Gérard Toupance

On the spatial distribution and seasonal variation of lower-troposphere ozone over Europe

Surface ozone data from 25 Europeanlow-altitude sites and mountain sites located between79°N and 28°N were studied. The analysiscovered the time period March 1989–February 1993.Average summer and winter O3 concentrations inthe boundary layer over the continent gave rise togradients that were strongest in the north-west tosouth-east direction and west-east direction, respectively. WintertimeO3 ranged from 19 to 27 ppbover the continent, compared to about 32 ppb at thewestern border, while for summer the continentalO3 values ranged between 39 and 56 ppb and theoceanic mixing ratios were around 37 ppb. In the lowerfree troposphere average wintertime O3 mixingratios were around 38 ppb, with only an 8 ppbdifference between 28°N and 79°N. For summerthe average O3 levels decreased from about 55 ppbover Central Europe to 32 ppb at 79°N. Inaddition, O3 and Ox(= O3 + NO2)in polluted and clean air were compared. Theamplitudes of the seasonal ozone variations increasedin the north-west to south-east direction, while thetime of the annual maximum was shifted from spring (atthe northerly sites) to late summer (at sites inAustria and Hungary), which reflected the contributionof photochemical ozone production in the lower partsof the troposphere.

Formation of prebiochemical compounds in models of the primitive Earth's atmosphere

In order to understand the formation of organic compounds in the primitive atmosphere, the first steps of evolution in models of the primitive atmosphere were investigated. Mixtures containing C−H−N elements were subjected to a low pressure silent electric discharge for several seconds, and the resulting effluents were analysed mainly by gas chromatography, infrared spectrometry and chemical analysis. The formation of hydrocarbons (i.e. ethylene, acetylene, methylacetylene) and of nitrogen containing compounds (i.e. hydrogen cyanide, cyanogen, saturated nitriles, acrylonitrile, cyanoacetylene) is reported. The influence of the initial mixture composition on the amount of compounds formed was systematically studied. The nature of the nitrogen source (N2 or NH3) in the primitive atmosphere has a great influence on the amount and on the very nature of the synthesized products. It is shown that important precursors such as cyanogen and cyanoacetylene are formed only in very rich N2 mediums. There results show the important role played by the nature of the primitive atmosphere in the determination of the chemical evolution pathways.

Organic syntheses from CH4−N2 atmospheres: Implications for Titan

Numerous experiments have already been performed, simulating the evolution of gaseous mixtures containing CH4 when submitted to energy flux. From their results, it appears that a variety of organic compounds, including unsaturated hydrocarbons and nitriles such as HCN, can be synthesized into noticeable amounts from CH4−N2 mixtures. In particular, systematic studies of the influence of the composition of the mixture on the nature and amount of synthesized compounds show that organic volatile nitriles, and particularly cyanoacetylene and cyanogen, are formed only in media rich in nitrogen. Those nitriles have been identified very recently in the atmosphere of Titan, and thus, data from such laboratory experiments may provide important indirect information on the organic chemistry occuring at the periphery of this satellite of Saturn. However, during these experiments, there is a continuous formation and accumulation of molecular hydrogen, which does not occur in the atmosphere of Titan, because of H2 escape. In order to reassess the data already available from this type of laboratory studies, experiments on CH4−N2 atmospheres, with and without H2 escape, have been recently performed. The influence of this parameter on the chemical evolution of the atmosphere and on the nature and relative quantities of organic compounds has been studied.After reviewing these experiments, implications of the obtained results on the organic chemistry at the periphery of Titan are discussed.

Formation of prebiochemical compounds in models of the primitive earth's atmosphere. I: CH4 NH3 and CH4 N2 atmospheres.

In order to understand the formation of organic compounds in the primitive atmosphere, the first steps of evolution in models of the primitive atmosphere were investigated. Mixtures containing C−H−N elements were subjected to a low pressure silent electric discharge for several seconds, and the resulting effluents were analysed mainly by gas chromatography, infrared spectrometry and chemical analysis. The formation of hydrocarbons (i.e. ethylene, acetylene, methylacetylene) and of nitrogen containing compounds (i.e. hydrogen cyanide, cyanogen, saturated nitriles, acrylonitrile, cyanoacetylene) is reported. The influence of the initial mixture composition on the amount of compounds formed was systematically studied. The nature of the nitrogen source (N2 or NH3) in the primitive atmosphere has a great influence on the amount and on the very nature of the synthesized products. It is shown that important precursors such as cyanogen and cyanoacetylene are formed only in very rich N2 mediums. There results show the important role played by the nature of the primitive atmosphere in the determination of the chemical evolution pathways.

Field studies of aldehyde chemistry in the Paris area

Abstract Measurements of carbonyl compound concentrations at different sites in the Paris area have been carried out. Interpretation of the results made use of the following data: general meteorological conditions, wind field analysis and type of primary pollutant sources. The principal phenomena observed were: a sharp formaldehyde decrease during rainfall; concentration levels of lower aldehydes in rural sites comparable to those found in the literature; an important variation in the ratio of primary aldehydes to secondary aldehydes depending on meteorological conditions; a significant increase in lower aldehyde concentrations downwind from the urban center despite vertical dispersion of the pollutants.

Abundance of organic compounds photochemically produced in the atmospheres of the outer planets

Abstract Organic photochemical syntheses in the Jovian atmosphere were simulated by irradiating, at 147 nm, gaseous mixtures of methane and ammonia with varying quantities of hydrogen. An excess of H 2 did not eliminate organic synthesis but did affect the yields quantitatively and qualitatively.

Infrared absorptivities and integrated band intensities for gaseous peroxyacetyl nitrate (PAN)

Abstract A quantitative reexamination of the i.r. spectrum of PAN using Fourier Transform Infrared Spectroscopy (FTIR) was undertaken. The influence on infrared absorptivities of a diluent gas (air or N 2 ) in the PAN samples was investigated for the first time, and the obtained results allowed us to establish the independence of the PAN absorptivities on the total pressure. The i.r. absorptivities and the integrated band intensities of the principal PAN bands were reevaluated and compared with literature data. Concerning the integrated band intensities, an important discrepancy was noticed between our results and the literature which is analyzed and discussed.

Far UV irradiation of model prebiotic atmospheres.

UV light has been the most important energy source on the primitive Earth. However, very few experiments have been performed to test directly the possible role of this energy source on the chemical evolution of the primitive atmosphere, mainly on account of experimental difficulties. Experiments are generally performed with other excitations, mainly electric discharge, and it is frequently assumed that UV irradiation would give similar results.As theoretical considerations make this assumption questionable, direct experimental controls have been undertaken: Model primitive atmospheres have been submitted to 147 nm UV light and the gaseous phase has been analysed. Preliminary qualitative results concerning CH4−NH3 atmospheres are reported.Irradiation of pure CH4 gives rise to the synthesis of a large number of hydrocarbons, mainly saturated hydrocarbons but including also unsaturated ones as, C2H2, C2H4, C3H6, C3H4. These insaturated hydrocarbons are synthetized at a very low rate when ammonia is present in the medium.Irradiations of CH4−NH3 mixtures give rise, in addition to hydrocarbons, to important amounts of HCN (about 0.1%) and to lesser amounts of CH3CN and C2H5CN. No unsaturated nitriles such as acrylonitrile and cyanoacetylene have been detected. Search for amines is in progress.These results evidence that UV irradiation may contribute largely to synthesis of HCN in CH4−NH3 atmospheres and, consequently to the synthesis of many biochemical compounds that can be derivated from HCN. However, synthesis of other compounds, such as pyrimidines, which can derivate from other nitriles, such as cyanoacetylene, cannot be initiated only by UV light, contrary to electric discharges. In addition, if electric discharges are very efficient for synthesis of nitriles in CH4−N2 atmospheres, there is not yet evidence that UV light is able to do so.

Rate constant measurement for the reactions of OH and Cl with peroxyacetyl nitrate at 298 K

The gas phase reactions of peroxyacetyl nitrate (PAN) with OH and Cl have been studied using the discharge-flow EPR method. The rate constants are found to be k3=(7.5±1.4)×10-14 and k4=(3.7±1.7)×10-13 cm3 molecule-1 s-1 at 298 K, respectively. These results confirm that the OH+PAN reaction will be the dominant sink of PAN in the middle and upper troposphere, whereas the reaction Cl+PAN will be negligible in contrast with previous estimations.

One-year measurements of PAN in the Paris basin: Effect of meteorological parameters

Abstract PAN has been measured continuously for 1 year in Creteil, a southeastern suburb of Paris Correspondence analysis was used to analyse the important PAN database obtained during this period, in relation to associated meteorological parameters and the location of precursors sources. This analysis revealed a complex relation (U-shaped curve) between PAN and temperatures, with a minimum in the range of 4–12°C and an increase in the lower and higher temperatures, arising from the competition between photochemical production and thermal decomposition. Long-range transport of PAN is associated with easterly winds and occurs under anticyclonic conditions in all seasons, particularly in winter. The highest PAN values (maximum 33.6 ppb) are observed in summer. They have a local origin and are associated with northwesterly to southwesterly winds, revealing the net impact, in summer, of nearby sources such as the urban conglomeration of Paris on PAN levels recorded in the surrounding areas.