pure ice surfaces and on liquid/solid ice mixture both doped with nitric acid (0.063, 0.63, and 6.3 wt %). Uptake of acetaldehyde on these surfaces was always found to be totally reversible whatever the experimental conditions were. The number of acetaldehyde molecules adsorbed per surface unit was conventionally plotted as a function of acetaldehyde concentration in the gas phase. Although the amounts of acetaldehyde adsorbed on solid ice surfaces (pure and HNO(3)-doped ice) were approximately similar and rather limited, the number of acetaldehyde molecules taken up on the HNO(3)-doped solid ice/liquid mixtures are significantly higher, up to 1 or 2 orders of magnitudes compared to pure ice surfaces. At 213 K for example and for low concentrations of acetaldehyde (<1 x 10(13) molecule cm(-3)), the amount of acetaldehyde molecules taken up on solid/liquid doped surfaces is 3.3 and 8.8 times higher than those measured on pure ice respectively for 0.063 and 0.63 wt % of HNO(3). The huge quantities of acetaldehyde taken up by liquid-/solid-doped mixtures are likely dissolved in the nonhomogeneous liquid part of the surfaces according to the Henrys law equilibrium. As a consequence, up to about 10% of acetaldehyde may be scavenged by supercooled liquid droplets of convective clouds in the upper troposphere.

Kinetic model for binary homogeneous nucleation in the H2O–H2SO4 system: Comparison with experiments and classical theory of nucleation

A kinetic model to predict nucleation rates in the sulfuric acid-water system is presented. It allows calculating steady-state nucleation rates and the corresponding time lag, using a direct solution of a system of kinetic equations that describe the populations of sub- and near-critical clusters. This kinetic model takes into account cluster-cluster collisions and decay of clusters into smaller clusters. The model results are compared with some predictions obtained with the classical nucleation theory (CNT) and also with available measurement data obtained in smog chambers or flow tubes. It is shown that in the case of slow nucleation processes, the kinetic model and the CNT as used by Shugard et al. [J. Chem. Phys. 75, 5298 (1974)] give the same results. However, in the case of intensive nucleation, a large part of the nucleation flux is due to cluster-cluster collisions and the CNT underestimates the nucleation rates.

Heterogeneous Chemistry of Nitryl Halides in Relation to Tropospheric Halogen Activation

The heterogeneous chemistry of nitryl chloride and nitryl bromide by salt containing solutions was studied as a function of temperature in the range from 275 to 293 K with the wetted-wall flowtube combined with FTIR and mass spectrometry detection. Uptake coefficients and values of the product Hk1/2 on these saline solutions have been determined. For nitryl halides interacting with NaI and NaBr solutions, the values of the product Hk1/2 are respectively 4384.7±326.7 and 103.1±18.7 M atm-1 s-1/2 for nitryl chloride at 275 K and 544.2±94.7 and 47.7±15.2 M atm-1 s-1/2 for nitryl bromide at 278 K. When reacting with NaI or NaBr solutions, these heterogeneous reactions release, as major products, the molecular forms of the halogen i.e., respectively I2 and Br2. A simplified reaction scheme explaining the formation of these products is presented and is inserted into a model simulating the chemistry in the marine boundary layer. The modelling effort showed Cl and BrO atoms concentrations up to 5×104 and 1.8×106 molecules cm-3 respectively, which are comparable to values actually measured in field campaigns.

Aldehydes and BTEX Measurements and Exposures in University Libraries in Strasbourg (France)

Aldehydes and BTEX concentrations were measured in 20 university libraries in Strasbourg (east of France) using Radiello passive sampling systems containing either DNPH or activated charcoal for aldehydes and BTEX, respectively. For the aldehydes, the conventional DNPH-derivatization method was used, followed by liquid chromatography coupled to UV detection while the BTEX were quantified by GC-PID. Aldehydes levels found inside the libraries ranged from 8.6 to 94.5 μg · m×3 for formaldehyde, 3.7 to 25.9 μg · m-3 for acetaldehyde, 2.1 to 58.8 μg · m-3 for hexanal, 0.2 to 5.3 μg · m-3 for benzaldehyde, and 0.7 to 16.3 μg · m-3 for propionaldehyde. Their mean values are 28.6± 18.8 μg · m-3 (formaldehyde), 10.2 ± 5.8 μg · m-3 (acetaldehyde) and 15.1 ± 12.1 μg · m-3 (hexanal), 1.1 ±1.1 for benzaldehyde and 2.8 ±3.3 for propionaldehyde, where the quoted errors correspond to 1σ level. Total BTEX concentrations were quantified in the same libraries and were usually relatively low (<20 μg · m -3) in all libraries, except for one where unusually high values (~295 μg · m-3) were found. Excluding this latter, the means BTEX concentrations were (in units of μg · m-3): 0.2 ±0.2 (benzene), 3.8 ±2.6 (toluene), 0.8 ±0.5 (ethyl benzene), 1.9 ± 1.2 (m- and p-xylenes), and 0.5 ±0.4 (o -xylene), where the quoted errors correspond again to 1σ.