Browsing by Author "Willis, Megan D., committee member"
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Item Open Access Climate model error in the evolution of sea surface temperature patterns affects radiation and precipitation projections(Colorado State University. Libraries, 2024) Alessi, Marc J., author; Rugenstein, Maria A.A., advisor; Barnes, Elizabeth A., committee member; Maloney, Eric D., committee member; Willis, Megan D., committee memberAtmosphere-ocean general circulation models (AOGCMs) are the primary tool climate scientists use in predicting the effects of climate change. While they have skill in reproducing global-mean temperature over the historical period, they struggle to replicate recently observed sea surface temperature (SST) trend patterns. In this dissertation, we quantify the impact of potential future model error in SST pattern trends on projections of global-mean temperature and Southwest U.S. (SWUS) precipitation. We primarily use a Green's function (GF) approach to identify which SST regions are most relevant for changes in these variables. Our findings demonstrate significant sensitivity of both global-mean temperature and SWUS precipitation to the pattern of sea surface warming, meaning that a continuation of AOGCM error in SST trend patterns adds uncertainty to climate projections which are currently not accounted for. In Chapter 1, we quantify the relevance of future model error in SST to global-mean temperature projections through convolving a GF with physically plausible SST pattern scenarios that differ from the ones AOGCMs produce by themselves. We find that future model error in the pattern of SST has a significant impact on projections, such as increasing total model uncertainty by 40% in a high-emissions scenario by 2085. A reversal of the current cooling trend in the East Pacific over the next few decades could lead to a period of global-mean warming with a 60% higher rate than currently projected. These SST pattern scenarios work through a destabilization of the shortwave cloud feedback to affect temperature projections. In Chapter 2, we focus on near-term projections of precipitation in the SWUS. The observed decrease in SWUS precipitation since the 1980s and heightened drought conditions since the 2000s have been linked to a cooling sea surface temperature (SST) trend in the Equatorial Pacific. Notably, climate models fail to reproduce this observed SST trend, and they may continue doing so in the future. In this chapter, we assess the sensitivity of SWUS precipitation projections to future SST trends using a GF approach. Our findings reveal that a slight redistribution of SST leads to a wetting or drying of the SWUS. A reversal of the observed cooling trend in the Central and East Pacific over the next few decades would lead to a period of wetting in the SWUS. In Chapter 3, we analyze SWUS precipitation sensitivity to SST patterns on long timescales (7+ years) according to a GF approach and a convolutional neural network (CNN) approach. The GF and CNN identify different SST regions as having greater influence on SWUS precipitation: the GF highlights the Central Pacific known from theory to be relevant, while the CNN highlights the South-Central Pacific. To determine if the South-Central Pacific has a physically meaningful and so far overlooked influence on SWUS precipitation, rather than just a statistical relationship, we force an atmosphere-only climate model with an SST anomaly inspired by an Explainable Artificial Intelligence (XAI) method. We find that SSTs in the South-Central Pacific influence SWUS precipitation through an atmospheric bridge dynamical pathway, justifying the CNN's sensitivity physically. The fact that we cannot fully trust the evolution of SST patterns in AOGCMs has many implications for the field of climate science and for how the world's governments and organizations respond to global warming. It is critical for climate change adaptation and mitigation assessments to consider this previously unaccounted for uncertainty in climate projections. Climate scientists can do this by developing SST pattern storylines based on theory, observations, and our understanding of the ocean-atmosphere system. If we fail to communicate known uncertainties for both global-mean and regional projections, the world could lose faith in the climate science community, resulting in less of a global response to climate change.Item Open Access Summertime ozone production at Carlsbad Caverns National Park, New Mexico: influence of oil and natural gas development(Colorado State University. Libraries, 2023) Marsavin, Andrey, author; Collett, Jeffrey L., Jr., advisor; Fischer, Emily V., committee member; Willis, Megan D., committee memberSoutheastern New Mexico's Carlsbad Caverns National Park (CAVE) has increasingly experienced summertime ground-level ozone (O3) levels surpassing the US Environmental Protection Agency's National Ambient Air Quality Standard (NAAQS) of 70 parts per billion by volume (ppbv). The park is located in the western part of the Permian oil and natural gas (O&G) basin, where production rates have more than tripled in the last decade. We investigate O3–precursor relationships by constraining a zero-dimensional (0-D) model to an hourly nitrogen oxides (NOx = NO + NO2) and speciated volatile organic compound (VOC) data set collected at CAVE during the summer of 2019. O&G-related VOCs dominated the calculated VOC reactivity with hydroxyl radicals (OH) on days when O3 concentrations were primarily controlled by local photochemistry. Radical budget analysis showed that NOx levels were high enough to impose VOC sensitivity on O3 formation in the morning hours, while subsequent NOx loss through photochemical consumption led to NOx-sensitive conditions in the afternoon. Daily maximum O3 was sensitive to both NOx and O&G-related VOC emission reductions, with NOx reductions generally being more effective. The model could not reproduce a 5-day high O3 episode when constrained to observed NOx and primary VOCs, likely due to influence from O3 produced during air mass transport from regional O&G basins as indicated by back-trajectory analysis, low i/n-pentane ratios consistent with O&G emissions, increased concentrations of secondary VOCs, and extensive oxidation of emitted NOx. Constraining the model with observed total oxidized reactive nitrogen (NOy), which approximates NOx at the time of emission, greatly improves model-observation agreement during this episode, reaffirming NOx-sensitive conditions in photochemically aged air masses.Item Open Access Using chemical ionization mass spectrometry to probe indoor and outdoor atmospheric chemistry(Colorado State University. Libraries, 2021) Mattila, James M., author; Farmer, Delphine K., advisor; Reynolds, Melissa M., committee member; Willis, Megan D., committee member; Carter, Ellison M., committee memberPeople spend the majority of their time in indoor environments. Knowledge of the sources, sinks, and chemistry of indoor pollutants is therefore imperative to indoor air quality and human health. We studied the indoor chemistry of cooking and cleaning at the House Observations of Microbial and Environmental Chemistry (HOMEChem) field campaign during summer 2018 at the University of Texas test house (UTest house) in Austin, TX. We performed measurements of several gas-phase cooking- and cleaning-related analytes using a fast (1 Hz), online chemical ionization mass spectrometry (CIMS) measurement technique utilizing iodide reagent ions. Combining these and other measurements of gas-phase analytes and particulate matter present in indoor air during HOMEChem enables us to piece together a holistic story of the indoor chemistry of cooking and cleaning. We observed enhanced levels of several chlorinated and nitrogenated compounds when cleaning indoors with a commercial bleach solution during HOMEChem. We observed production of several inorganic chlorinated and nitrogenated pollutants from bleaching, including hypochlorous acid, chlorine gas, and chloramines. Levels of hypochlorous acid and nitrogen trichloride observed during cleaning are likely detrimental to human health. Bleach cleaning indoors also lead to the production of secondary organic aerosol—a common outdoor atmospheric pollutant associated with respiratory and cardiovascular issues—as well as potentially harmful organic isocyanates, cyanogen chloride, and chlorocarbons. These results collectively demonstrate bleach cleaning as a source of indoor pollution which impacts indoor air quality and occupant health. We characterized indoor reactive organic carbon (ROC) emissions from cooking and cleaning during HOMEChem, and directly compared resultant chemical complexity of indoor air to outdoors. Cooking indoors greatly impacts ROC concentrations and physiochemical properties, and thus carbon reactivities and lifetimes. Cleaning indoors yielded relatively insubstantial changes. Consistently higher indoor ROC concentrations compared to outdoors demonstrated that indoor emissions were a net source of reactive carbon to the outdoor atmosphere, following their removal by ventilation. ROC dominated indoor and outdoor oxidant reactivity compared to other atmospheric carbon species, thereby greatly influencing secondary pollutant formation, including carbon dioxide, ozone, and secondary particulate matter. Most oxidation chemistry to produce these secondary pollutants likely took place outdoors following the ventilation of ROC species, given the low oxidant levels typical of indoor environments. Moving outdoors, we demonstrated the efficacy of a CIMS instrument utilizing acetate ionization toward quantifying various gas-phase acids in the troposphere. Here, we performed measurements during the Front Range Air Pollution and Photochemistry Experiment (FRAPPE) field campaign in summer 2014. Diurnal increases in mixing ratios were consistent with photochemical sources of nitric, isocyanic, formic, propionic, butyric, valeric, and pyruvic acid. Vertical profiles taken on the 300 m Boulder Atmospheric Observatory tower demonstrated net surface-level emissions of alkanoic acids, but net surface deposition of nitric and pyruvic acid. Nearby traffic emissions and agricultural activity were a primary source of propionic, butyric, and valeric acids, and likely contributed photochemical precursors to nitric and isocyanic acids. The combined diel and vertical profiles of the alkanoic acids and isocyanic acid were inconsistent with dry deposition and photochemical losses being the only sinks, suggesting additional loss mechanisms.Item Open Access Using laboratory and airborne measurements to investigate the role of ice nucleating particles in ice and mixed-phase clouds(Colorado State University. Libraries, 2023) Patnaude, Ryan John, author; Kreidenweis, Sonia M., advisor; DeMott, Paul J., advisor; van den Heever, Susan C., committee member; Chui, J. Christine, committee member; Willis, Megan D., committee memberIce may be present in the atmosphere either in cirrus or mixed-phase cloud regions, each with their own distinctly different characteristics and formation mechanisms. The former is characterized by the presence of only ice crystals at temperatures < -38 °C, while the latter includes the coexistence of both supercooled liquid cloud droplets and ice crystals between temperatures of 0 °C and -38 °C. Cirrus clouds represent an important cloud type as they are ubiquitous in the atmosphere and their radiative effects depend upon their microphysical properties. Their formation mechanisms may proceed via homogeneous or heterogeneous nucleation, and whether one or the other or both occur determines the size and number of ice crystals. The ocean represents one of the largest sources of aerosols into the atmosphere, and sea spray aerosols (SSA), if they are lofted to the upper troposphere, may act as ice nucleating particles (INPs) to initiate heterogeneous nucleation under cirrus conditions. Although a number of previous studies have investigated the ice nucleating behavior of SSA proxies such as sodium chloride (NaCl), or SSA generated from commercially-available artificial seawater products, ice nucleation under cirrus conditions of SSA generated from natural seawater had not been examined at the inception of this research program. Additionally, whether secondary marine aerosols (SMA), which form via the gas-to-particle conversion of ocean-emitted gas-phase species, may act as an INP in cirrus clouds is currently unknown. The first half of this dissertation highlights two laboratory studies that investigated the role and characteristics of SSA and SMA to act as INPs at cirrus cloud temperatures. The first study compared ice nucleation results for submicron SSA and NaCl particles and examined whether particle size affected the low temperature ice nucleation. Results showed that both SSA and NaCl initiated heterogeneous nucleation strongly at temperatures below 220 K, and that the size of the particles did not affect the ice nucleating ability of SSA. The similarities between the freezing behaviors of SSA and NaCl particles suggested the salt components were controlling heterogeneous ice nucleation. The second study used a more realistic aerosol generation method, utilizing a Marine Aerosol Reference Tank (MART) that was filled with natural seawater, and investigated the effects of atmospheric oxidation on SSA using an oxidation flow reactor (OFR), which was also used to generate SMA from gaseous emissions released in the MART. SMA alone were also examined for their ice nucleation behavior at cirrus temperatures. Results from this study indicated that atmospheric oxidation did not hinder low temperature ice nucleation of SSA, and that SMA are not efficient ice nucleating particles at cirrus temperatures, but could participate in homogeneous nucleation. Finally, the similarities between the findings from the two studies indicated that the generation method of SSA, and any impacts on SSA organic aerosol content, did not affect the ice nucleating behavior of SSA at cirrus temperatures. Ice in mixed-phase clouds (MPCs), on the other hand, forms initially via heterogeneous nucleation at a wide range of temperatures and relative humidity conditions, depending on the abundance and characteristics of available INPs. Secondary ice production (SIP) may follow heterogeneous nucleation in MPCs, where new ice crystals form either during the heterogeneous freezing event, or through subsequent interactions between the pre-existing liquid cloud droplets and ice crystals. SIP may lead to enhanced ice crystal number concentrations via a number of proposed mechanisms, especially in convective environments. Despite decades of study toward developing better understanding of ice formation in MPCs, the freezing pathways of ice crystals over the course of cloud lifetimes, and the conditions that favor the various proposed SIP pathways, are not fully resolved. The third study in this dissertation reports and interprets observations of INPs during an airborne campaign over the U.S. Central Great Plains during the Secondary Production of Ice in Cumulus Experiment (SPICULE) campaign that primarily sampled cumulus congestus clouds. Coincident measurements of INP and ice crystal number concentrations in cumulus congestus clouds were used to infer the ice formation pathway, either through heterogeneous nucleation or SIP. Warmer cloud base temperatures and faster updrafts were found to facilitate environmental conditions favorable for SIP. Further, the fragmentation of freezing droplets (FFD) SIP mechanism was found to be critical in the enhancement of observed ice crystal number concentrations during the earliest stages of the cloud lifetime. Numerical model simulations of an idealized, single congestus cloud, designed to mimic the clouds sampled during SPICULE, were conducted with newly-implemented SIP mechanisms, added to the existing Hallet-Mossop (HM) rime-splintering mechanism. The model results indicated that HM dominated the production of ice crystals, but without the FFD and ice-ice collisional breakup (BR) SIP mechanisms, the model could not accurately resolve ice crystal number concentrations compared to observations. Competing results in the dominant SIP mechanisms underscore the need for improved mechanistic understanding of these SIP processes, either through laboratory or observational studies, in order to close this gap between model prediction and observations. The final portion of this dissertation describes airborne observations of INPs during a field campaign along the U.S. Gulf Coast, also aimed at investigating the impacts of various aerosol-cloud interaction mechanisms on development of convective clouds. During this campaign, a widespread and prolonged Saharan Air Layer (SAL) event took place and INP characteristics during this event are reported and contrasted with INP characteristics prior to the arrival of the SAL. The INP concentrations at temperatures below -20 °C were enhanced by 1–2 orders of magnitude compared to the flights prior to the dust intrusion, and showed good agreement with one previous study of Saharan dust near Barbados, but lower INP concentrations than another study off the coast of western Africa. The INP concentrations in the SAL also generally overlapped with or exceeded INP concentrations during SPICULE, but only for INPs active temperatures < -25 °C. These observations were the first airborne measurements in nearly two decades tagging INP concentrations to North African dust that had been transported all the way to the United States. Further, they provide the most comprehensive description of these INPs yet recorded, and suggest a common natural INP perturbation in the southeastern U.S. and Gulf regions in early summer, with implications for cloud processes that warrant further study.