Marie-Louise Riba

Diurnal variation in the concentration of α- and β-pinene in the landes forest (France)

Abstract The study deals with the identification and the determination of the main monoterpenes (α- and β-pinene) released by the pines in the Landes Forest in France. The measurements were taken after preconcentration using TENAX-GC and thermal desorption. The diurnal variation of atmospheric concentration of α- and β-pinene is established; it is less than 0.5 ppbV in the daytime and varies from 2 to 4 ppbV in the night-time. These variations are explained by the fact that the photochemical reactions and the turbulence phenomena to which the terpenes are exposed change quite radically from day to night. The terpene concentration, higher at 5 m than at 25 m, would seem to confirm the existence of a terpene source at ground level.

Preconcentration of Atmospheric Terpenes on Solid Sorbents

Abstract Hydrocarbons emitted by vegetation, such as isoprene and monoterpenes, seem to play an important role in atmospheric chemistry. A better estimation of their biogeochemical cycle requires that the levels be testable at any time. The extremely low concentration of these chemicals in the atmosphere necessitates a preconcentration step at the time of smpling. This enrichment was done on 13 solid adsorbents (Tenax GC and TA, Chromosorb 101, 103, 105 and 107, Porapak R and Q, XAD-2, 4 and 7, Ambersorb-XE-340 and Carbosphere). This study allowed us to determine the breakthrough volumes of isoprene and the principal monoterpenes on these adsorbents. Of the adsorbents tested, Tenax GC and TA were those the best adapted to the desorption of compounds as reactive as the terpenes.

Preconcentration and analysis of atmospheric isoprene and monoterpenes System automation

Abstract As part of a study on terpene hydrocarbons in the biosphere-atmosphere exchange processes, an entirely automatic device has been elaborated for sampling, preconcentration and analysis of isoprene and the principal atmospheric monoterpenes. This device comprises mainly a gas chromatograph for analysis and a programmable controller for the management of the operations. It permits measurement of the concentration and flux rate of terpenes. Automatic and continuous cycles (sampling-analysis) were carried out over several days. They were characterized by a reproducibility lower than 1%; their analytical characteristics were quite acceptable.

Preconcentration of Volatile Sulphur Compounds on Solid Sorbents

Abstract The development of industry and urbanisation is at the origin of ever-increasing levels of anthropogenic pollution. Emissions of sulphur-containing compounds are partly composed of volatile organic compounds which add to those generated through natural processes. For low concentrations, direct assaying is problematic and a preconcentration step is necessary at the time of sampling. In order to select efficient solid sorbents for use in preconcentration of volatile organic sulphur-containing compounds, the breakthrough volumes of a certain number of mercaptans and sulphides have been determined on 9 porous polymers (Chromosorb 101, 103, 107,—Porapak Q, T, N,—XAD-4, XAD-7 and Tenax G.C.) and on Carbosphere. The breakthrough volumes were determined using a direct method with an apparatus based on diffusion tubes dynamically generating standard mixtures of carrier gas and sulphur-containing compounds.

Automated Determination of Monoterpenes in a Forested Area

Abstract We have developed an automated analytical device, equipped with a preconcentration/thermodesorption module, in order to monitor natural hydrocarbons that occur in the atmosphere. The objectives of this study were to test the performances of the automated device on a Bavarian experimental forested site, and to determine the diurnal variation in atmospheric concentration of α-and β-pinene at 2 and 21 m. The terpenic concentrations ranged between 6 pptV (36 ng m−3) and 216 pptV (1300 ng m−3).

Efficiency of adsorption-desorption of volatile compounds : Application to isoprene on Tenax GC

Abstract A method called “gaseous deposit” has been developed for the determination of the adsorption-desorption efficiency of volatile compounds. When applied to isoprene on Tenax GC it gives an efficiency of 96.5%. The reliability of the “gaseous deposit” technique is high since, for α-pinene, results obtained were extremely close to those obtained by more widely known methods. The advantages and disadvantages of the “gaseous deposit” method compared to other methods are discussed.

A Simple, Efficient Device for Determining Terpenes Present as Traces in the Atmosphere

Abstract Up to the present it has been established that natural hydrocarbons, such as isoprene and the monoterpenes, play an important role in the physical chemistry of the atmosphere because of their high reactivity. However, their true contribution to the natural chemical system has not yet been clearly defined as the seasonal variations in their concentrations are not sufficiently accurately known due to their low content in the atmosphere (a few hundredths of a ppb to a few ppb). We have developed an analysis procedure which enables quantitative trapping to be carried out together with excellent chromatographic separation. It consists of enrichment on adsorbents followed by very rapid thermal desorption and instantaneous chromatographic injection without further cryogenic trapping. Our adsorption-desorption cycle does not reduce the efficiency of the chromatographic separation at all and does not bring about any decomposition of these highly reactive plant oils.

Trace analysis. Determination of the “breakthrough volume”; (B.T.V.) of atmospheric isoprene

This paper presents the first results of a study of B.T.V. on Tenax TA, at various flow rates of sampling, with standard isoprene gaseous mixtures whose concentration ranging from 0.7 ppbV to 4 ppbV was very similar to the concentrations found in natural atmosphere. The method used is based on the exploitation of chromatographic signals obtained from a preconcentration—thermodesorption —analysis cycle carried out on a standard isoprene mixture. The B.T.V. of isoprene on Tenax TA does not show a significant change at very low concentrations of isoprene.

BIOGENIC EMISSIONS BY OAK TREES COMMON TO MEDITERRANEAN ECOSYSTEMS

The experimental site was a Mediterranean type forest located in Viols-en-Laval near Montpellier (France). The principal species studied were Quercus ilex and Quercus pubescens. The determination of biogenic emissions was carried out by the enclosure method, which consists in enclosing an intact branch in a Teflon cuvet. The evolution of global terpenic emissions were recorded on June 1995.For Quercus ilex it appears that most terpenic emissions take place during the diurnal period. The evolution of terpenic emission rates versus light (PAR), internal temperature within the cuvet, CO2 exchange and transpiration (H2O) were carried out. A close relation between terpene emission and light-triggered physiological activities, was always found. It is obvious that there was no simple correlation between the leaf temperature and the terpenic emission. The temperature of the leaves does not seem to play a significant role in the regulation of the monoterpenic emission. This behaviour allows us to conclude that light with its effect on assimilation and stomatal behaviour is the most important factor for monoterpenic emission by Quercus ilex like for Quercus pubescens which is an isoprene emitter.

Isoprenoïc Compounds: Emission and Atmospheric Concentration Measurements

We have developed an automated analytical device, equipped with a preconcentration/thermo-desorption module, in order to monitor natural hydrocarbons that are emitted into the atmosphere.