Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

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Subject: search.filters.subject.Atmospheric Chemistry

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Now showing 1 - 6 of 6
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    Identification and Quantification of Regional Aerosol Trends and Impact on Clouds over the North Atlantic Ocean
    (2017) Jongeward, Andrew; Li, Zhanqing; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Aerosols and clouds contribute to atmospheric variability and Earth’s radiative balance across local, regional, and global scales. Originating from both natural and anthropogenic sources, aerosols can cause adverse health effects and can interact directly with solar radiation as well as indirectly through complex interactions with clouds. Aerosol optical depth (AOD) has been observed from satellite platforms for over 30 years. During this time, regional changes in emissions, arising from air quality policies and socioeconomic factors, have been suggested as causes for some observed AOD trends. In the United States, the Clean Air Act and amendments have produced improvements in air quality. In this work the impacts of improved air quality on the aerosol loading and aerosol direct and indirect effects over the North Atlantic Ocean are explored using satellite, ground, and model datasets on the monthly timescale during 2002 to 2012. It is established that two trends exist in the total AOD observed by MODIS over the North Atlantic. A decreasing AOD trend between −0.02 and −0.04 per decade is observed over the mid-latitude region. Using the GOCART aerosol model it is shown that this trend results from decreases in anthropogenic species. Ground based aerosol networks (AERONET and IMPROVE) support a decreasing trend in AOD and further strengthen links to anthropogenic aerosol species, particularly sulfate species. This anthropogenic decrease occurs primarily during spring and summer. During the same time period, MODIS also observes an increasing AOD trend of 0.02 per decade located in the sub-tropical region. This trend is shown to occur during summer and is the result of natural dust aerosol. Changes in the North African environment seen in the MERRA reanalysis suggest an accelerated warming over the Saharan Desert leads to changes in the African Easterly Jet, related Easterly Waves, and baroclinicity playing a role in an increase and northward shift in African dust. Both the direct and indirect impacts of the aerosol trends are investigated. Using the SBDART radiative transfer model, estimates of the shortwave direct radiative forcing are calculated. The decrease in anthropogenic AOD produces an increase of 2.0 ± 0.3 W/m2 per decade in the Earth-system absorbance over the mid-latitude site (37.5ºN, −68.5ºE). The increase in natural AOD results in a decrease of −1.1 ± 0.2 W/m2 per decade in the Earth-system absorbance over the sub-tropical site (23.5ºN, −55.5ºE). Evaluation of the first indirect effect demonstrates agreement with Twomey theory when considering the North Atlantic domain on the whole. A regional analysis reveals the existence of counter-Twomey behavior along the U.S. Atlantic coast. Using a daily dataset during summertime with focus on warm, non-precipitating clouds, it is found that aerosol-cloud interaction in this coastal region is sensitive to vertical velocity and aerosol size. Cases experiencing updrafts (ω < 0 Pa/s) and cases of mainly coarse-mode aerosol demonstrate good agreement with Twomey theory. Additionally, cases with low specific humidity near the cloud base show non-Twomey behavior for clouds with low liquid water path.
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    Diagnosing Clouds and Hazes in Exoplanet Atmospheres
    (2015) Fraine, Jonathan David; Deming, Leo D; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Exoplanet atmospheres provide a probe into the conditions on alien worlds, from hot Jupiters to Super-Earths. We can now glimpse the behaviour of extreme solar systems that defy our understanding of planet formation and capture our imaginations about the possibilities for understanding planets and life in our universe. I combined multi-epoch, multi-instrument observations from both space and ground based facilities. I developed observational techniques and tools to constrain exoplanetary atmospheric compositions, temperature profiles, and scale heights over a span of planetary masses and wavelengths, that provided a probe into the properties of these diverse planetary atmospheres. I led a team that used the Spitzer Space Telescope, with the IR Array Camera (IRAC), to observe the well known transiting Super-Earth, GJ 1214b (~2.7 R_Earth). My precisely constrained infrared transit depth, error ~ O(40 ppm), significantly constrained the lack of any molecular detections out to a wavelength of 5 microns. The significance of this null detection challenges self-consistent models for the atmosphere of this super-Earth. Models must invoke thick, grey opacity clouds that uniformly cause the atmosphere to be opaque at all wavelengths. My team and I used the Hubble Space Telescope Wide Field Camera 3 (HST-WFC3) to spectroscopically probe the atmosphere of the transiting warm Neptune, HAT-P-11b (~4.5 R_Earth), and detected the first molecular signature from a small exoplanet (R_planet < R_Saturn), inferring the presence of a hydrogen rich atmosphere. The average densities of many transiting exoplanets are known, but the degree to which atmospheric composition -- abundance of Hydrogen relative to other atoms and molecules -- correlates with the bulk composition has not yet been established. In an effort to characterize the atmospheric metallicity in greater detail, my team observed HAT-P-11 using warm Spitzer IRAC at 3.6 and 4.5 microns. The non-detections of eclipses HAT-P-11b provided upper limits on the temperature profile at 3.6 and 4.5 microns. I am one of the founding members of the ACCESS collaboration (Arizona-CfA-Catolica Exoplanet Spectroscopy Survey), a ground based observational campaign to spectroscopically survey a catalogue of exoplanetary atmospheres using major optical telescopes. I observed several of our targets from the 6.5m Magellan-Baade telescope. The results of my first observation provided low signal-to-noise constraints on the cloud properties of the hot Jupiter WASP-4b, as well as the UV radiation environment produced by its host star, WASP-4. The combination of these observational constraints provided greater insight into the end-products of the planet formation process, and developed the knowledge base of our community for both cloudy and clear worlds.
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    An evaluation of a severe smog episode in the Eastern U.S. using regional modeling and satellite measurements
    (2011) Yegorova, Elena Andreyevna; Dickerson, Russell R; Allen, Dale J; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    An ensemble of regional chemical modeling (WRF/Chem with RADM2) simulations, satellite, ozonesonde, and surface observations during July 7-11, 2007 was used to examine the horizontal and vertical signature of one of the worst smog events in the eastern U.S. in the past decade. The general features of this event -- a broad area of high pressure, weak winds and heavy pollution, terminated by the passage of a cold front -- were well simulated by the model. Average 8-hr maximum O3 has a mean (±Σ) bias of 0.59 (±11.0) ppbv and a root mean square error of 11.0 ppbv. WRF/Chem performed the best on poor air quality days, simulating correctly the spatial pattern of surface O3. Yet the model underpredicted O3 maxima by 5-7 ppbv in the Northeast and overpredicted by 8-11 ppbv in the Southeast. High O3 biases in the Southeast are explained by overpredicted temperatures in the model (>1.5°C). Sensitivity simulations with 1) accelerated O3 dry deposition velocity and 2) suppressed multiphase nitric acid formation pushed the model closer to observations. Simulated O3 vertical profiles over Beltsville, MD showed good agreement with ozonesonde measurements, but the modeled boundary layer depth was overpredicted on July 9, contributing to the low bias over this region. During this severe smog episode, space-borne TES detected high total tropospheric column ozone (TCO) over the Western Atlantic Ocean off the coast near North and South Carolina. The standard product (OMI/MLS) missed the magnitude of these local maxima, but the level-2 ozone profile (OMI) confirmed the TES observations. HYSPLIT back trajectories from these O3 maxima intersected regions of strong convection over the Southeast and Great Lakes regions. When lightning NO emissions were implemented in WRF/Chem, the high concentrations of NOx and O3 off the coast were well reproduced, showing that the exported O3 was produced by a combination of natural NO and pollutants lofted from the lower atmosphere. Lastly, WINTER MONEX O3 data from 1978 are presented for the first time here in discussion of open cell convection over Indonesia.
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    STATE-RESOLVED QUENCHING DYNAMICS IN COLLISIONS OF VIBRATIONALLY EXCITED MOLECULES
    (2010) Du, Juan; MULLIN, AMY S; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The collisional relaxation of highly excited molecules plays a very important role in many chemistry processes. The work presented in this thesis studies the collisional quenching dynamics of highly vibrationally excited molecules using high–resolution transient IR absorption spectroscopy. This work investigates “weak” (small energy transfer) and “strong” (large energy transfer) collisions between donor and bath molecules. The experimental results illustrate how the properties of donor molecules influence the collisional energy transfer. These properties include the molecular structure, internal energy, state density. In several weak collision studies, this thesis studies the vibration–rotation/translation pathway for pyrazine/DCl, pyrazine/CO2 with different internal energies and for three excited alkylated pyridine molecules/CO2 systems. A single–exponential rotational distribution and J–dependent translational energy distributions of scattered DCl molecules are observed. For CO2 collisions, the scattered CO2 has a biexponential rotational distribution and J–dependent translational energy distributions for all collision pairs. Recoil velocities scale with product angular momenta. The observed collision rates for these collision pairs match Lennard–Jones rates. The full energy transfer distribution for these pairs is determined by combining data for weak and strong collisions. Lowering the internal energy of donor molecules reduces the amount of rotational and translational energy transfer to CO2. Reducing the internal energy of pyrazine decreases the probabilities of strong collision and increases the probabilities of weak collision. The average energy transfer reduces by ∼ 50% when the internal energy is decreased by only 15%. The collision rates are independent on the internal energy for these systems. Methylation of donor molecules decreases the magnitude of V—RT energy transfer. The collision results are affected by the number of methyl–groups, and not by the position of the group. Increasing the number of methyl groups increases the ratio of the measured collision rate to the Lennard–Jones collision rate. In the strong collision studies, the effects of alkylation and internal energy are studied. In collisions with alkylated pyridine donors with E ∼ 39000 cm−1, CO2 molecules gain less energy from alkylpyridine than from pyridine. The alkylated donors undergo strong collisions with CO2 via a less repulsive part of the intermolecular potential compared to pyridine. For azulene/CO2 collisions with two different internal energies, scattered CO2 molecules gain double the amount of rotational and translational energy when the azulene energy is doubled. The rate of strong collisions increases four times when the internal energy is doubled.
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    Analysis of Air Quality with Numerical Simulation (CMAQ), and Observations of Trace Gases
    (2009) Castellanos, Patricia; Ehrman, Sheryl H; Dickerson, Russell R; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Ozone, a secondary pollutant, is a strong oxidant that can pose a risk to human health. It is formed from a complex set of photochemical reactions involving nitrogen oxides (NOx) and volatile organic compounds (VOCs). Ambient measurements and air quality modeling of ozone and its precursors are important tools for support of regulatory decisions, and analyzing atmospheric chemical and physical processes. I worked on three methods to improve our understanding of photochemical ozone production in the Eastern U.S.: a new detector for NO2, a numerical experiment to test the sensitivity to the timing to emissions, and comparison of modeled and observed vertical profiles of CO and ozone. A small, commercially available cavity ring-down spectroscopy (CRDS) NO2 detector suitable for surface and aircraft monitoring was modified and characterized. The CRDS detector was run in parallel to an ozone chemiluminescence device with photolytic conversion of NO2 to NO. The two instruments measured ambient air in suburban Maryland. A linear least- squares fit to a direct comparison of the data resulted in a slope of 0.960±0.002 and R of 0.995, showing agreement between two measurement techniques within experimental uncertainty. The sensitivity of the Community Multiscale Air Quality (CMAQ) model to the temporal variation of four emissions sectors was investigated to understand the effect of emissions' daily variability on modeled ozone. Decreasing the variability of mobile source emissions changed the 8-hour maximum ozone concentration by ±7 parts per billion by volume (ppbv). Increasing the variability of point source emissions affected ozone concentrations by ±6 ppbv, but only in areas close to the source. CO is an ideal tracer for analyzing pollutant transport in AQMs because the atmospheric lifetime is longer than the timescale of bound- ary layer mixing. CO can be used as a tracer if model performance of CO is well understood. An evaluation of CO model performance in CMAQ was carried out using aircraft observations taken for the Regional Atmospheric Measurement, Mod- eling and Prediction Program (RAMMPP) in the summer of 2002. Comparison of modeled and observed CO total columns were generally in agreement within 5-10%. There is little evidence that the CO emissions inventory is grossly overestimated. CMAQ predicts the same vertical profile shape for all of the observations, i.e. CO is well mixed throughout the boundary layer. However, the majority of observations have poorly mixed air below 500 m, and well mixed air above. CMAQ appears to be transporting CO away from the surface more quickly than what is observed. Turbulent mixing in the model is represented with K-theory. A minimum Kz that scales with fractional urban land use is imposed in order to account for subgrid scale obstacles in urban areas and the urban heat island effect. Micrometeorological observations suggest that the minimum Kz is somewhat high. A sensitivity case where the minimum Kz was reduced from 0.5 m2/s to 0.1 m2/s was carried out. Model performance of surface ozone observations at night increased significantly. The model better captures the observed ozone minimum with slower mixing, and increases ozone concentrations in the residual layer. Model performance of CO and ozone morning vertical profiles improves, but the effect is not large enough to bring the model and measurements into agreement. Comparison of modeled CO and O3 vertical profiles shows that turbulent mixing (as represented by eddy diffusivity) appears to be too fast, while convective mixing may be too slow.
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    Assimilation of trace gas retrievals with the Local Ensemble Transform Kalman Filter
    (2009) Kuhl, David Derieg; Kalnay, Eugenia; Szunyogh, Istvan; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Over the 50 year history of Numerical Weather Prediction (NWP), the focus has been on the modeling and prediction of meteorological parameters such as surface pressure, temperature, wind, and precipitation. However, due to concerns over pollution and to recent advancements in satellite technologies, an increasing number of NWP systems have been upgraded to include capabilities to analyze and predict the concentration of trace gases. This dissertation explores some of the specific issues that have to be addressed for an efficient modeling of the concentration of the trace gases. These issues include modeling the effects of convective mixing on the concentration of the trace gases and the multivariate assimilation of space-based observations of the concentration of the trace gases. In this dissertation, we assimilate observations of the concentration of trace gases with an implementation of the Local Ensemble Transform Kalman Filter (LETKF) data assimilation system on the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) NWP model. We use a modified version of the NCEP GFS model that was operational in 2004 at resolution T62/L28. We modify the model by adding parameterization for the process of convective mixing of the trace gases. We consider two specific trace gases: ozone (O3) and carbon monoxide (CO). We incorporate these gases into the model by using 3-dimensional time-dependent O3 and CO production-loss values from the Real-time Air Quality Modeling System (RAQMS) global chemical model. The O3 observations we assimilate are from the Solar Backscatter UltraViolet generation 2 (SBUV/2) satellite instrument (version 8) flown on the NOAA 16 and 17 satellites. The CO observations we assimilate are from the Measurements Of Pollution In The Troposphere (MOPITT) instrument (version 3) flown on the NASA TERRA satellite. We also develop a new observation operator for the assimilation of retrievals with the LETKF. We carry out numerical experiment for the period between 000UTC 1 July 2004 to 000UTC 15 August in the summer of 2004. The analysis and forecast impact of the assimilation of trace gas observations on the meteorological fields is assessed by comparing the analyses and forecasts to the high resolution operational NCEP GFS analyses and to radiosonde observations. The analysis and forecast impact on the trace gas fields is assessed by comparing the analyzed and predicted fields to observations collected during the Intercontinental Chemical Transport Experiment (INTEX-A) field mission. The INTEX-A field mission was conducted to characterize composition of pollution over North America, thus providing us with ozonesonde and aircraft based verification data. We find that adding the process of convective mixing to the parameterization package of the model and the assimilation of observations of the trace gases improves the analysis and forecast of the concentration of the trace gases. In particular, our system is more accurate in quantifying the concentration of O3 in the troposphere than the original NCEP GFS. Also, our system is competitive with the state-of-the-art RAQMS atmospheric chemical model in analyzing the concentration of O3 and CO throughout the full atmospheric model column. The assimilation of O3 and CO observations has a mixed impact on the analysis and forecast of the meteorological fields. We find that most of the negative impact on the meteorological fields can be eliminated, without much reduction to the positive impact on the trace gas fields, by inflating the prescribed variance of the trace gas observations. The appendices of this dissertation reproduces two papers on related research. The first paper covers the northward front movement and rising surface temperatures in the great planes. The second paper covers the assessment of predictability with a Local Ensemble Kalman Filter.