Browsing by Author "Leong, Yu Jun"

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    Apportioned primary and secondary organic aerosol during pollution events of DISCOVER-AQ Houston
    (Elsevier, 2021) Yoon, Subin; Ortiz, Stephanie M.; Clark, Adelaide E.; Barrett, Tate E.; Usenko, Sascha; Duvall, Rachelle M.; Ruiz, Lea Hildebrandt; Bean, Jeffrey K.; Faxon, Cameron B.; Flynn, James H.; Lefer, Barry L.; Leong, Yu Jun; Griffin, Robert J.; Sheesley, Rebecca J.
    Understanding the drivers for high ozone (O3) and atmospheric particulate matter (PM) concentrations is a pressing issue in urban air quality, as this understanding informs decisions for control and mitigation of these key pollutants. The Houston, TX metropolitan area is an ideal location for studying the intersection between O3 and atmospheric secondary organic carbon (SOC) production due to the diversity of source types (urban, industrial, and biogenic) and the on- and off-shore cycling of air masses over Galveston Bay, TX. Detailed characterization of filter-based samples collected during Deriving Information on Surface Conditions from Column and VERtically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) Houston field experiment in September 2013 were used to investigate sources and composition of organic carbon (OC) and potential relationships between daily maximum 8 h average O3 and PM. The current study employed a novel combination of chemical mass balance modeling defining primary (i.e. POC) versus secondary (i.e. SOC) organic carbon and radiocarbon (14C) for apportionment of contemporary and fossil carbon. The apportioned sources include contemporary POC (biomass burning [BB], vegetative detritus), fossil POC (motor vehicle exhaust), biogenic SOC and fossil SOC. The filter-based results were then compared with real-time measurements by aerosol mass spectrometry. With these methods, a consistent urban background of contemporary carbon and motor vehicle exhaust was observed in the Houston metropolitan area. Real-time and filter-based characterization both showed that carbonaceous aerosols in Houston was highly impacted by SOC or oxidized OC, with much higher contributions from biogenic than fossil sources. However, fossil SOC concentration and fractional contribution had a stronger correlation with daily maximum 8 h average O3, peaking during high PM and O3 events. The results indicate that point source emissions processed by on- and off-shore wind cycles likely contribute to peak events for both PM and O3 in the greater Houston metropolitan area.
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    Characterization of Atmospheric Nitrogen Chemistry and the Formation/ Evolution of Particulate Matter in Houston, TX
    (2015-12-04) Leong, Yu Jun; Griffin, Robert; Cohan, Daniel; Biswal, Sibani L
    This thesis covers laboratory experiments to study the homogeneous reduction of nitric acid (HNO₃) to nitrous acid (HONO) in the presence of volatile organic compounds that are surrogates for those emitted by motor vehicles. The results presented in this study focus on the impact of environmental variables on the rate of formation of HONO in this process. The homogeneous conversion of HNO₃ to HONO has significant atmospheric implications due to the “renoxification” of less reactive HNO₃ into more reactive HONO. Consecutively, this thesis describes particulate matter (PM) data collected from a month-long (September 2013) field project in Houston, TX. A mobile laboratory containing state-of-the-art PM instrumentation and auxiliary measurements was deployed. The main focus for the thesis work was to utilize this dataset to better characterize PM pollution in the city of Houston. This was achieved by several analysis approaches including cluster analysis, back-trajectory analysis, and principal component analysis to describe spatial and temporal variations in submicron PM in the Houston region. Finally, this work describes the use of a statistical source apportionment technique, positive matrix factorization, on the field dataset to apportion important constituents of atmospheric aerosols in Houston. This technique allowed the apportionment of four organic aerosol factors, two of which were associated with organic nitrates from biogenic sources. Submicron PM plume events from on-road, industrial, and biomass burning sources in Houston also were chemically characterized. Because sources of PM pollution are still poorly understood, particularly in the highly industrial and urban city of Houston, the results from this thesis will advance PM modeling capabilities and allow improved PM control strategies in polluted urban areas similar to Houston.
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    Composition and Sources of Particulate Matter Measured near Houston, TX: Anthropogenic-Biogenic Interactions
    (MDPI, 2016) Bean, Jeffrey K.; Faxon, Cameron B.; Leong, Yu Jun; Wallace, Henry William; Cevik, Basak Karakurt; Ortiz, Stephanie; Canagaratna, Manjula R.; Usenko, Sascha; Sheesley, Rebecca J.; Griffin, Robert J.; Ruiz, Lea Hildebrandt
    Particulate matter was measured in Conroe, Texas (~60 km north of downtown Houston, Texas) during the September 2013 DISCOVER-AQ campaign to determine the sources of particulate matter in the region. The measurement site is influenced by high biogenic emission rates as well as transport of anthropogenic pollutants from the Houston metropolitan area and is therefore an ideal location to study anthropogenic-biogenic interactions. Data from an Aerosol Chemical Speciation Monitor (ACSM) suggest that on average 64 percent of non-refractory PM1 was organic material, including a high fraction (27%–41%) of organic nitrates. There was little diurnal variation in the concentrations of ammonium sulfate; however, concentrations of organic and organic nitrate aerosol were consistently higher at night than during the day. Potential explanations for the higher organic aerosol loadings at night include changing boundary layer height, increased partitioning to the particle phase at lower temperatures, and differences between daytime and nighttime chemical processes such as nitrate radical chemistry. Positive matrix factorization was applied to the organic aerosol mass spectra measured by the ACSM and three factors were resolved—two factors representing oxygenated organic aerosol and one factor representing hydrocarbon-like organic aerosol. The factors suggest that the measured aerosol was well mixed and highly processed, consistent with the distance from the site to major aerosol sources, as well as the high photochemical activity.