Düsseldorf, Univ., Diss., 2002. / Computerdatei im Fernzugriff.

Düsseldorf, Universiẗat, Diss., 2002.

Düsseldorf, Universiẗat, Diss., 2002.

Techn. University, Diss., 2005--Berlin.

The regulated control of pollutants is essential to maintaining good air quality in urban areas. A major concern is the formation of tropospheric ozone, which can be especially harmful to those with lung conditions and has been linked to the occurrence of asthma. Ozone is formed through reactions of oxidized volatile organic compounds with nitrogen oxides, and the accurate modeling of the process is necessary for smart and effective regulations. Ambient measurements are important to understanding the mechanisms involved in tropospheric chemistry. This dissertation describes the characterization of a novel instrument for the ambient measurement of dinitrogen pentoxide, N2O5, and the results of several field studies. This is an important intermediate in the major nighttime loss pathway of nitrogen oxides. The understanding of this process requires correct modeling formation, as any nitrogen oxides not removed at night will result in increased ozone formation at sunrise. Calibration studies have been performed in order to quantify the loss of reactive species within the instrument, and the sampling flow and N2O5 detection have been well characterized. The results of the laboratory measurements are presented. Results are presented from the SHARP Field Study in Houston, TX in the spring of 2009. N2O5 measurements are compared to measurements of other species, including nitric acid and nitryl chloride, which were performed by other research groups. Mixing ratios exceeding 300 ppt were observed following ozone exceedance days, and a dependence of the concentration on both wind speed and direction was noticed. There was a strong correlation determined between N2O5 with HNO3 and ClNO2 indicating both a fast heterogeneous hydrolysis and N2O5 as the primary source of the species. Observed atmospheric lifetimes for N2O5 were short, ranging from several seconds to several minutes. We have also investigated the presence of N2O5 in College Station, TX. Low mixing ratios peaking at approximately 20 ppt were observed, with longer atmospheric lifetimes of up to several hours. The role of biogenic emissions in the NO3-N2O5 equilibrium is discussed.

Development of effective pollution control strategies for urban areas requires accurate predictive models. The ability of models to correctly characterize the atmospheric chemistry, meteorology, and deposition rely on accurate data measurements, both as input and verification of output. Therefore, the measurement techniques must be sensitive, accurate, and capable of resolving the spatial and temporal variations of key chemical species. The application of a sensitive in situ optical absorption technique, known as cavity ring-down spectroscopy, will be introduced for simultaneously measuring the nitrate radical and dinitrogen pentoxide. The cavity ring-down spectrometer was initially designed and constructed based on the experiments by Steven Brown and Akkihebal Ravishankara at the National Oceanic and Atmospheric Administration. The instrument design has since undergone many revisions before attaining the current instrumentation system. Laboratory observations provide verification of accurate N2O5 and NO3 detection with measurements of the nitrate radical absorption spectrum centered at 662 nm, effective chemical zeroing with nitric oxide, and efficient thermal decomposition of N2O5. Field observations at a local park provided further confirmation of the instruments capability in measuring N2O5 and NO3. However, detection limits were too high to detect ambient NO3. Effective and frequent zeroing can easily improve upon the sensitivity of the instrument. Determination of the source of the polluted air masses detected during these studies was unknown since the typical southerly winds from Houston were not observed. Since deployment in the field, instrumentation modifications and laboratory measurements are underway for preparation of the SOOT campaign in Houston, Texas starting April 15, 2009. Current modifications include automation of the titration with a solenoid valve and an automated filter changer. Wall losses and filter transmission for NO3 and N2O5 will be determined through laboratory measurements in coincidence with and ion-drift chemical ionization mass spectrometer prior to the SOOT project. Potential modifications to improve upon the instrument are suggested for future endeavors.

Brandenburgische Techn. University, Diss., 2003--Cottbus.

Universiẗat, Diss., 2004--Kiel.

Investigation of aerosol chemistry and growth under atmospheric conditions in a novel rotating aerosol suspension chamber with cavity ring-down spectroscopy provided key insight into the effect of pollutants and other vapors on the overall atmospheric lifetime of particulate matter. The Atmospheric Cloud Simulation Instrument (ACSI) creates a well-defined and controllable atmosphere of suspended particles, analyte gases, and background gas molecules, which remains stable up to several days. Preliminary studies have shown that monodisperse polystyrene latex (dp = 0.994 µm) and polydisperse ammonium sulfate (CMD dp = 100 nm) particles remain suspended for at least 22 hours while the chamber rotates at 2 RPM. Further investigation into the aerosol dynamics showed the coagulation efficiency of high concentration particle suspensions (>10^6 particles/cm3) depends on particle phase state and composition. The coagulation efficiency decreased with increased humidity in the model atmosphere and with increased ion concentrations in the aerosols. The decrease in efficiency is attributed to repulsive forces from like-charges on the particle surfaces. In addition to humidity, the spectroscopy integrated into the main chamber monitors the real-time response to a perturbation in the model atmosphere, such as the introduction of a gas-phase reactant. Cavity ring-down spectroscopy, performed in situ along the center axis, records mid-infrared spectra (1010 cm-1 to 860 cm-1) to identify gas species evolved from gas-particle heterogeneous chemistry. In accord with previous studies, my results show that a known reaction between monomethyl amine and ammonia occurs readily on suspended ammonium sulfate particles in >50% RH and the extent of the reaction depends on the humidity of the model atmosphere. Acidic ammonium bisulfate aerosols also produced a detectable amount of ammonia upon exposure to monomethyl amine in a model atmosphere with >50% RH. Overall, the new ACSI approach to atmospheric science provides the opportunity to study the influence of interfacial chemistry on particle growth, aging, and re-admission of gas-phase compounds. / Doctor of Philosophy / "Molecules don't have a passport." - Carl Sagan. Gas molecules and particles emitted into the atmosphere in one area can travel thousands of kilometers over the course of hours to days, even weeks for some compounds. The gas-solid interactions that occur over the lifetime of particulate matter are largely unknown. I focused my doctorate on bridging the knowledge gap between traditional environmental monitoring research and highly controlled laboratory experiments. To do so, I designed a new instrument capable of creating stable model atmospheres that more accurately simulate the gas-particle interactions in Earth's atmosphere than previous environmental chambers. The Atmospheric Cloud Simulation Instrument design included a rotating chamber to increase the duration of stable particle suspensions in a laboratory and a multi-pass infrared spectrometer to monitor gas-phase reactions in situ. I explored the effect of humidity and particle composition on particle-particle coagulation and gas-particle reactions. For example, liquid aerosols at humidities higher than 35% RH do no coagulate as fast as a solid particle with the same composition in <35% RH. Similarly, the same liquid aerosols produced more gaseous product during a heterogeneous reaction with a 'pollutant' gas than solid particles. Overall, the ACSI will be an important tool for future experiments exploring individual aspects of complex atmospheric processes.

Un spectromètre utilisant la technique CRDS a été développé entre 2.00 et 2.35 µm afin de réaliser la spectroscopie en absorption de molécules d’intérêt atmosphérique et planétologique avec une très grande sensibilité et à haute résolution spectrale. Cette région du spectre correspond à une fenêtre de transparence de la vapeur d’eau et du dioxyde de carbone. Ces fenêtres sont des zones de très faible absorption utilisées pour le sondage des atmosphères terrestre et vénusienne dans lesquelles la vapeur d’eau et le dioxyde de carbone représentent respectivement les absorbants gazeux principaux dans l’infrarouge.La technique CRDS consiste à injecter des photons dans une cavité optique de haute finesse et à mesurer la durée de vie des photons dans cette cavité. Celle-ci est mesurée en interrompant l’injection des photons dans la cavité optique lors du passage en résonance du laser avec l’un des modes longitudinaux. Cette durée de vie dépend de la réflectivité des miroirs et des pertes intra-cavité comme celles induites par un gaz qui absorbe. Mesurer ces pertes en fonction de la longueur d’onde permet d’obtenir le spectre d’absorption du gaz en question. L’extrême réflectivité des miroirs permet d’atteindre dans une cavité d’un peu plus d’1 m de longueur une sensibilité équivalente à celle qui serait obtenue classiquement avec une cellule d’absorption longue de plusieurs milliers de kilomètres.Trois diodes laser DFB émettant autour de 2.35, 2.26 et 2.21 µm ont été utilisées avec ce spectromètre. Grâce à une rétro-action optique provenant d’une cavité externe, certaines de ces diodes ont pu être affinées, ce qui a permis de mieux injecter la cavité haute finesse et ainsi de réduire le niveau de bruit du spectromètre. Parallèlement grâce à une collaboration avec l’Institut d’Electronique (IES, UMR 5214) à Montpellier et la société Innoptics nous avons pu tester le prototype d’un VECSEL (Vertical-External-Cavity Surface-Emitting-Laser). Ce laser a permis de couvrir une gamme spectrale de 80 cm-1, entre 4300 et 4380 cm-1, équivalente à quatre diodes laser DFB. La sensibilité obtenue en routine avec ce spectromètre, correspondant au coefficient minimum détectable, est typiquement de 1×10-10 cm-1. Le chapitre introductif (Chapitre 1) fait le point sur les différentes techniques permettant d’acquérir des spectres en absorption dans la gamme spectrale étudiée et sur les sensibilités atteintes. A notre connaissance l’instrument développé ici est le plus sensible dans cette région du spectre. Le fonctionnement de ce spectromètre CRDS est détaillé dans le chapitre 2.Pour démontrer les performances obtenues avec notre instrument celui-ci a été utilisé pour enregistrer des transitions quadrupolaires donc de très faible intensité. Ainsi la transition S(3) de la bande 1–0 de HD a été enregistrée pour la première fois et son intensité mesurée (S=2.5×10-27 cm/molecule). La sensibilité obtenue en routine a encore pu être améliorée en réalisant une moyenne d’une centaine de spectres sur une gamme spectrale réduite pour atteindre 1×10-11 cm-1. Grâce à cela nous avons pu mesurer la position et l’intensité de la raie quadrupolaire électrique O(14) de la bande 2–0 de N2 qui est très fortement interdite avec une intensité de 1.5×10-30 cm/molecule. Ces mesures font l’objet du chapitre 3 de cette thèse.Les deux derniers chapitres sont dédiés à la caractérisation de l’absorption du CO2, au centre de la fenêtre de transparence, et à celle de la vapeur d’eau. Dans les deux cas, les transitions permises du monomère et la contribution du continuum ont été étudiées. Ce dernier correspond à une absorption variant lentement avec la longueur d’onde. Les sections efficaces du « self-continuum » de la vapeur d’eau ont notamment été mesurées en plusieurs points de la fenêtre de transparence avec une incertitude beaucoup plus faible que les mesures existantes. Elles représentent un jeu de données décisif pour tester les modèles décrivant ce continuum. / A cavity ring down spectrometer has been developed in the 2.00-2.35 µm spectral range to achieve highly sensitive absorption spectroscopy of molecules of atmospheric and planetologic interest and at high spectral resolution. This spectral region corresponds to a transparency window for water vapor and carbon dioxide. Atmospheric windows, where absorption is weak, are used to sound the Earth’s and Venus’ atmospheres where water vapor and carbon dioxide represent the main gaseous absorbers in the infrared, respectively.The CRDS technique consists of injecting photons inside a high finesse optical cavity and measuring the photon’s life time of this cavity. This life-time depends on the mirror reflectivity and on the intra-cavity losses due to the absorbing gas in the cavity. Measuring these losses versus the wavelength allow obtaining the absorption spectrum of the gas. The extreme reflectivity of the mirrors allows reaching, for a 1-meter long cavity, a sensitivity equivalent to the one obtained classically with absorption cells of several thousands of kilometers.Three DFB laser diodes emitting around 2.35, 2.26, 2.21 µm were used with this spectrometer giving access to the 4249-4257, 4422-4442 and 4516-4534 cm-1 interval, respectively. Thanks to optical feedback from an external cavity, two of these diodes were spectrally narrowed leading to a better injection of the high finesse cavity thus reducing the noise level of the spectrometer. In parallel, we tested a VECSEL (Vertical-external-Cavity, Surface Emitting laser) through a collaboration with the Institu d’Electronique (IES, UMR 5214) in Montpellier and the Innoptics firm. This laser source is able to cover a 80 cm-1 spectral range centered at 4340 cm-1, equivalent to four DFB laser diodes. In routine the achieved sensitivity with this spectrometer, corresponding to the minimum detectable coefficient is typically of 1×10-10 cm-1. The introductive chapter (Chapter 1) makes the point on the different techniques allowing absorption spectra recordings in the studied spectral region and on their sensitivity. The experimental set-up, the characteristics and performances by the CRD spectrometer developed in this work are detailed in Chapter 2. To our knowledge this instrument is the most sensitive in the considered spectral region.In Chapter 3, detection of quadrupolar electric transitions of HD and N2 illustrate the level of sensitivity reached: (i) the S(3) transition in the 1-0 band of HD has been recorded for the first time and its intensity measured (S=2.5×10-27 cm/molecule), (ii) the position and intensity of the highly forbidden O(14) quadrupolar electric transition of the 2-0 band of N2 have also been newly determined.The two last chapters are devoted to the characterization of the CO2 absorption, in the centre of the transparency window, and of the water vapor absorption. In both cases, we not only studied the allowed transitions of the monomer, but also the continuum absorption. This latter correspond to a weak background absorption varying slowly with the wave length. The self-continuum cross-sections of the water vapor continuum were measured in many spectral points through the transparency window with a much better accuracy compared to existing measurements. These CRDS data constitute a valuable data set to validate the reference model (MT_CKD) for the continuum which is implemented in most of the atmospheric radiative transfer codes.