Tropospheric ozone columns (TCOs) derived from differences between the Dutch-Finnish Aura Ozone Monitoring Instrument (OMI) measurements of the total atmospheric ozone column and the Aura Microwave Limb Sounder (MLS) measurements of stratospheric ozone columns are discussed. Because the measurements by these two instruments are not spatially coincident, interpolation techniques, with emphasis on mapping the stratospheric columns in space and time using the relationships between lower stratospheric ozone and potential vorticity (PV) and geopotential heights (Z), are evaluated at mid-latitudes. It is shown that this PV mapping procedure produces somewhat better agreement in comparisons with ozonesonde measurements, particularly in winter, than does simple linear interpolation of the MLS stratospheric columns or the use of typical coincidence criteria at mid-latitudes. The OMI/MLS derived tropospheric columns are calculated to be 4 Dobson units (DU) smaller than the sonde measured columns at mid-latitudes. Standard deviations between the derived tropospheric columns and those measured by ozonesondes are 9 DU (30%) annually but they are just 6 DU (15%) in summer. Uncertainties in the interpolated MLS stratospheric columns are likely to be the primary cause of these standard deviations. An important advantage of the PV mapping approach is that it works well when MLS data are missing (e.g., when an orbit of measurements is missing). In the comparisons against ozonesonde measurements, it provides up to twice as many comparisons compared to the other techniques. The OMI/MLS derived tropospheric ozone columns have been compared with corresponding columns based on the Tropospheric Emission Spectrometer (TES) measurements, and Regional chEmical trAnsport Model (REAM) simulations. The variability of tropospheric ozone columns has been examined for spring and summer 2005 over North America and the surrounding oceans. Comparisons of monthly mean distributions show good agreements between OMI/MLS tropospheric ozone columns, REAM columns, and TES columns. Two six-day periods in March have been selected to study the periodic TCO enhancements in two regions, around the Baja peninsula (Mexico) and over the West Coast of California. Thirteen-day back trajectories and daily maps of carbon monoxide (CO) and ozone from GEOS-CHEM and OMI/MLS have been used to investigate the influence of cross-Pacific transport. It is concluded that in the first period of the case study, the high ozone concentrations in mid and lower troposphere over the West Coast of California have been under the influence of cross-Pacific transport. Meteorological fields indicate that the high ozone concentrations in the upper troposphere over the West Coast of California and the high TCOs over the Baja peninsula are associated with stratospheric intrusions through a deep Rossby wave breaking event. The correlations between REAM TCOs and surface ozone from Environmental Protection Agency ground network measurements indicate that the TCO enhancement over the West Coast is associated with an increase of surface ozone. The correlations of REAM TCOs with geopotential height, wind fields, and tropopause height during the case study period suggest that TCO enhancement is best characterized in springtime by decreases of geopotential height on the 500 mb surface.

This dissertation investigates the impact of bromine on tropospheric ozone, OH, and mercury in the preindustrial and present-day atmosphere through use of modeling and observations from satellite. We developed bromine simulation capabilities coupled to oxidant-aerosol chemistry in the GEOS-Chem chemical transport model (CTM). Standard gas-phase mechanisms for bromine chemistry were unable to reproduce recent estimates of tropospheric BrO from satellite. Agreement was improved significantly after imposing HBr+HOBr heterogeneous chemistry in the model. Under present-day conditions, we find that bromine decreases ozone by 6.5%, < 1 – 8 ppb, and global mean OH by 4%. Most ozone loss is due to HOBr production and photolysis, with additional contributions from \(NO_x\) and ozone loss through \(BrNO_3\) hydrolysis. Simulations of the pre-industrial atmosphere are important as baselines for ozone air quality and radiative forcing calculations. However, standard models for the pre-industrial overestimate ozone observations taken a century ago at Montsouris and cannot reproduce the observed aseasonality. We find that bromine chemistry significantly improves this agreement. However, bromine chemistry has negligible impact on the ozone radiative forcing, as concentrations of BrO remain similar. Despite the small change in BrO concentrations, lower ozone in the preindustrial leads to a 40% greater Br mixing ratios. We estimate that this change may have increased the lifetime of atmospheric Hg(0) against oxidation to Hg(II) by 70% since the pre-industrial, making atmospheric mercury a more global pollutant. Additionally, we develop a retrieval algorithm for stratospheric profiles of BrO number density from SCIAMACHY limb near-UV observations. Zonal means of our BrO profile retrievals throughout April 2008 show common features expected from stratospheric photochemistry and dynamics. We apply simulated \([BrO]/[Br_y]\) ratios to the BrO profile retrievals and estimate a stratospheric loading of \(23.5 \pm 6 ppt Br_y\). This supports the 23 ppt stratospheric \(Br_y\) assumed in the satellite-derived climatology of tropospheric BrO that we used to evaluate our GEOS-Chem simulation. Our results imply \(7 \pm 6 ppt\) Br from short-lived bromocarbons, at the higher end of the 3 – 8 ppt range suggested by observations. / Engineering and Applied Sciences

No description available.

No description available.

Thesis (Ph. D.)--University of Adelaide, 1986. / Includes bibliographical references (leaves 163-172).

Thesis (Ph. D.)--Earth and Atmospheric Sciences, Georgia Institute of Technology, 2005. / Russell, Armistead G., Committee Chair ; Chameides, William L., Committee Member ; Wang, Yuhang, Committee Member ; Noonan, Douglas, Committee Member ; Chang, Michael E., Committee Member. Vita. Includes bibliographical references.

Dans le domaine de l’Aviation Civile, les motivations de recherches sont souvent guidées par la volonté d’améliorer la capacité de l’espace aérien grâce à la modernisation des moyens de navigation aérienne existants et aux nouvelles infrastructures. Ces buts peuvent être atteints en développant les services qui permettent des opérations d’approche et d’atterrissage plus robustes et plus fiables. La navigation par satellite, grâce au Global Navigation Satellite System (GNSS), a été reconnue comme un moyen performant de fournir des services de navigation aérienne [1] [2]. Le concept du GNSS requiert l’utilisation de moyen d’augmentations pour fournir une fonction de contrôle d’intégrité au vu des exigences [1] relatives aux applications critiques de type aviation civile. Un de ces moyen est le GBAS (Ground Based Augmentation System) et est standardisé par l’OACI pour fournir un service de navigation incluant les approches de précision allant jusqu’à la catégorie I incluse, en utilisant les constellations GPS ou GLONASS [3]. Des études sont en cours pour permettre d’étendre ce service jusqu’à la catégorie II/III avec le GPS L1 C/A, cependant des contraintes sont apparues lors de la surveillance de la ionosphère. Grâce à la modernisation du GPS et GLONASS et aux futures constellations Galileo et Beidou, les futurs GNSS utilisant de multiples constellations et de multiples fréquences (MC/MF) sont étudiés. Les activités de recherches européennes se sont appuyées sur la constellation GPS et sur la future constellation Galileo. Ce MC/MF GBAS devrait permettre de nombreuses améliorations comme un meilleur modèle des retards atmosphériques. Cependant, des challenges doivent être résolus avant d’atteindre les bénéfices potentiels. Dans ce travail de thèse, 2 principaux sujets en rapport avec le GBAS ont été traités, la transmission des données de corrections avec le MC/MF GBAS et l’impact des biais troposphériques avec le SC/SF et MC/MF GBAS. Dû aux contraintes portant sur le format des messages transmis à l’utilisateur via l’unité VDB [4], une nouvelle approche est nécessaire pour permettre l’élaboration du MC/MF GBAS. Une des solutions proposée dans cette thèse est de transmettre les corrections et les données d’intégrité à l’utilisateur dans des messages séparés à des fréquences différentes. De plus, ce travail de thèse remet en question la modélisation de l’atmosphère et particulièrement celle de la troposphère dans des conditions nominales que non-nominales en se concentrant d’abord sur le calcul du pire gradient troposphérique avant de développer les précédents travaux pour borner cette menace dans le but de protéger l’utilisateur. En vue du futur MC/MF GBAS, une nouvelle approche s’est avérée nécessaire. Ainsi, dans ce projet de thèse, des modèles météorologiques numériques (NWMs) sont utilisés pour estimer intégralement la composante horizontale du pire gradient troposphérique. Une méthode innovante pour rechercher les pires gradients troposphériques horizontaux est utilisée pour déterminer les biais qu’ils induisent impactant les avions visant une approche de Cat II/III avec le GBAS. Un modèle de ces pires biais de mesures troposphériques différentiels horizontaux dépendant de l’élévation des satellites pour 2 régions européennes est alors développé. La composante verticale est aussi modélisée grâce à une étude statistique qui compare les données réelles au modèle standard. Un modèle du biais différentiel total non corrigé est développé et doit être introduit dans le calcul des niveaux de protections sous des conditions nominales. Pour borner l’impact de la troposphère sur l’erreur de position tout en se focalisant sur le souhait d’avoir un nombre de données transmises à l’utilisateur faible, différentes solutions conservatives ont été développées où au minimum 3 paramètres, définis selon leur région géographique d’utilisation, doivent être transmis à l’utilisateur. / In the Civil Aviation domain, research activities aim to improve airspace capacity and efficiency whilst meeting stringent safety targets. These goals are met by improving performance of existing services whilst also expanding the services provided through the development of new Navigation Aids. One such developmental axe is the provision of safer, more reliable approach and landing operations in all weather conditions. The Global Navigation Satellite System (GNSS) has been identified as a key technology in providing navigation services to civil aviation users [1] [2] thanks to its global coverage and accuracy. The GNSS concept includes the provision of an integrity monitoring function by an augmentation system to the core constellations. This is needed to meet the required performances which cannot be met by the stand-alone constellations. One of the three augmentation systems developed within civil aviation is the GBAS (Ground Based Augmentation System) and is currently standardized by the ICAO to provide precision approach navigation services down to Cat I using the GPS or GLONASS constellations [3]. Studies on-going with the objective to extend the GBAS concept to support Cat II/III precision approach operations with GPS L1 C/A, however some difficulties have arisen regarding ionospheric monitoring. With the deployment of Galileo and Beidou alongside the modernization of GPS and GLONASS, it is envisaged that the GNSS future will be multi-constellation (MC) and multi-frequency (MF). European research activities have focused on the use of GPS and Galileo. The MC/MF GBAS concept should lead to many improvements such as a better modelling of atmospheric effects but several challenges must be resolved before the potential benefits may be realized. Indeed, this PhD has addressed two key topics relating to GBAS, the provision of corrections data within the MC/MF GBAS concept and the impact of tropospheric biases on both the SC/SF and MC/MF GBAS concepts. Due to the tight constraints on GBAS ground to air communications link, the VDB unit, a novel approach is needed. One of the proposals discussed in the PhD project for an updated GBAS VDB message structure is to separate message types for corrections with different transmission rates. Then, this PhD argues that atmospheric modelling with regards to the troposphere has been neglected in light of the ionospheric monitoring difficulties and must be revisited for both nominal and anomalous scenarios. The thesis focuses on how to compute the worst case differential tropospheric delay offline in order to characterize the threat model before extending previous work on bounding this threat in order to protect the airborne GBAS user. In the scope of MC/MF GBAS development, an alternative approach was needed. Therefore, in this PhD project, Numerical Weather Models (NWMs) are used to assess fully the worst case horizontal component of the troposphere. An innovative worst case horizontal tropospheric gradient search methodology is used to determine the induced ranging biases impacting aircraft performing Cat II/III precision approaches with GBAS. This provides as an output a worst case bias as a function of elevation for two European regions.The vertical component is also modelled by statistical analysis by comparing the truth data to the GBAS standardized model for vertical tropospheric correction up to the height of the aircraft. A model of the total uncorrected differential bias is generated which must be incorporated within the nominal GBAS protection levels. In order to bound the impact of the troposphere on the positioning error and by maintaining the goal of low data transmission, different solutions have been developed which remain conservative by assuming that ranging biases conspire in the worst possible way. Through these techniques, it has been shown that a minimum of 3 parameters may be used to characterize a region’s model.

No description available.

This thesis aims to improve the understanding of transport and critical layer mixing in the troposphere and stratosphere. A dynamical approach is taken based on potential vorticity which has long been recognised as the essential field inducing the flow and thermodynamic structure of the atmosphere. Within the dynamical framework of critical layer mixing of potential vorticity, three main topics are addressed. First, an idealised model of critical layer mixing in the stratospheric surf zone is examined. The effect of the shear across the critical layer on the critical layer evolution itself is investigated. In particular it is found that at small shear barotropic instability occurs and the mixing efficiency of the critical layer increases due to the instability. The effect of finite deformation length is also considered which extends previous work. Secondly, the dynamical coupling between the stratosphere and troposphere is examined by considering the effect of direct perturbations to stratospheric potential vorticity on the evolution of midlatitude baroclinic instability. Both zonally symmetric and asymmetric perturbations to the stratospheric potential vorticity are considered, the former representative of a strong polar vortex, the latter representative of the stratospheric state following a major sudden warming. A comparison of these perturbations gives some insight into the possible influence of pre or post-sudden warming conditions on the tropospheric evolution. Finally, the influence of the stratospheric potential vorticity distribution on lateral mixing and transport into and out of the tropical pipe, the low latitude ascending branch of the Brewer-Dobson circulation, is investigated. The stratospheric potential vorticity distribution in the tropical stratosphere is found to have a clear pattern according to the phase of the quasi-biennial oscillation (QBO). The extent of the QBO influence is quantified, by analysing trajectories of Lagrangian particles using an online trajectory code recently implemented in the Met Office's Unified Model.

The study presented in this thesis was motivated by the large uncertainty on the concentration of atmospheric electrical discharges to the global nitrogen budget. This uncertainty is partly due to the fact that information concerning the NOx production efficiency of electrical discharges having current signatures similar to those of lightning flashes is not available in the literature. Another reason for this uncertainty is the fact that energy is used as a figure of merit in evaluating NOx production from lightning flashes even though insufficient knowledge is available concerning the energy dissipation in lightning flashes. The third reason for this uncertainty is the lack of knowledge concerning the contribution of discharge processes other than return strokes to the NOx production in the atmosphere. Lightning is not the only process in the atmosphere that causes ionisation and dissociation of atmospheric air. Cosmic rays continuously bombard the Earth with high energetic particles and radiation causing ionization and dissociation of air leading to the production of NOx in the atmosphere. The work carried out in this thesis is an attempt to improve the current knowledge on the way in which these processes contribute to the global NOx production. Experiments have been conducted in this thesis to estimate the NOx production efficiency of streamer discharges, laser-induced plasma, laboratory sparks having current signatures similar to those of lightning flashes, alpha particle impact in air and finally with the lightning flash itself. The results obtained from laboratory electrical discharges show the following: (a) The NOx production efficiency, in terms of energy, of positive streamer discharges is more or less similar to those of hot discharges. (b) The NOx production efficiency of an electrical discharge depends not only on the energy but also on the peak and the shape of the current waveform. (c) The current signature is a better figure of merit in evaluating the NOx yield of electrical discharges. As a part of this thesis work a direct measurement of NOx generated by lightning flashes was conducted and the results show that slow discharge processes such as continuing currents could be the main source of NOx in lightning flashes. Concerning NOx production by other ionisation processes such as alpha particle impacts in the atmosphere, the data gathered in this thesis show that each ionising event in air leads to the creation of one NOx molecule. In terms of energy the NOx production efficiency of alpha particles is similar to that of electrical discharges. The theoretical studies conducted within this thesis indicate that M-components contribute more than the return strokes to the NOx production. The calculations also show that the contribution to the global NOx budget by return stroke is not as high as that assumed in the current literature.