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Analysis of the physical and chemical properties of atmospheric aerosol at the Puy de Dôme station / Analyse des propriétés physiques et chimiques de l’aérosol atmosphérique à la station du Puy de DômeFarah, Antoine 19 December 2018 (has links)
Les particules d'aérosol sont importantes en raison de leurs impacts directs et indirects sur le climat. Dans la couche limite (CL), ces particules ont une durée de vie relativement courte en raison de leur élimination fréquente par dépôt humide. En revanche, lorsque les aérosols sont transportés dans la troposphère libre (TL), leur durée de vie dans l'atmosphère augmente de manière significative, ce qui les rend représentatifs de vastes zones spatiales. Dans le cadre de ces travaux de thèse, nous avons utilisé une combinaison de mesures in situ effectuées à la station PUY (Puy de Dôme, 45 ° 46 'N, 2 ° 57'E, 1465 m d'altitude), ainsi que des profils LIDAR obtenus depuis Clermont-Ferrand pour identifier les conditions de TL et caractériser davantage les propriétés physiques et chimiques des aérosols dans cette zone de l'atmosphère peu documentée. Dans un premier temps, une combinaison de quatre critères a été utilisée pour déterminer le positionnement de la station PUY en TL ou en CL. Les résultats montrent que la station est située en CL avec des fréquences allant de 50% en hiver à 97% en été. Cette classification a ensuite été utilisée pour évaluer, sur un an de mesures, les différences qui existent entre la TL et la CL en termes de caractéristiques physique (distribution en taille) et chimique (fraction non réfractaire) de l’aérosol, et vis-à-vis des concentrations en carbone suie (BC). Sur la base de cette ségrégation, nous avons observé pour la plupart des saisons que les concentrations en particules des modes Aitken et accumulation ainsi que la concentration en BC sont plus élevées dans la CL que dans la TL. Cette observation est cohérente avec le fait que la majorité des sources d’aérosol sont situées dans la CL. Au contraire, des concentrations plus élevées en particules dans les modes Aitken et accumulation (notamment organiques) et en BC sont observées en TL au printemps. Ces aérosols organiques ont été identifiés comme étant âgés / moins âgés ; ils coïncident avec la présence de fortes concentrations en sulfate et en BC et sont probablement originaires de processus de combustion de biomasse, à la suite desquels ils sont directement injectés en TL sous l’effet d’une convection thermique intense. Aucune différence significative entre les concentrations de CL et de TL n'a été observée pour les particules du mode nucléation, et ce quelle que soit la saison, ce qui suggère une source supplémentaire continue de particules du mode nucléation dans la TL en hiver et en automne. Les concentrations en particules du mode grossier sont en revanche plus élevées dans la TL que dans la CL pour toutes les saisons, et en particulier en été. Cela indique un transport longue distance efficace des grosses particules dans la TL depuis des sources lointaines (marines et désertiques), probablement favorisé par les vitesses de vent accrues dans la TL par rapport à la CL. Nous avons ensuite calculé les rétro-trajectoires des masses d'air que nous avons combinées aux estimations de hauteur de couche limite du modèle ECMWF ERA-Interim pour estimer le temps passé par les masses d’air dans la TL depuis leur dernier contact avec la CL, et pour évaluer l'impact de ce paramètre sur les propriétés des aérosols. Nous avons observé que même après 75 heures sans aucun contact avec la CL, les aérosols de la TL conservent les propriétés spécifiques du type de masse d'air auquel ils appartiennent. Ce manuscrit présente également une étude des mesures simultanées au PUY et à une station urbaine à basse altitude, AtmoAura. Les résultats montrent que lorsque le PUY est en TL, les concentrations des PM1 (particules de diamètre inférieur à 1 µm) sont plus faibles au PUY qu’à AtmoAura, ce qui confirme notre classification. Lorsque le PUY est en CL, la composition en aérosol est similaire pour les deux sites, ce qui a permis de quantifier la contribution de la pollution urbaine locale issue de la ville de Clermont-Ferrand. / Aerosol particles are important due to their direct and indirect impacts on climate. Within the planetary boundary layer (BL), these particles have a relatively short lifetime due to their frequent removal process by wet deposition. When aerosols are transported into the free troposphere (FT), their atmospheric lifetime increases significantly, making them representative of large spatial areas. In this work, we use a combination of in situ measurements performed at the high altitude PUY (Puy de Dôme, 45°46’ N, 2°57’E, 1465 m asl) station, together with LIDAR profiles at Clermont-Ferrand for characterizing FT conditions, and further characterize the physical and chemical properties of aerosol in this poorly documented area of the atmosphere. First, a combination of four criteria was used to identify whether the PUY station lies within the FT or within the BL. Results show that the PUY station is located in BL with frequencies ranging from 50% during the winter, up to 97% during the summer. Then, the classification is applied to a year-long dataset of particle size distribution and NR-PM1 data’s to study the differences in particle physical and chemical characteristics and BC concentrations between the FT and the BL. Based on this segregation, we observed higher concentrations in the BL compared to FT for BC, Aitken and accumulation mode particle concentrations for most seasons, as expected from larger sources originating from the surface. However, BC, Aitken mode, accumulation mode and organic aerosols concentrations were higher in the FT compared to BL during spring. These organic aerosols were identified as aged/less aged, and were correlated with sulphate and BC and we suspect that the higher concentrations of particles observed in the FT compared to BL during spring originate from direct injection of BB aerosols in the FT through strong heat convection. No significant difference between the BL and the FT concentrations was observed for the nucleation mode particles for all seasons, suggesting a continuous additional source of nucleation mode particles in the FT during winter and autumn. Coarse mode particle concentrations were found higher in the FT than in the BL for all seasons and especially during summer. This indicates an efficient long-range transport of large particles in the FT from distant sources (marine and desert) due to higher wind speeds in the FT compared to BL. For FT air masses, we used 204-h air mass back-trajectories combined with boundary layer height estimations from ECMWF ERA-Interim to assess the time they spent in the FT since their last contact with the BL and to evaluate the impact of this parameter on the aerosol properties. We observed that even after 75 h without any contact with the BL, FT aerosols preserve specific properties of their air mass type. This manuscript is also presenting a study of simultaneous measurements at PUY and an urban low altitude station AtmoAura. Results show that when the PUY is influenced by FT air masses, the PM1 species are lower at the PUY compared to AtmoAura confirming our classification. When the PUY is predicted to lay within the BL, the aerosol composition was similar among the two sites for several species, which allowed for a quantification of the local urban pollution contribution for the species enhanced within the city of Clermont-Ferrand.
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Use of wind profilers to quantify atmospheric turbulenceLee, Christopher Francis January 2011 (has links)
Doppler radar wind profilers are already widely used to measure atmospheric winds throughout the free troposphere and stratosphere. Several methods have been developed to quantify atmospheric turbulence with such radars, but to date they have remained largely un-tested; this thesis presents the first comprehensive validation of one such method. Conventional in-situ measurements of turbulence have been concentrated in the surface layer, with some aircraft and balloon platforms measuring at higher altitudes on a case study basis. Radars offer the opportunity to measure turbulence near continuously, and at a range of altitudes, to provide the first long term observations of atmospheric turbulence above the surface layer. Two radars were used in this study, a Mesosphere-Stratosphere-Troposphere (MST) radar, at Capel Dewi, West Wales, and the Facility for Ground Based Atmospheric Measurements (FGAM) mobile boundary layer profiler. In-situ measurements were made using aircraft and tethered-balloon borne turbulence probes. The spectral width method was chosen for detailed testing, which uses the width of a radar's Doppler spectrum as a measure of atmospheric velocity variance. Broader Doppler spectra indicate stronger turbulence. To obtain Gaussian Doppler spectra (a requirement of the spectral width method), combination of between five and seven consecutive spectra was required. Individual MST spectra were particularly non-Gaussian, because of the sparse nature of turbulence at its observation altitudes. The width of Gaussian fits to the Doppler spectrum were compared to those from the `raw' spectrum, to ensure that non-atmospheric signals were not measured. Corrections for non-turbulent broadening, such as beam broadening, and signal processing, were investigated. Shear broadening was found to be small, and the errors in its calculation large, so no corrections for wind shear were applied. Beam broadening was found to be the dominant broadening contribution, and also contributed the largest uncertainty to spectral widths. Corrected spectral widths were found to correlate with aircraft measurements for both radars. Observing spectral widths over time periods of 40 and 60 minutes for the boundary layer profiler and MST radar respectively, gave the best measure of turbulence intensity and variability. Median spectral widths gave the best average over that period, with two-sigma limits (where sigma is the standard deviation of spectral widths) giving the best representation of the variability in turbulence. Turbulent kinetic energies were derived from spectral widths; typical boundary layer values were 0.13 m 2.s (-2) with a two-sigma range of 0.04-0.25 m 2.s (-2), and peaked at 0.21 m 2.s (-2) with a two-sigma range of 0.08-0.61 m 2.s (-2). Turbulent kinetic energy dissipation rates were also calculated from spectral widths, requiring radiosonde measurements of atmospheric stability. Dissipation rates compared well width aircraft measurements, reaching peaks of 1x10 (-3) m 2.s (-3) within 200 m of the ground, and decreasing to 1-2x10 (-5) m 2.s (-3) near the boundary layer capping inversion. Typical boundary layer values were between 1-3x10 (-4) m 2.s (-3). Those values are in close agreement with dissipation rates from previous studies.
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