The freezing kinetics of binary nitric acid/water aerosols is of fundamental importance to the modelling of polar stratospheric clouds and the role they in ozone depletion over the Arctic/Antarctic regions. Cirrus clouds are also often composed of nitric acid solutions, hence an understanding of freezing process in these aerosols also aids in modelling the earth's radiation budget and global warming. This thesis explores the kinetic phase diagram of nitric acid/water aerosols with sizes ranging between 0. 2 and 1. 5 mm in radius and concentrations ranging between pure water and 0. 45 mole fraction HNO3. Although the kinetic phase diagram has now been studied between 0. 46 mole fraction HNO3 and pure water, more data is needed in the region between 0. 18 and 0. 25 mole fraction HNO3 to confirm the results reported. The project described in this thesis are a continuation of a project begun by Allan Bertram. The measurements involving aerosols with compositions greater than 0. 25 mole fraction HNO3 were carried out as part of Allan Bertram's Ph. D. thesis (see ref. 20) These data were later examined using a more comprehensive data analysis method (as presented in this thesis) in an effort to obtain a more complete understanding of this system.

The freezing kinetics of binary nitric acid/water aerosols is of fundamental importance to the modelling of polar stratospheric clouds and the role they in ozone depletion over the Arctic/Antarctic regions. Cirrus clouds are also often composed of nitric acid solutions, hence an understanding of freezing process in these aerosols also aids in modelling the earth's radiation budget and global warming. This thesis explores the kinetic phase diagram of nitric acid/water aerosols with sizes ranging between 0. 2 and 1. 5 mm in radius and concentrations ranging between pure water and 0. 45 mole fraction HNO3. Although the kinetic phase diagram has now been studied between 0. 46 mole fraction HNO3 and pure water, more data is needed in the region between 0. 18 and 0. 25 mole fraction HNO3 to confirm the results reported. The project described in this thesis are a continuation of a project begun by Allan Bertram. The measurements involving aerosols with compositions greater than 0. 25 mole fraction HNO3 were carried out as part of Allan Bertram's Ph. D. thesis (see ref. 20) These data were later examined using a more comprehensive data analysis method (as presented in this thesis) in an effort to obtain a more complete understanding of this system.

An examination of the seasonal and spatial distribution of Polar Stratospheric Clouds (PSCs) inferred from standard temperature profiles in the lower-middle atmosphere above Antarctica, as derived from the Earth Observing System (EOS) Aura Microwave Limb Sounder (MLS) satellite observations and NCEP/NCAR assimilations, is provided. Chemical volume mixing ratio (VMR) observations of EOS Aura MLS v2.2 hydrogen chloride (HCl) were used to show the interannual variability of PSC formation with respect to stratospheric chlorine partitioning during five Southern Hemisphere Antarctic seasons from 2005 to 2009. A remarkable first set of results, obtained from an algorithm developed for modelling HCl depletion areas in the Antarctic polar vortex region, and based on satellite observations, is presented. In particular, the analysis of HCl concentration data obtained from 2006 indicated that the area processed for HCl was larger than the area of PSC during some periods of Antarctic winter, and that this result was robust with respect to the various PSC formation and HCl depletion thresholds utilized. The results suggest that an underestimation in chlorine activation area can occur when temperature thresholds for PSC formation thresholds are employed. The work presented here also evaluated chlorine activation via sulfate aerosol (SA) in the Southern Hemisphere 2006 stratosphere, based on satellite measurements of water vapor (H2O) and constant values of SA, by implementing the TACL formula of Drdla and Müller [2010] in contrast to the TNAT formula of Hanson and Mauersberger [1988]. The results indicated that the former formula was not completely sufficient for accurately modeling areas of depleted HCl and chlorine deactivation for all pressure surfaces in the Antarctic stratosphere. Based on the results of this study, the role of SA in chlorine activation appears to be more important at lower altitudes than for areas higher in the stratosphere.

Measuring the composition of the stratosphere, and understanding the processes regulating it, have become,in the last few decades, top priorities in the scientific community, particularly since the discovery ofthe ozone hole in the 1980s. While a lot has indeed been done in monitoring ozone, other constituents also influence the stratosphere’s composition, and interfere namely with ozone, affecting its chemical and dynamical balance. Among these is nitric acid (HNO3 ) which is a reservoir for ozone depleting NOx ,but also a key player in the formation of polar stratospheric clouds which, by turning inert species into active radicals, enhance the ozone depletion further. The nadir-viewing IASI instrument is a very good means of obtaining simultaneous data of nitric acid and ozone. Indeed, it measures the radiation of the Earth’s atmosphere in the thermal infrared spectral range, which allows it to measure even at night. This is crucial to the study of polar processes, since they occur mostly during the polar winter, when no light reaches these latitudes. Thanks to its design and its technical characteristics, the IASI instrument provides data all-year round, for every location on the Earth. The purpose of this work is to use this unique set of IASI data to understand what drives the variability of HNO3 in the stratosphere. No study so far has focused on the factors affecting the time and spatial distributions of nitric acid to the extent and scale we propose here. We aim to identify and quantify these factors, and to compare them with the drivers of ozone variability. Nitric acid data are thus obtained for the 10 years of IASI observation (2008 − 2017), and vertical profiles are retrieved in near-real time thanks to the FORLI algorithm developed at ULB. The first part of the present work provides a detailed characterization of the IASI FORLI-HNO3 data set in terms of vertical sensitivity and errors. We show that the HNO3 maximum is found around 20 km altitude, where we also find the maximum sensitivity of the measurements to the vertical profile. The analysis of the averaging kernels shows us that only one level of information can be extracted from the vertical profile, which constrains the rest of our analyses to the use of a total (or almost total) column. We also find that the IASI measurements tend to overestimate slightly the HNO3 column in the upper troposphere/lower stratosphere region of the profile. The data set is validated against ground-based FTIR measurements at different latitudes: we find good agreement between IASI and the FTIR data, which confirms that IASI manages to reproduce the HNO3 columns and their seasonality accurately. Comparisons with a state of the art atmospheric model data are also shown, and suggest that improvement is still largely needed in models to represent the HNO3 distributions accurately. The use of a data-assimilated model (BASCOE) shows a much better agreement with the IASI observations. The next part of the work describes the geophysical analyses carried out, and details the first time series and global distributions of HNO3 from IASI. After describing the various (mostly polar) processes at play observed in the time series, the question of the formation of the polar stratospheric clouds is raised, and further results are shown about the temperature at which these form. While a fixed threshold (195 K) is usually used for geophysical analyses, we find from the observational IASI data set that this fixed temperature can vary substantially depending on local conditions and on altitude. The last sections use multivariate linear regressions to fit the HNO3 and O3 time series, featuring various chemical and dynamical variables in order to identify what factors are responsible for their respective variability. We include the variables most commonly used in such kind of study, i.e. a linear trend, harmonical terms to account for the annual seasonality, and proxies for the quasi-biennial oscillation, the multivariate ENSO index, and the Arctic and Antarctic oscillations. The novelty of our work resides in the addition of a proxy for the volume of polar stratospheric clouds to account for the strong denitrification observed in the HNO3 time series in polar regions. We find that the annual cycle, encompassing the solar seasonality and the Brewer-Dobson circulation, is the factor explaining most of the variability of both HNO3 and O3 ,at almost all latitudes. In the polar regions, however, the volume of polar stratospheric clouds is a key factor contributing the most to their variability. Globally, the same factors explain the same portion of both HNO3 and O3 variability. In the last part of the thesis, we conclude and provide a preliminary co-analysis of HNO3 and O3 from the 10-year IASI data. The results are encouraging and highlight the potential of the IASI measurements to monitor the polar processes on various scales. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

<p>The Auroral Large Imaging System (ALIS) was proposed in 1989 by Åke Steen as a joint Scandinavian ground-based nework of automated auroral imaging stations. The primary scientic objective was in the field of auroral physics, but it was soon realised that ALIS could be used in other fields, for example, studies of Polar Stratospheric Clouds (PSC), meteors, as well as other atmospheric phenomena.</p><p>This report describes the design, operation and scientic results from a Swedish prototype of ALIS consisting of six unmanned remote-controlled stations located in a grid of about 50 km in northern Sweden. Each station is equipped with a sensitive high-resolution (1024 x 1024 pixels) unintensified monochromatic CCDimager. A six-position filter-wheel for narrow-band interference filters facilitates absolute spectroscopic measurements of, for example, auroral and airglow emissions. Overlapping fields-of-view resulting from the station baseline of about 50 km combined with the station field-of-view of 50° to 60°, enable triangulation as well as tomographic methods to be employed for obtaining altitude information of the observed phenomena.</p><p>ALIS was probably one of the first instruments to take advantage of unintensi- fied (i.e. no image-intensifier) scientific-grade CCDs as detectors for spectroscopic imaging studies with multiple stations of faint phenomena such as aurora, airglow, etc. This makes absolute calibration a task that is as important as it is dificult.</p><p>Although ALIS was primarily designed for auroral studies, the majority of the scientific results so far have, quite unexpectedly, been obtained from observations of HF pump-enhanced airglow (recently renamed Radio-Induced Aurora). ALIS made the first unambiguous observation of this phenomena at high-latitudes and the first tomography-like inversion of height profiles of the airglow regions. The scientific results so far include tomographic estimates of the auroral electron spectra, coordinated observations with satellite and radar, as well as studies of polar stratospheric clouds. An ALIS imager also participated in a joint project that produced the first ground-based daytime auroral images. Recently ALIS made spectroscopic observations of a Leonid meteor-trail and preliminary analysis indicates the possible detection of water in the Leonid.</p>

The Auroral Large Imaging System (ALIS) was proposed in 1989 by Åke Steen as a joint Scandinavian ground-based nework of automated auroral imaging stations. The primary scientic objective was in the field of auroral physics, but it was soon realised that ALIS could be used in other fields, for example, studies of Polar Stratospheric Clouds (PSC), meteors, as well as other atmospheric phenomena. This report describes the design, operation and scientic results from a Swedish prototype of ALIS consisting of six unmanned remote-controlled stations located in a grid of about 50 km in northern Sweden. Each station is equipped with a sensitive high-resolution (1024 x 1024 pixels) unintensified monochromatic CCDimager. A six-position filter-wheel for narrow-band interference filters facilitates absolute spectroscopic measurements of, for example, auroral and airglow emissions. Overlapping fields-of-view resulting from the station baseline of about 50 km combined with the station field-of-view of 50° to 60°, enable triangulation as well as tomographic methods to be employed for obtaining altitude information of the observed phenomena. ALIS was probably one of the first instruments to take advantage of unintensi- fied (i.e. no image-intensifier) scientific-grade CCDs as detectors for spectroscopic imaging studies with multiple stations of faint phenomena such as aurora, airglow, etc. This makes absolute calibration a task that is as important as it is dificult. Although ALIS was primarily designed for auroral studies, the majority of the scientific results so far have, quite unexpectedly, been obtained from observations of HF pump-enhanced airglow (recently renamed Radio-Induced Aurora). ALIS made the first unambiguous observation of this phenomena at high-latitudes and the first tomography-like inversion of height profiles of the airglow regions. The scientific results so far include tomographic estimates of the auroral electron spectra, coordinated observations with satellite and radar, as well as studies of polar stratospheric clouds. An ALIS imager also participated in a joint project that produced the first ground-based daytime auroral images. Recently ALIS made spectroscopic observations of a Leonid meteor-trail and preliminary analysis indicates the possible detection of water in the Leonid.

The Auroral Large Imaging System (ALIS) was proposed in 1989 by Åke Steen as a joint Scandinavian ground-based nework of automated auroral imaging stations. The primary scientic objective was in the field of auroral physics, but it was soon realised that ALIS could be used in other fields, for example, studies of Polar Stratospheric Clouds (PSC), meteors, as well as other atmospheric phenomena. This report describes the design, operation and scientic results from a Swedish prototype of ALIS consisting of six unmanned remote-controlled stations located in a grid of about 50 km in northern Sweden. Each station is equipped with a sensitive high-resolution (1024 x 1024 pixels) unintensified monochromatic CCDimager. A six-position filter-wheel for narrow-band interference filters facilitates absolute spectroscopic measurements of, for example, auroral and airglow emissions. Overlapping fields-of-view resulting from the station baseline of about 50 km combined with the station field-of-view of 50° to 60°, enable triangulation as well as tomographic methods to be employed for obtaining altitude information of the observed phenomena. ALIS was probably one of the first instruments to take advantage of unintensi- fied (i.e. no image-intensifier) scientific-grade CCDs as detectors for spectroscopic imaging studies with multiple stations of faint phenomena such as aurora, airglow, etc. This makes absolute calibration a task that is as important as it is dificult. Although ALIS was primarily designed for auroral studies, the majority of the scientific results so far have, quite unexpectedly, been obtained from observations of HF pump-enhanced airglow (recently renamed Radio-Induced Aurora). ALIS made the first unambiguous observation of this phenomena at high-latitudes and the first tomography-like inversion of height profiles of the airglow regions. The scientific results so far include tomographic estimates of the auroral electron spectra, coordinated observations with satellite and radar, as well as studies of polar stratospheric clouds. An ALIS imager also participated in a joint project that produced the first ground-based daytime auroral images. Recently ALIS made spectroscopic observations of a Leonid meteor-trail and preliminary analysis indicates the possible detection of water in the Leonid.