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Browsing by Author "DeMott, Paul, advisor"

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    Characteristics of atmospheric ice nucleating particles associated with biomass burning in the US: prescribed burns and wildfires
    (Colorado State University. Libraries, 2013) McCluskey, Christina S., author; Kreidenweis, Sonia, advisor; DeMott, Paul, advisor; Pierce, Jeffrey, committee member; Volckens, John, committee member
    Insufficient knowledge regarding the sources and number concentrations of atmospheric ice nucleating particles (INP) leads to large uncertainties in understanding the interaction of aerosols with cloud processes, such as cloud life time and precipitation rates. An increasingly important source of aerosol in the United States is biomass burning, particularly in the form of prescribed burns and wildfires in the southeastern and western U.S., respectively. Prior field and laboratory observations have suggested that biomass burning can be a source of INP. However, emissions from biomass burning are complex, varying with combustion efficiency, fuel type, plume age and dilution. Thus, this potentially important source of INP is poorly characterized. This study utilizes measurements of INP from a diverse set of biomass burning events to better understand INP associated with biomass burning in the U.S. Prescribed burns in Georgia and Colorado, two Colorado wildfires and two laboratory burns were monitored for INP number concentrations (nINP) using the Colorado State University continuous-flow diffusion chamber (CFDC) to activate INP in the condensation/immersion freezing nucleation mode. Additional measurements included total particle number concentrations, number concentrations of particles with diameters larger than 500 nm, aerosol mass concentrations, carbon monoxide concentrations and chemically-speciated bulk aerosol filter samples. Additionally, activated INP were collected onto TEM grids downstream of the CFDC, isolating INP for single particle chemical and morphological analyses. These fires varied by fuel type, including wiregrass, longleaf pine and ponderosa pine, and also varied by combustion efficiency, ranging from highly flaming to a mixture of flaming and smoldering. Additionally, plume histories were different between the fires including aged plumes from the wildfires and freshly emitted smoke from the prescribed and laboratory burns. The relationship between nINP and total particle number concentrations, evident within prescribed burning plumes, was degraded within aged smoke plumes from the wildfires, limiting the utility of this relationship for comparing laboratory and field data. Larger particles, represented by n500nm, are less vulnerable to plume processing and have previously been evaluated for their relation to nINP. Our measurements indicated that for a given n500nm, nINP associated with the wildfires were nearly an order of magnitude higher than nINP found in prescribed fire emissions. That is, nINP represented a much larger fraction of n500nm in wildfires as compared with prescribed fires. Further, an existing parameterization for "global" nINP that relates INP abundance to n500nm largely under-predicted and over-predicted nINP emitted from wildfires and prescribed burns, respectively. Reasons for the differences between INP characteristics in these emissions were explored, including variations in combustion efficiency, fuel type, transport time and environmental conditions. Combustion efficiency and fuel type were eliminated as controlling factors by comparing samples with contrasting combustion efficiencies and fuel types. Transport time was eliminated because the expected impact would be to reduce n500nm, thus resulting in the opposite effect from the observed change. Bulk aerosol chemical composition analyses support the potential role of elevated soil dust particle concentrations during the fires, contributing to the population of INP, but the bulk analyses do not target INP composition directly. Predictions from the Naval Aerosol Analysis and Prediction System model further indicate elevated dust mass concentrations during the wildfire periods, suggesting impact of mineral dust from long-range transport (LRT). It is hypothesized that both hardwood burning and soil lofting are responsible for the elevated production of INP in the Colorado wildfires in addition to LRT of mineral dust. The chemical compositions of INP were probed directly via TEM imaging. Single particle analyses of residual INP showed that they comprised various C-containing particle types, but with a higher abundance of mineral and metal oxide containing INP in emissions from flaming phase combustion. Fractal soot was found as an INP type comprising up to 60% of collected INP in young smoke emissions from the Georgia prescribed burns. In a series of laboratory combustion experiments, the use of a new instrumental set up, pairing the CFDC with a single particle soot photometer, revealed up to a 60% decrease in active INP after the removal of refractory black carbon from smoke aerosol emitted from a highly flaming burn of wiregrass, supporting that soot particles serve as INP in fire emissions. The presence of soil minerals was clearly evident in TEM images of samples taken during the wildfires in addition to tarballs, carbon balls most commonly associated with aged smoke plumes. These results demonstrate that the ice nucleating particles observed in the wildfires were influenced by other factors not represented in the smoke emitted from the laboratory or prescribed burns. Finally, an INP parameterization was developed based on the temperature dependent relationship between nINP and n500nm, following methods used by previous studies. This parameterization is likely only representative of the Hewlett and High Park wildfires due to the apparent impact of non-biomass-burning aerosol. However, all wildfires are typically associated with vigorous localized convection and arid soils, required for the lofting of the soils and dusts similar to these wildfires. It will be useful to compare future wildfires in various regions to the proposed parameterization.
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    Constraining marine ice nucleating particle parameterizations in atmospheric models using observations from the Southern Ocean
    (Colorado State University. Libraries, 2020) Moore, Kathryn A., author; Kreidenweis, Sonia, advisor; DeMott, Paul, advisor; Farmer, Delphine, committee member; Pierce, Jeffrey, committee member
    The limited anthropogenic and terrestrial aerosol sources impacting the Southern Ocean (SO) make it a unique site to study the production of primary sea spray aerosols (SSA) and their role in modifying cloud properties. Previous observations of low ice nucleating particle (INP) concentrations and recent modeling work support the idea that the SO INP population is dominated by SSA. These marine INPs are hypothesized to strongly influence the lifetime, formation, and optical properties of the supercooled and mixed phase clouds that are common in the region, though direct observational evidence for this is lacking. This study focuses on improving our understanding of INP emissions in the marine boundary layer over the SO, with applicability to other ocean regions, and to provide in situ measurements with which to validate and improve INP parameterizations in global and cloud resolving models. Measurements of INPs and aerosols in the marine boundary layer were made during the Clouds, Aerosols, Precipitation Radiation and atmospherIc Composition Over the southeRN ocean 2 (CAPRICORN-2) study on the R/V Investigator during Jan. - March 2018. An initial focus of this thesis was on increasing speed and reproducibility of processing online INP measurements, as well as improving the determination of statistical significance and uncertainty bounds. Different approaches to parameterizing INPs in models are explored for SO aerosols, including the use of aerosol surface area and number concentrations. With an eye towards augmenting global datasets of INPs, a comparison of particle surface area measurements from four different techniques is presented, for use in developing and testing INP parameterizations for different sources and atmospheric conditions. Surface area concentrations derived from Wideband Integrated Bioaerosol Sensor (WIBS) and nephelometer observations are strongly correlated with direct particle size distribution measurements, and can be used in their stead. Uncertainty bounds for both techniques and a scaling factor for WIBS measurements are provided to aid in these estimates. INP concentrations observed during CAPRICORN-2 are very low across the entire temperature range measured (to -30 °C), even compared to previous measurements of marine-dominated airmasses. Unlike INPs from other sources, Southern Ocean marine INPs appear most correlated with accumulation, rather than coarse mode, particles, and are dominated by submicron particles. Commonly used relationships between coarse mode particle number and total aerosol surface area show no significant correlation with SO INP concentrations, indicating a different functional form or different independent variable may be needed to accurately parameterize marine INPs in models.
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    Fire and ice: analyzing ice nucleating particle emissions from western U.S. wildfires
    (Colorado State University. Libraries, 2019) Barry, Kevin Robert, author; Kreidenweis, Sonia, advisor; DeMott, Paul, advisor; Barnes, Elizabeth, committee member; Farmer, Delphine, committee member
    Wildfires in the western U.S. can have impacts on health and air quality and are forecasted to increase in the future. Some of the particles released from wildfires can affect cloud formation through serving as ice nucleating particles (INPs). INPs are necessary for heterogenous ice formation in mixed-phase clouds at temperatures warmer than about -38 °C and can have climate implications from radiative impacts on cloud phase and by affecting cloud lifetime. Wildfires have been shown to be a potential source of INPs from previous ground-based measurement studies, but almost no data exist at the free tropospheric level that is relevant for cloud formation. The Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN) campaign that was conducted in summer 2018 utilized the NSF/NCAR C-130 to sample many smoke plumes of various ages in the free troposphere and aged smoke in the boundary layer. INP measurements were made with the CSU Continuous Flow Diffusion Chamber (CFDC) and with aerosol filter collections to analyze offline with the CSU Ice Spectrometer (IS). The results presented in this thesis indicate a contribution of smoke to the INP number concentration budget over the plume-background air, but much variability exists in concentrations and in INP composition among fires. Treatments of the filter suspensions show a dominant organic influence in all plume filters analyzed while a biological INP population is evident in several cases. For the South Sugarloaf fire, which had a primary fuel of sagebrush shrubland, the highest INP concentrations of the campaign were measured, and the unique INP temperature spectrum suggests lofting of material from uncombusted plant material. Normalization of INP concentrations measured in WE-CAN confirms that smoke is not an especially efficient source of ice nucleating particles, however emissions impacts may still occur regionally. The determination of a Normalized Excess Mixing Ratio (NEMR) of INP emissions for the first time will permit modeling of such impacts, and possible INP in-plume production will be explored in future research.
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    Ice nucleating particles in the Arctic: measurement and source tracking
    (Colorado State University. Libraries, 2024) Barry, Kevin Robert, author; Kreidenweis, Sonia, advisor; DeMott, Paul, advisor; van den Heever, Susan, committee member; Fischer, Emily, committee member; Trivedi, Pankaj, committee member
    The Arctic landscape is rapidly changing in a warming climate, with sea ice melting and permafrost thawing. Its near-surface air temperature is warming 3.8 times faster than other regions around the world. This rapid warming is known as Arctic amplification. Clouds contribute to this amplification, with their presence and phase is important for determining the surface energy budget. Arctic mixed-phase clouds can last for several days but are not represented well in climate models. Special aerosols, called ice nucleating particles (INPs) trigger ice formation in the atmosphere at temperatures warmer than -38 °C, and thus are important for determining the initiation, lifetime, and radiative properties of these clouds. Observations of INPs, especially over the central Arctic, are limited, and many sources are unknown. This dissertation has the overarching goal of increasing understanding of Arctic INPs. This is achieved through first presenting a full year of INP measurements in the central Arctic, as well as a full year of their composition, using coincident sampling of bacteria and fungi to gain insight into airmass origin. Next, some of the potentially most active Arctic INP sources are explored. Permafrost, which was known previously to contain high levels of INPs, was tested for its activity and persistence in water, and ability to be aerosolized through bubble bursting over several weeks. Then, sources of INPs were surveyed in a region that is controlled by permafrost (a thermokarst landscape). This included field measurements of permafrost, vegetation, sediment, active layer soil, water, and aerosol samples. A high temperature heat test was developed as a diagnostic tool to differentiate sources. Coincidentally, clean working methods to measure INPs were optimized, as efforts to reduce contamination are needed to accurately sample in this region. The main findings from this work suggest a regionally relatively homogenous population of Arctic INPs at most times of year, which is encouraging for efforts to represent them in numerical models across scales and understand their changes in the future. Permafrost-sourced INPs showed high activity and were enhanced near the coast. Unexpectedly, other components of the thermokarst landscape were found to be rich, organic INP reservoirs, emphasizing that the Arctic tundra is a diverse collection of potential contributors to the aerosol.