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

Düsseldorf, Universiẗat, Diss., 2002.

Düsseldorf, Universiẗat, Diss., 2002.

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

Cavity ring-down (CRD) spectroscopy has emerged as a sensitive analytical technique. In this method, a laser pulse is injected through one of two highly-reflective mirrors which form a stable optical cavity and the rate that the light leaves the cavity is monitored by a detector placed behind the second mirror. In this research a CRD spectrometer has been designed and constructed. The light exiting the cavity is collected via a fiber optic cable which is then directed toward a photo multiplier tube (PMT) detector. The signal is digitized and averaged by an oscilloscope and the data are transferred by an I 488 interface to a personal computer where the data are analyzed. Instrument command and data acquisition are controlled by a Visual Basic computer program. A short review of several attempts to measure liquid samples using CRD spectroscopy is presented; most discuss the necessity for the incorporation of Brewster’s angle at the liquid interface. This study integrates a 1 cm standard quartz cuvette at normal incidence. It was determined that there are significant losses from scattering and reflection; however, these losses were not so large as to negate the efficacy of the technique. The hypothesis tested here is that the light “lost” as reflections are collected by the cavity mirrors and redirected back into the cavity. Rhodamine 6G was used as the primary model absorber in these studies. Absorbance measurements were extracted from the measured ring-down times and a detection limit was obtained. Four cavity lengths were constructed to determine the effect on the scattering losses with varying cavity lengths. The calculated detection limit for the CRD spectrometer used in this study was found to be in the range of 4-5 nM. It was found that the detection limit of the CRD spectrometer was 36 times lower than that of the commercial instrument. Aligning the cavity mirrors at longer cavity lengths proved to be more difficult; however, there were no significant additional losses observed by incorporating longer cavities.

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.