UMD Theses and Dissertations
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Item EXAMINATION OF TROPOSPHERIC OZONE AND ITS PRECURSORS WITHIN AN AIR QUALITY MODEL AND IMPLICATIONS FOR AIR QUALITY AND CLIMATE(2021) Hembeck, Linda; Salawitch, Ross J; Canty, Timothy P; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Elevated levels of tropospheric ozone (O3) caused by emissions of NOx and VOCs negatively impact human health, crops, and ecosystems. Even if precursor emissions are reduced below current levels, predicted higher temperatures due to increased greenhouse gas emissions could impede resulting air quality benefits. Air quality models simulate the complex relationships that form O3 and are used to guide policy decisions directed at improving O3. The body of this work encompasses three projects related to improvements in the representation of O3 and precursors in air quality models. First, I examine the role of O3 and its precursors in air quality and climate change by evaluating ozone production efficiency (OPE) and O3 precursors within models. I modified a chemical mechanism and the emissions of NOx to accurately represent NOx, the reactivity of NOx with peroxy radicals, HCHO, isoprene, as well as organic and inorganic NOy reservoir species. Implementation of these modifications increased confidence in model simulations. Results indicate accepted inventories overestimated NOx emissions but underestimate total VOC reactivity and OPE. Second, I examined the dependence of surface O3 on temperature (climate penalty factor (CPF)) throughout a period of 11 years within an air quality model and measurements. Future increases in temperature could offset benefits from future reductions in the emission of O3 precursors. Determining and understanding the CPF is critical to formulating effective strategies to reduce future exceedances. I have demonstrated that the model can reproduce O3 sensitivity to temperature reasonably well. By controlling emissions specifically of NOx mankind has reduced its vulnerability. Third, I compare satellite-observed and modeled ammonia (NH3) under varying chemical environments over East Asia. Regulation of O3 precursor concentrations in the atmosphere has an indirect effect on NH3 concentrations. Air quality policy to reduce NOx and through that also nitric acid (HNO3) in the atmosphere can result in an increase in the concentration of NH3 because of its neutralizing ability. Therefore, a less acidic atmosphere sequesters less NH3. This preliminary work exposes different areas that need to be addressed to gain greater insight into NH3 emissions and chemistry.Item Air Pollutant Concentrations and Trends over the Eastern U.S. and China: Aircraft Measurements and Numerical Simulations(2012) He, Hao; Dickerson, Russell R.; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the last several decades, efforts have been made to mitigate air pollution all around the world. With surface observations showing substantial decrease of criteria pollutants, including O3, NOx, CO and SO2, the long-term aircraft measurements over the eastern U.S. provide a unique opportunity to study the trend of the air pollutant column contents and the regional transport in the free troposphere. Analyses of the historical data indicated ~2.0 Dobson Unit/decade decrease in tropospheric O3 columns over the eastern U.S. with a similar decreasing trend of CO. The statistical analysis also showed a significant decreasing trend for tropospheric SO2. Analyses of the EPA CEMS emission data showed parallel reductions. A case study of tropospheric O3 and SO2 over downwind area of Baltimore showed that the regional transport by westerly wind from Ohio and Pennsylvania play an important role in the local air quality issues. As the second largest economy in the world, China's rapid economic growth in the last decade lead to a dramatic increase in energy demand, which relied heavily on coal burning. The enormous amount of SO2 emissions caused severe environmental issues including acid deposition and particulate matter pollution. To mitigate these air quality problems, strict control measures and regulations were applied to abate sulfur emissions, especially before and during the 2008 Beijing Olympics. Aircraft measurements of tropospheric SO2 were conducted over central China in spring 2008, where intense measurements are lacking. A substantial amount of SO2 was observed in the free troposphere, which is important to regional transport and remote sensing. I successfully validated the SO2 columns with satellite retrievals, and proved that the new OMI SO2 algorithm performs better than the conventional algorithm. An emission inventory was evaluated through a combination of model simulations and satellite products. Between 2006 and 2008, the SO2 emissions had been reduced substantially over middle and eastern China. I also analyzed the model simulations, and find the SO2 lifetime is ~ 38 h during spring in China and that ~50% of Chinese emissions are exported to the western Pacific.Item RETRIEVAL OF TROPOSPHERIC AEROSOL PROPERTIES OVER LAND FROM INVERSION OF VISIBLE AND NEAR-INFRARED SPECTRAL REFLECTANCE: APPLICATION OVER MARYLAND(2007-04-26) Levy, Robert; Dickerson, Russell R.; Remer, Lorraine A; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Aerosols are major components of the Earth's global climate system, affecting the radiation budget and cloud processes of the atmosphere. When located near the surface, high concentrations lead to lowered visibility, increased health problems and generally reduced quality of life for the human population. Over the United States mid-Atlantic region, aerosol pollution is a problem mainly during the summer. Satellites, such as the MODerate Imaging Spectrometer (MODIS), from their vantage point above the atmosphere, provide unprecedented coverage of global and regional aerosols over land. During MODIS' eight-year operation, exhaustive data validation and analyses have shown how the algorithm should be improved. This dissertation describes the development of the 'second-generation' operational algorithm for retrieval of global tropospheric aerosol properties over dark land surfaces, from MODIS -observed spectral reflectance. New understanding about global aerosol properties, land surface reflectance characteristics, and radiative transfer properties were learned in the process. This new operational algorithm performs a simultaneous inversion of reflectance in two visible channels (0.47 and 0.66 μm) and one shortwave infrared channel (2.12 μm), thereby having increased sensitivity to coarse aerosol. Inversion of the three channels retrieves the aerosol optical depth (τ) at 0.55 μm, the percentage of non-dust (fine model) aerosol (η) and the surface reflectance. This algorithm is applied globally, and retrieves τ that is highly correlated (y = 0.02 + 1.0x, R=0.9) with ground-based sunphotometer measurements. The new algorithm estimates the global, over-land, long-term averaged τ ~ 0.21, a 25% reduction from previous MODIS estimates. This leads to reducing estimates of global, non-desert, over-land aerosol direct radiative effect (all aerosols) by 1.7 W·m-2 (0.5 W·m-2 over the entire globe), which significantly impacts assessment of aerosol direct radiative forcing (contribution from anthropogenic aerosols only). Over the U.S. mid-Atlantic region, validated retrievals of τ (an integrated column property) can help to estimate surface PM2.5 concentration, a monitored criteria air quality property. The 3-dimensional aerosol loading in the region is characterized using aircraft measurements and the Community Multi-scale Air Quality Model (CMAQ) model, leading to some convergence of observed quantities and modeled processes.