Etude des échanges à long-terme de composés organi-ques volatils d’origine biogénique par un écosystème forestier

The terrestrial biosphere, especially forest ecosystems, emits large quantities of volatile organic compounds (VOCs) which have a significant impact on the atmosphere’s chemical and physical characteristics. In particular, VOCs are precursors in the formation of ozone and sec-ondary organic aerosols. Isoprene and monoterpenes dominate the total VOC emissions, and methanol is one of the most abundant atmospheric VOCs due to its longer half-life than the other two. The main objective of this thesis was to investigate (using the eddy covariance technique and a proton-transfer-reaction mass spectrometer) the mechanisms of VOC (isoprene, monoterpene and methanol) emission and/or deposition at the scale of a temperate climate forest ecosystem (Vielsalm, Belgium) comprising several species (Fagus sylvatica, Abies alba, Picea Abies and Pseudotsuga menziessi). The eddy covariance technique is very suitable for studying VOC emission/deposition mechanisms at ecosystem level as it does not interfere with the functioning of the ecosystem and it has very good temporal resolution (half an hour). It was used for several months at the Vielsalm site without any major interruption to the measurements. The first measurement period ran from early July to late November 2009 and the second from late March to late November 2010. As well as measuring the VOC exchanges by eddy covari-ance, the climate parameters controlling the exchange mechanisms were also measured. During both these periods the methanol, acetaldehyde, acetone, isoprene, methyl vinyl ke-tone/methacrolein, monoterpene, acetic acid (2010) and formic acid (2010) fluxes were meas-ured. The highest emission levels observed were isoprene and monoterpenes along with methanol, which unlike the first two also showed depositions. The thesis therefore naturally focused on studying these three fluxes, in view of the important role played by these three compounds in atmospheric chemistry and hence the scientific community’s interest in refining the parametrisation of these compounds’ ecosystem/atmosphere exchange models. The study of the isoprene, monoterpene and methanol fluxes has been written up in three original articles which form the main body of this thesis. Because of the heterogeneity of the ecosystem studied, the first essential study concerned the identification of VOC-emitting species. This was done with the aid of a flux footprint model combined with a map of the species occurring on the site. This analysis showed that the main monoterpene emitter was Fagus sylvatica followed, to a lesser extent, by Abies alba, Picea Abies and Pseudotsuga menziessi. In contrast to the literature, the analysis showed Abies alba to be a probable isoprene emitter but the presence of Picea Abies, a known isoprene emitter, ruled out absolute certainty on that point. The isoprene fluxes were observed by day only, unlike the monoterpene fluxes which were observed both day and night. Diurnal flux analysis clearly showed temperature and light to be the two main variables controlling emissions. Combining this analysis with a study of the close relationship between isoprene/monoterpene emissions and photosynthesis revealed the plants’ de novo biosynthetic production mechanisms, an original aspect at ecosystem scale. From the occurrence of nocturnal monoterpene emissions it was possible to determine that de novo monoterpene production emitted directly into the atmosphere (as in the case of isoprene) was not the only source of the emissions observed. Withdrawals from monoterpene sinks located in plant organs or in the soil can also be monoterpene sources. Studying the relationship between isoprene/monoterpene fluxes and light, distinguishing between cloudy and sunny conditions, showed that for the same light intensity the emissions were higher in cloudy con-ditions than in sunshine. Similarly, a study of the relationship between isoprene fluxes and photosynthesis in cloudy/sunny conditions suggested that de novo isoprene production is greater in leaves above the canopy than in leaves within the canopy. Long-term measurement of isoprene and monoterpene emissions enabled seasonal changes in the mechanisms observed to be studied and more fully understood. As well as providing an understanding of the mechanisms, this research also resulted in quantification of the seasonal changes in the key parameters for modelling isoprene/monoterpene emissions. Methanol exchanges were generally positive (emissions) by day and negative (depositions) at night. Overall, methanol depositions were predominant in summer and autumn but in the mi-nority in spring. On average, the Vielsalm site behaved like a methanol sink, which contradicts all the other research published to date. An original model was developed for identifying the mechanisms responsible for short-term and long-term methanol emissions/depositions. The consistency between the measurements and the model simulations suggested that the main processes controlling methanol exchanges in summer could be attributed, in the short term, to (water-soluble) methanol adsorption/desorption occurring in the films of water on leaf surfaces and/or on the soil surface and, in the long term, to methanol destruction by a biological and/or chemical degradation process also occurring on the surface of leaves and/or the soil. A study of the difference between the measurements and the model, in spring, indicated the possibility of biosynthetic methanol production by the plants. This production was apparently controlled mainly by temperature, but it could not be shown in summer when methanol adsorption/desorption processes dominated. The literature on ecosystem-atmosphere exchanges of isoprene, monoterpenes and, to a lesser extent, methanol is extensive. Nevertheless, what makes this research original is the spatio-temporal scale used. We are in fact working at ecosystem scale, and not at leaf or branch scale as in most other cases. Moreover, our measurements cover a timescale from half an hour to a full growing season, which is rarely found in the literature. This has resulted in a better under-standing of these compounds’ production and exchange mechanisms. To be precise, the methanol flux study is currently unique in its description and understanding of the deposition mechanisms

Seasonal variation of LAI in the footprint of a flux measurement tower

C

Characterisation of light-absorbing atmospheric particles in the Brussels sub-urban atmosphere

The particle composition is important for air quality studies in the urban atmosphere. In particular, particles smaller than 1 μm are increasingly in the focus with respect to human health because of they are inhalable deeply into the human lung. A relevant part of such (ultra-)fine particles are light-absorbing particles. Important sources for such particles in the cities and residential areas are emissions from traffic (mainly soot), but also emissions from wood-burning stoves in private households. The relative contributions of these sources to the atmospheric particle load are important to know in order to be able to reduce hazardous emissions by specific measures. The Royal Meteorological Institute of Belgium (RMI) gathers ambient aerosol data in Brussels with a 7-wavelengths aethalometer (mass concentration and absorption coefficient of light-absorbing aerosol). The measurement site is located in the sub-urban, rather residential southern part of Brussels and measurements are representative for the urban background. At the same site, the boundary layer height and atmospheric stability parameter are derived from a co-located ceilometer and eddy-covariance system. Further, RMI uses the multi-scale chemical transport model CHIMERE, coupled to the high resolution regional numerical weather prediction limited area model ALARO. In this context, CHIMERE will be used to evaluate emission scenarios. The wavelength dependency of the measured aerosol parameters revealed distinct variations of aerosol composition, both on a daily, weekly, and seasonal scale. The Absorption Angstrom Exponent (AAE; spectral dependency of the absorption coefficient) revealed values around 1.3 during winter and around 1.0 during summer months. As fresh soot has a flat spectrum, (i.e., AAE around 1.0), higher AAE values during winter meant that other light-absorbing compounds, which absorb stronger in the UV, increased in importance (e.g., wood burning aerosol). The multi-wavelengths data is exploited to derive the relative contribution of fresh soot (thus traffic emissions) and other sources to the amount of light-absorbing aerosol. The influence of boundary layer height and atmospheric stability on the aerosol data will be presented

Flux de Composés Organiques Volatils (COV) au-dessus d'une forêt belge: De la mesure par eddy covariance à la recherche des mécanismes d'émission et de déposition.

Les écosystèmes forestiers à l'échelle globale sont les principaux émetteurs de COV (Composés Organiques Volatils). La quantité de COV émise et leur haute réactivité avec les principaux oxydants de l'atmosphère contribuent de manière non négligeable à la chimie de l'atmosphère (ozone). Dans le cadre d'un projet fédéral belge, des mesures continues de flux de COV par eddy-covariance au moyen d'un PTR-MS sont réalisées sur le site forestier de Vielsalm. Une première campagne de mesures a été effectuée pendant la période Juillet-Octobre 2009 et une seconde campagne a débutée au mois de Mars 2010. Les émissions les plus importantes observées ont été celles de l'isoprène, des monoterpènes et du méthanol. Des dépositions importantes de méthanol ont également été observées. L'exposé présentera ces mesures et leur exploitation avec pour objectif principal d'étudier la variabilité saisonnière des flux de COVs en relation avec leurs principales variables de contrôle. Les mécanismes potentiellement responsables de cette variabilité seront discutés.IMPECVOC: Impact of Phenology and Environmental Conditions on BVOC Emissions from Forest Ecosystem

Etude des flux de CO2 échangés par une jeune forêt mixte dans la région de Jalhay au lieu-dit ''La Robinette''.

Ce travail a été réalisé au Laboratoire de Physique Atmosphérique et Planétaire (LPAP) de l'université de Liège (ULg). La mise au point du traitement des données de covariance de turbulences ainsi que l'étude des flux de CO2 du site de La Robinette (Jalhay, Belgique) sont les buts de ce mémoire. En raison du rôle important de la biosphère au sein du cycle global du carbone, il est essentiel d'étudier les échanges de CO2 à travers l'interface entre l'atmosphère et les écosystèmes terrestres. Le site de La Robinette est une ancienne pessière mise à blanc en 1996. Par la suite, diverses essences forestières à dominance de feuillus caducifoliés tempérés ont été replantées sur le site. La technique de la covariance de turbulences est la technique micrométéorologique la plus directe permettant notamment de mesurer des flux verticaux turbulents de CO2 et de vapeur d'eau. Cette technique est valide si différentes corrections sont appliquées sur les flux turbulents. Les plus importantes sont la correction du décalage temporel, la correction de fréquence, la rotation des coordonnées et la correction des fluctuations de densité de l'air. De plus, des erreurs dans la mesure du flux peuvent survenir pendant des périodes de faibles turbulences où des phénomènes d'advection et de stockage peuvent être importants par rapport aux flux turbulents verticaux mesurés. Afin d'éviter d'éventuelles erreurs instrumentales, une analyse statistique est effectuée sur les données. Un test de stationnarité et de similarité est également appliqué aux données afin de vérifier la validité des mesures. Dans ce travail, nous avons déterminé une fonction de transfert du CO2 et de la vapeur d'eau nécessaire à la correction de fréquence des flux turbulents verticaux ainsi qu'une vitesse de friction limite. Nous avons pu mettre en évidence la dépendance du décalage temporel de la vapeur d'eau vis-à-vis de l'humidité relative. De plus, nous observons que le logiciel EDDYFLUX utilisé pour calculer les flux turbulents sous-estime le décalage temporel de la vapeur d'eau lorsque l'humidité relative est supérieure à 85%. La précision de l'anémomètre sonique nous a permis de mettre en évidence sur le site de La Robinette la présence de vents locaux. Ces vents sont liés à la vallée de La Soor qui se situe à proximité du site. En conditions de sécheresse, nous avons pu mettre en évidence un flux de vapeur d'eau à l'intérieur du sol. Il est dû à l'existence d'un gradient thermique entre la surface et l'intérieur du sol. Nous avons également étudié la réponse du flux de CO2 aux conditions météorologiques en dissociant le flux de CO2 diurne et nocturne

Use of the daily differencing approach to evaluate uncertainties affecting eddy covariance measurements

The eddy covariance technique is recognised to be the most adapted micrometeorological method to study the exchange processes between terrestrial ecosystems and the atmosphere. Like all other methods, it is submitted to systematic and random measurement errors. A thorough analysis of these errors is necessary in order to set the limits of validity of the method and to quantify the uncertainty that affects net carbon exchange computed with this method. In this presentation, we’ll concentrate on the random errors using the dailydifferencing approach (DDA) developed by Hollinger and Richardson (2005). The interest of this approach is that it requests only single tower measurements and is thus applicable to all flux tower sites. In this approach, uncertainties are estimated by comparing flux values taken at two successive days at the same hour and under similar meteorological conditions. The analysis may be applied to sensible heat, latent heat and CO2 flux densities. It was applied here to the eddy-covariance data from the Vielsalm mixed forest site (10 years of data) and from the Lonzée agricultural site (4 years of data). Both sites are situated in Belgium and are part of the Carboeurope IP network. The study is developed in the frame of the European IMECC project. The absolute and relative random error was quantified for both sites. Their daily evolution and their dependencies on different climate conditions (magnitude of the flux, PPFD, Rnet, wind velocity, wind direction, clarity index) were analysed. For both sites, the absolute random error increases linearly with the absolute value of flux. This is the principal factor controlling the random error. More particularly the response of the random error to wind velocity was analysed. For the CO2 flux, the absolute random error decreases with increasing wind speed. This effect is more important for the agricultural site than for the forest site. The behaviour of the relative random error with wind speed is more contrasted: it generally decreases with increasing wind speed at low velocities but, for some directions may increase with wind speed at high velocities. In addition, the random error was found very dependent on wind direction at the forest site probably as a result of site heterogeneity.IMECC: Infrastructure for Measurements of the European Carbon Cycl

Use of the daily differencing approach (DDA) to evaluate the impact of EC computation procedures.

IMECC: Infrastructure for Measurements of the European Carbon Cycl

Use of the daily-differencing approach to evaluate uncertainties in eddy covariance measurements

IMECC (Infrastructure for Measurements of the European Carbon Cycle