The depletion of fossil fuel resources, pollution concerns and the challenge of energy security are driving the search for renewable energy sources. The use of lignocellulosic plant biomass as an energy source is increasing in the United Kingdom and worldwide. In the UK, up to 0.35MHa (6% of total arable land) may be planted with perennial bioenergy crops by 2020 in order to meet renewable energy and CO2 reduction targets. Several plant species that produce high biomass from low inputs have been identified. The most promising for the UK climate are the genus Miscanthus, a perennial rhizomatous grass which can grow up to 3.5m in a year, and short rotation coppice (SRC) willow (Salix spp.), plantations of which can remain viable for up to 30 years. Although bioenergy crops are perceived as “carbon neutral”, changes in land use can have a wider impact on atmospheric composition than through CO2 alone. This study compares vegetation fluxes of methyl halides (CH3Br and CH3Cl) and biogenic volatile organic compounds (BVOCs) from perennial bioenergy crops and annual arable crops at three sites in the UK. Methyl halides are the most abundant natural vectors of bromine and chlorine into the stratosphere and play an important role in stratospheric ozone destruction. BVOCs affect atmospheric oxidising capacity and are a major precursor to the formation of ozone and secondary organic aerosols in the troposphere. Although terrestrial vegetation is an important source of these trace gases there are very few previous measurements of these reactive gases from bioenergy crops. This study describes measurements conducted at two SRC willow sites in Scotland, and one site in England planted with adjacent perennial bioenergy crops and annual arable crops, to quantify and characterise natural methyl halide and BVOC fluxes from vegetation. Measurements were conducted with branch chambers, using static enclosure techniques to measure methyl halide fluxes and dynamic enclosures to measure BVOCs such as isoprene and -pinene. Fluxes were calculated from the concentration difference between background/inlet samples and after enclosure/outlet samples. Methyl halide concentrations were determined by sampling gas from static enclosures followed by analysis using an oxygen-doped GC-ECD with a custom-built pre-concentration unit. Samples for BVOC analysis were collected onto adsorbent tubes and a thermal desorption GC-MS was used to determine BVOC concentrations. Potentially influential environmental variables such as photosynthetically active radiation (PAR), total solar radiation, air temperature, soil temperature, internal chamber temperature and soil moisture were recorded in parallel to the enclosures to determine their potential relationships to fluxes. Long-term environmental data was also available from on-site or nearby weather stations. Long-term measurements were carried out for 2 1 2 years at a site in Lincolnshire, England where adjacent fields are planted with Miscanthus, SRC willow and annual arable crops (wheat and oilseed rape crop rotation). Vegetation measurements were made almost every month throughout the period, with more intensive measurements such as full diurnal cycle carried out during the summer. Ten sampling points are sampled in each field and semi-diurnal measurements are taken regularly. Long-term measurements were also carried out at two sites in Scotland planted with SRC willow, one in Arnot, Perth & Kinross which was sampled for a year and one in East Grange, Fife which was sampled for half a year. Up to 30 sampling points were employed in Arnot and another 10 in East Grange. The bioenergy crops and wheat emitted low fluxes of methyl halides in comparison to the oilseed rape. Mean annual net fluxes of CH3Br and CH3Cl from Miscanthus were 1.8 ng g (dry weight)-1 h-1 and 11 ng g-1 h-1, respectively. At the three willow sites, mean annual net fluxes of CH3Br and CH3Cl ranged between 0.6 - 1.7 ng g-1 h-1, and 1.7 - 12 ng g-1 h-1, respectively. Negligible fluxes of methyl halides were measured from wheat but oilseed rape was found to emit large fluxes of methyl halides with mean annual net fluxes of CH3Br and CH3Cl of 20 ng g-1 h-1and 144 ng g-1 h-1, respectively. The largest BVOC fluxes were measured from SRC willow at the Brattleby site, with high mean annual net fluxes of isoprene (77 μ g g-1 h-1), α -pinene (46 g μg-1 h-1), β -pinene (5.5 μ g g-1 h-1), limonene (3.7 μ g g-1 h-1) and δ -3-carene (11 μ g g-1 h-1). However, mean annual net BVOC fluxes measured at Arnot and East Grange were much lower. High fluxes of - pinene were measured from Miscanthus in 2010 (giving a mean annual net flux of 65 μ g g-1 h-1) but no other significant BVOC fluxes were distinguished. Negligible fluxes of isoprene were measured from wheat but fluxes of monoterpenes were high with mean annual net fluxes of 65 μ g g-1 h-1 and 25 μg g-1 h-1 for α -pinene and limonene, respectively. No significant fluxes of BVOCs were measured from the oilseed rape. All fluxes demonstrated a strong seasonal trend with higher emissions during the summer growing season and low to zero emissions over winter. The high spatial variability was captured by sampling from many points in each field. Some diurnal measurements exhibited a clear pattern of higher emissions during the day and low to zero emissions at night. Some positive correlations between fluxes and environmental variables such as PAR and air temperature were observed. An experiment carried out on willow cuttings in the greenhouse found no clear increase or decrease in fluxes of BVOCs in response to N fertiliser treatment, instead fluxes were found to vary significantly with some of the observed environmental variables. UK estimates were derived by extrapolating measured fluxes using the current respective land cover areas for Miscanthus and SRC willow. Estimated UK annual fluxes of CH3Br and CH3Cl from Miscanthus were 0.01Mgy-1 and 0.05Mgy-1, respectively and from SRC willow were 0.06Mgy-1 and 0.4Mgy-1, respectively, accounting for a very small percentage of total global annual turnover of CH3Br and CH3Cl. Estimated UK annual fluxes of the BVOCs isoprene and α -pinene from Miscanthus were 0.01Mgy-1 and 0.4Mgy-1, respectively and from SRC willow were 3Mgy-1 and 1Mgy-1, respectively. Future planting of SRC willow to meet the UKs energy needs could lead to a potential annual isoprene flux of 150 MgMgy-1.

Biogenic volatile organic compounds (BVOC) are a wide-ranging group of trace gas components in the atmosphere which are emitted naturally from Earth’s surface. It is now recognised that biogenically sourced VOCs are far more significant on a global scale than those from anthropogenic sources, with up to 10 times greater emissions. Very few field-based studies of fluxes from plant canopies have been undertaken, particularly for non-terpenoid compounds. This thesis presents mixing ratio and flux measurements of BVOC from a range of temperate plant canopies: Douglas fir, short-rotation coppice willow, Miscanthus and mixed peatland vegetation. The virtual disjunct eddy covariance technique (vDEC) using a proton transfer reaction mass spectrometer (PTR-MS) as a fast VOC sensor was used for all measurements except for peatlands, where grab samples were collected on adsorbent sampling tubes for later chromatographic analysis. The PTR-MS was also utilised for measuring the rate of degradation of VOCs during laboratory chamber experiments. Mixing ratios and fluxes of VOCs measured within and above a Douglas fir forest were the first canopy-scale measurements for this species. Fluxes of monoterpenes were comparable to previous studies while isoprene was also detected (standard emissions factors up to 1.15 μg gdw -1 h-1 and 0.18 μg gdw -1 h-1, respectively). Emissions of oxygenated VOCs were also found to be significant, highlighting the importance of quantifying a wider variety of VOCs from biogenic sources, other than isoprene and monoterpenes. Results for bioenergy crops Miscanthus and willow showed that willow was a high isoprene emitter (20 μg gdw -1 h-1), but no measureable VOCs were detected from Miscanthus. This indicates that future expansion of bioenergy crops, and hence species selection, should take resultant air quality and human health impacts – due to changing VOC emissions – into account. Fluxes of BVOC from a Scottish peatland are the first reported measurements for this ecosystem in a temperate climate. Additionally, to assess the impact of nitrogen deposition on VOC fluxes, BVOC measurements were taken from sample plots in a pre-existing, long-term field manipulation study to assess impacts of wet nitrate or ammonium deposition on peatland. The peatland was found to be a significant source of isoprene and monoterpenes (590 and 1.5 μg m-2 h-1 respectively) and there was evidence that emissions were affected by wet nitrogen treatment. Isoprene emissions were reduced by both nitrate and ammonium treatment, while nitrate increased β- pinene fluxes. Increasing atmospheric nitrogen concentrations are therefore predicted to have an impact on VOC emission. Chamber studies showed that the rate of loss of α-pinene from the gas-phase during oxidation – and hence potential formation of secondary organic aerosol (SOA) – decreased with increasing isoprene mixing ratio. This was not observed for limonene. These results show that as isoprene mixing ratios increase with increasing global temperatures, negative feedback on radiative forcing from SOA particles may be suppressed. Results from this thesis provide valuable experimental data for a range of temperate plant canopies, which will help constrain modelled predictions of future VOC emissions. Additionally, the importance of understanding the effects of land use and environmental change on VOC emissions was demonstrated.

Recent research has shown that secondary organic aerosols (SOA) are major contributors to ultrafine particle growth to climatically relevant sizes, increasing global cloud condensation nuclei (CCN) concentrations within the continental boundary layer (BL). This thesis contains two separate studies investigating SOA characteristics and the implications of SOA on global climate. The first study investigates two critical, but uncertain, characteristics of SOA: (1) the amount of SOA available to condense and (2) the volatility or condensational behavior of SOA. The second study investigates the effect of biological volatile organic compound (BVOC) emission changes on SOA formation from preindustrial to present day, and the effect on CCN concentrations using BVOC emission estimates over the last millennium.

Les Composés Organiques Volatils biogènes (COVB) sont des molécules issues du métabolisme secondaire des végétaux, dont l'émission peut être modulée par les conditions environnementales. Parmi ces composés, l'isoprène a été très étudié du fait des flux d'émission important et de son implication dans la photochimie troposphérique. Cependant, les mécanismes d'action des facteurs environnementaux sont encore mal connus, et notamment celui de l'impact du stress hydrique. Dans le contexte de changements climatiques, ce type de stress va particulièrement impacter la région méditerranéenne.Nous avons étudié l'impact du stress hydrique sur les émissions d'isoprène de Quercus pubescens Willd. Cette espèce, très présente dans cette région, serait la seconde source d'isoprène en Europe.Deux étude ont été menées.La première, effectuée en pépinière, a consisté à appliquer un stress hydrique modéré et sévère d'avril à octobre. Une augmentation des émissions d'isoprène des arbres modérément stressés a été observée alors qu'il n'y a eu aucune modification des émissions pour les arbres très stressés.La seconde a consisté à faire un suivi saisonnier du stress hydrique au sein d'une chênaie pubescente. Un stress hydrique amplifié a été appliqué par un système d'exclusion de pluie, permettant de diminuer la quantité de pluie de 30%. Nous avons observé que le stress hydrique amplifié augmentait les facteurs d'émission d'isoprène des arbres.Cette base de données a permis le développement, par Réseau de Neurones Artificiels (RNA), d'un algorithme d'émission d'isoprène. Nous avons ainsi mis en évidence l'impact prédominant du contenu en eau du sol sur les émissions d'isoprène. / Biogenic Volatile Organic Compounds (BVOC) are plants secondary-metabolism-molecules. Their emissions are modulated by environmental conditions. Among these compounds, isoprene has been particularly studied due to its intense emission fluxes as well as its major contribution to tropospheric photochemistry. However, the impacts of environmental constraints on isoprene emission are still not yet well known. In particular, water stress impact is still a contradictory issue. In a world facing multiple climatic changes, models expect this kind of stress to hit Mediterranean area.This work focused on the impact of water stress on Quercus pubescens Willd. isoprene emissions. This species, widely spread in this area, is the second isoprene emitter in Europe.Two types of study were used.First, during an experimental carried out in a nursery, Q. pubescens saplings were grown under a moderate and severe water stress from April to October. This experimentation highlighted an increase of isoprene emissions for mid-stressed trees, while no emission changes were observed for the highly stressed trees.Secondly, an experimentation was conducted on a pubescent oak forest with trees acclimated to long lasting stress periods. We followed, during a whole season, the impact, on isoprene emissions, of a water stress created by artificially reducing 30% of the rains by means of a specific deploying roof. Isoprene emission factors were observed to increase under water stress.The database thus obtained was used in an Artificial Neural Network (ANN) to develop an appropriate isoprene emission algorithm. We underlined the predominant impact of soil water content on isoprene emissions.

Les Composés Organiques Volatils d’origine Biogénique (COVB) émis par la végétation représentent 1PgC.an-1 à l’échelle globale. Ces COVB, une fois émis dans l’atmosphère, peuvent participer à la formation d’ozone troposphérique ainsi qu’à la formation d’aérosols organiques secondaires et donc à la pollution atmosphérique. C’est pourquoi, il est important de quantifier le plus précisément possible les taux d’émissions de COVB et de mieux comprendre quels sont les facteurs environnementaux qui contrôlent ces émissions. Il est bien connu que les émissions de COVB sont contrôlées par la lumière et/ou la température mais elles peuvent également être influencées par d’autres facteurs comme le stress hydrique, bien que son impact soit encore mal compris. En effet, il a été montré que le déficit hydrique pouvait augmenter ou diminuer les émissions de COVB selon son intensité, sa durée et l’espèce étudiée. Dans le cadre du changement climatique, une intensification de la sécheresse est attendue en région Méditerranéenne avec une augmentation de la température, une diminution des pluies ainsi qu’une prolongation de la période de sécheresse. Ce changement climatique pourrait donc modifier les émissions de COVB. De plus, les effets d’une sécheresse appliquée sur plusieurs années sont encore mal connus. Dans cette étude, nous nous sommes intéressés à la réponse des émissions de COVB du chêne pubescent (Quercus pubcescens Willd.)face au stress hydrique attendu en région méditerrannéenne avec le changement climatique. / Biogenic Volatile Organic Compounds (BVOC) emitted by vegetation represent 1PgC.yr-1 at the global scale. These BVOC, once emitted into the atmosphere, can participate in the troposheric ozone formation as well as secondary oragnic aerosols and, consequently, on the atmospheric pollution. That’s why, it is very important to quantify, as accurately as possible, the BVOC emissions and to improbe the knowledge about the environmental factors which drive these emissions. It is well known that BVOC emissions are controlled by the light and the temperature but they can be impacted by other factors such as water stress. Nevertheless, these mechanisms are not well understood yet, since it has been shown that water stress can increase or decrease BVOC emissions according to the intensity and the duration of stress. In a context of climate change, we can expected an intensification of summer drought in Mediteranean area with an incerase of temperature, a decrease of rainfall as well as an elongation of stress period. This climate change could modify BVOC emissions. Moreover, the effects of a water stress applied during several years are not known. In this study, we wanted to evaluate the impact of water stress, expected with climate change, on BVOC emitted by Downy oak (Quercus pubescens Willd.), main isoprene emitter of Mediterranean region.