University, Diss., 2004--Tübingen.

University, Diss., 2004--Osnabrück.

Universiẗat, Diss., 2003--Bremen.

Clouds play an important role in precipitation, solar radiation budget and electrification of Earth's atmosphere. The presence of small ice crystals in clouds and their morphology can complicate parametrisation and climate modelling, consequently leading to a net cooling feedback on climate. In situ airborne measurements provide single particle characterisation with high temporal and spatial resolution allowing better understanding of atmosphericprocesses of ice nucleation and growth. Simulations of the preindustrial clouds and accurate characterisation and comparison of the instruments require a well-controlled and often pristine environment. The experimental chamber setup allows simulations of these and other conditions. The microphysical features of the micrometric ice particles in clouds were examined in a laboratory setup, at numerous sub-zero temperatures [-10 to -50 ⁰C]. The following instruments were sampling the content of the CLOUD chamber air volume: Cloud and Aerosol Spectrometer with Polarisation (CASPOL), Particle Phase Discriminator mark 2 (PPD-2K, Karlsruhe edition), 3-View Cloud Particle Imager (3V-CPI), and the Scattering-Intensity-Measurements-for-the-Optical-detectioN-of-icE (SIMONE-Junior). Cluster analysis was applied to the data collected with CASPOL and compared with results from the other probes. We were able to discriminate and map the aerosol and cloud particles in the pristine chamber environment using polarisation ratios (Dpol/Backscatter and Dpol/Forwardscatter) of the scattered light. We demonstrate the sensitivity of the instruments in detecting secondary organic aerosol (SOA) phase transitions. Then, we show the ability of the viscous SOA to nucleate ice in a series of SPectrometer for Ice Nuclei (SPIN) measurements. The detected viscous SOA ice nucleation efficiency may affect global modelling and estimations of ice water content in the atmosphere. Subsequently, the analysis and discrimination technique used in the CLOUD chamber was applied to airborne measurements to test its efficiency and to retrieve the composition of clouds. Data from four flight campaigns on board of the FAAM BAe-146 were analysed: Aerosol-Cloud Coupling and Climate Interactions in the Arctic (ACCACIA), COnvective-Precipitation-Experiment (COPE) in south England, CIRrus Coupled Cloud-Radiation EXperiment (CIRCCREX), and PIKNMIX in Scotland. In these and other flights, we were able to identify unique clusters such as salts, minerals, organics, volcanic ash, water and ice, confirming some of the offline laboratory elemental analysis results, and providing complementary information. Single particle polarisation measurements were compared with bulk depolarisation, diffraction patterns, and imaging. Most of the optical instruments still suffer from ambiguity in phase derivation (i.e. water/ice) of optically spherical small shapes. We discuss some of the limitations of optical cloud particle discrimination in different ambient conditions and offer possible solutions to reduce the uncertainty, e.g., surface complexity derivation from scatteringpatterns. Our findings will help to develop better instruments and improve the models which are used for weather forecasts and climate change predictions.

Accurate representations of clouds are required in large-scale weather and climate models to make detailed and precise predictions of the Earth's weather and climate. Representations of clouds within these models are limited by the present understanding of the role of aerosols in the microphysical processes responsible for cloud formation and development. As part of a NERC funded CASE studentship with the Met Office, this thesis aims to test new aerosol-dependent mixed-phase cloud parametrizations by obtaining extensive cloud microphysical measurements in-situ and comparing and contrasting them with model simulations. Cloud particle concentrations were measured during the Ice NUcleation Process Investigation And Quantification (INUPIAQ) field campaign at Jungfraujoch in Switzerland. A new probe was used to separate droplet and small ice concentrations by using depolarisation ratio and size thresholds. Whilst the new small ice crystal and droplet number concentrations compared favourably with other instruments, the size and depolarisation ratio thresholds were found to be subjective, and suggested to vary from cloud to cloud. An upwind site was chosen to measure out-of-cloud aerosol particle concentrations during INUPIAQ. During periods where the site was out-of-cloud and upwind of Jungfraujoch, several large-scale model simulations were run using the aerosol concentrations in an aerosol-dependent ice nucleation parametrization. The inclusion of the parametrization failed to increase the simulated ice crystal number concentrations, which were several orders of magnitude below those observed in-situ at Jungfraujoch. Several possible explanations for the high observed ice crystal number concentrations at Jungfraujoch are tested using further model simulations. Further primary ice nucleation was ruled out, as the inclusion of additional ice nucleating particles in the model simulations suppressed the liquid water content, preventing the simulation of the mixed-phase clouds observed during INUPIAQ. The addition of ice crystals produced via the Hallett-Mossop process upwind of Jungfraujoch into the model only infrequently provided enough ice crystals to match the observed concentrations. The inclusion of a simple surface flux of hoar crystals into the model simulations was found to produce ice crystal number concentrations of a similar magnitude to those observed at Jungfraujoch, without depleting the simulated liquid water content. By confronting models with in-situ observations of cloud microphysical process, this thesis highlights interactions between surface ice crystals and mixed-phase clouds, and their potential impact on large-scale models.

L’objectif de ce travail est d’approfondir les connaissances sur les phénomènes mis en jeu lors de la combustion de carburant liquide dispersé sous forme d’aérosol. De nombreux propulseurs (moteurs de fusée, turbines à gaz, moteurs à combustion interne..) reposent sur la combustion de combustibles initialement sous forme liquide. Or, la combustion diphasique est un phénomène très complexe faisant intervenir de nombreux processus: atomisation, interaction entre gouttelettes, vaporisation, écoulement diphasique, cinétique chimique, propagation de flamme. Tous ces phénomènes étant couplés, une description complète est seulement possible par le biais de simulations numériques ; mais les investigations expérimentales sont nécessaires pour fournir des données aussi quantitatives que possible dans des configurations simples afin de proposer ou de confirmer des modèles prenant en compte ces couplages. Ainsi, des études théoriques, expérimentales et numériques, doivent être menées en parallèle pour accroître les connaissances dans le domaine de la combustion diphasique et améliorer le fonctionnement des applications actuelles. Ce travail a été initié dans le cadre du Groupement de Recherche « Micropesanteur Fondamentale et Appliquée » du CNRS et du CNES, en 2008. Il est la suite des précédents travaux sur la vaporisation et la combustion des gouttes en microgravité conduites à ICARE et soutenus par le CNES durant de longues années. Cette nouvelle étude a essentiellement porté sur la détermination expérimentale des vitesses de propagation des flammes dans un aérosol (ou brouillard). Elle s’est appuyée sur l'utilisation d’une chambre de combustion haute pression développée à ICARE. Les expériences ont été principalement conduites sous des conditions de gravité réduite, pour éviter les problèmes de sédimentation de la phase dispersée. Cet appareillage a été conçu et élaboré pour être utilisé à bord de l’Airbus A300-0g du CNES. / Spray and aerosol cloud combustion accounts for 25% of the world’s energy use, and yet it remains poorly understood from both a fundamental and a practical perspective. Realistic sprays have a liquid breakup region, a dispersed multiphase flow, turbulent mixing processes, and various levels of flame interactions through the spray. Idealization of spray configurations in a quiescent environment (the starting point for models) has been impossible in 1 g due to the settling of large droplets and the buoyant pluming of post combustion gases. Testing in microgravity conditions relates to the possibility of creating aerosols without sedimentation effects. This research was to determine experimentally the flame propagation velocity in aerosols. First, the size of droplets in the aerosol was characterized using a laser diffraction particle size analyzer “Sympatec-HELOS”, and using ethanol as fuel. Second, high-Pressure combustion studies were performed using a high-Pressure combustion chamber (max pressure 12 MPa). These pieces of equipment were designed to be used in microgravity while aboard the Airbus A300-0g of the CNES. After ground tests, five parabolic flight campaigns were conducted. A systematic comparative analysis for identical initial conditions in both normal and reduced gravity was performed. The effects of initial temperature and pressure on the droplet diameter distribution of the aerosol, the effects of gravity on the flame behavior for both vapor-Air and droplet-Vapor-Air mixtures, and the effect of drops size on the flame speed and structure were all studied.

In this thesis, we assess various climatic impacts of stratospheric aerosol injection (SAI), a geoengineering proposal that aims to cool Earth by enhancing the sunlight-reflecting aerosol layer in the lower stratosphere. To this end, we employ simpleradiative transfer models, a detailed radiative transfer code (SOCRATES), and two Hadley Centre general circulation models (HadGEM2-CCS and HadGEM2-ES). We find that the use of a light-absorbing aerosol (black carbon) for SAI would result in significant stratospheric warming and an unprecedented weakening of the hydrological cycle. Conversely, we find that SAI with sulphate or titania aerosol could counteract many of the extreme climate changes exhibited by a business-as-usual scenario (RCP8.5) by the end of this century. In a separate investigation, we show that volcanic aerosol dispersion following low-altitude volcanic eruptions can exhibit high sensitivity to the ambient weather state. Volcanic aerosol may get 'trapped' in a single hemisphere or transported to the opposite hemisphere depending simply on the meteorological conditions on the day of the eruption. In a final study, we investigate the impacts of SAI on North Atlantic tropical storm frequency. We find that SAI exclusively promoted in the southern hemisphere would increase North Atlantic storm frequency, and vice versa for northern hemisphere SAI. The results of this thesis should promote further research into SAI, which could conceivably be deployed to maintain global-mean temperature below the COP21 target of +1.5 K above pre-industrial levels, whilst society transitions onto a sustainable energy pathway. Conversely, the possibility of SAI being weaponised, for instance, to specifically increase North Atlantic tropical storm frequency, should motivate policymakers to implement effective regulation and governance to deter unilateral SAI deployments.

Atmospheric particles play a crucial role in climate, visibility, air pollution, and human health. Reactions between gas-phase molecules and particles (heterogeneous reactions) affect not only the particle composition and morphology, but also the composition of the atmosphere. This thesis investigates the heterogeneous chemistry of organic mixtures and inorganic solutions coated with organic monolayers as proxies for atmospheric particles. The first topic of interest was the reaction between N₂0₅ and aqueous inorganic solutions coated with organic monolayers. The goal of this work was to better understand how organic monolayers on aqueous particles affect the mass transport and kinetics of N₂0₅ uptake by aqueous aerosols, and consequently what effectthe monolayer can have on predicted concentrations of N₂0₅ in the atmosphere. To investigate heterogeneous reactions of inorganic solutions coated with an organic monolayer a new rectangular channel flow reactor was developed. This newly developed flow reactor was described in detail and validated. Subsequently, the new flow reactor was used to study the reactive uptake of N₂0₅ on sulfuric acid solutions in the presence of a variety of 1- and 2-component monolayers with varying functional groups, solubilities, chain lengths, surface pressures, and molecular surface areas. Reactive uptake of N₂0₅ on aqueous sulfuric acid solutions was found to correlate most strongly with the molecular surface area or packing density of the monolayer. These results provide a good foundation for determining the influence of monolayers on heterogeneous reactions in the atmosphere, and highlight the need for characterization of monolayer surface properties of organic monolayers present on atmospheric particles. The second topic of interest was reactions between 0₃ and proxies for meat cooking aerosols with the goal to better understand the effect of the phase and microstructure of the mixtures on the lifetime of oleic acid (OA) in atmospheric particles. The reactive uptake of 0₃ was approximately 1 order of magnitude slower on binarysolid-liquid mixtures and multicomponent mixtures that closely represent compositions of meat-cooking aerosols compared to the liquid solutions. Lifetimes up to 75 min were obtained for these mixtures. / Science, Faculty of / Chemistry, Department of / Graduate

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