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Browsing by Author "DeMott, Paul J., advisor"

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    Evidence for a biological control on emissions of marine ice nucleating particles: laboratory, field and modeling results
    (Colorado State University. Libraries, 2017) McCluskey, Christina Song, author; Kreidenweis, Sonia M., advisor; DeMott, Paul J., advisor; Collett, Jeffrey L., committee member; Pierce, Jeffrey R., committee member; Mykles, Donald L., committee member
    To view the abstract, please see the full text of the document.
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    Marine ice nucleating particles: sources, composition, emissions, and model parameterizations
    (Colorado State University. Libraries, 2023) Moore, Kathryn A., author; Kreidenweis, Sonia M., advisor; DeMott, Paul J., advisor; Farmer, Delphine K., committee member; Pierce, Jeffrey R., committee member; van den Heever, Susan C., committee member
    Sea spray aerosol has received increasing attention over the last decade as a source of ice nucleating particles (INPs) to the atmosphere. Sparse measurements in remote marine regions indicate both marine INP concentrations and ice nucleating efficiency are several orders of magnitude lower than those of mineral or soil dusts, which dominate the INP budget on a global scale. The Southern Ocean (SO) surrounding Antarctica is thought to be the only region where marine INPs are the predominant INP type due to its remoteness from continental and anthropogenic aerosol sources and persistent strong westerlies, although several recent studies have suggested this may also be true of the high Arctic seasonally or intermittently. INPs are critical for initiating cloud glaciation at temperatures warmer than ~-36 °C and can thus have an outsize effect on cloud phase and related climate feedbacks due to their relative scarcity. This is particularly true over the polar oceans, where low and mid-level mixed phase and supercooled clouds are ubiquitous and especially sensitive to aerosols due to the generally low background particle concentrations. The research presented here aimed to improve our understanding of the factors influencing marine INP emissions and the sources and composition of INPs in remote marine regions, as well as to evaluate and improve current INP model parameterizations. This was accomplished using observations made in the Southern Ocean, one of the few remaining pristine aerosol environments, during the Southern Ocean Cloud Radiation Aerosol Transport Experimental Study (SOCRATES) aircraft campaign on the NSF/NCAR G-V, and the second Clouds, Aerosols, Precipitation, Radiation and atmospherIc Composition Over the southeRN ocean (CAPRICORN-2) ship campaign on the R/V Investigator in 2018. Ambient observations were supplemented by measurements from the CHaracterizing Atmosphere-Ocean parameters in SOARS (CHAOS) mesocosm experiment in the new Scripps Ocean-Atmosphere Research Simulator (SOARS) wind-wave channel. CHAOS measurements allowed for isolation of the role of wind speed in marine INP production, which had not previously been characterized through controlled experiments. SOCRATES and CAPRICORN-2 are notable for collecting the first vertically resolved INP measurements over the Southern Ocean, including the first in situ observations in and above cloud in the region. Both aerosol and INP concentrations showed excellent agreement between G-V and R/V Investigator observations during overflights of the ship, supporting the use of such a multi-platform measurement approach for future campaigns interested in aerosol and INP vertical profiles. New techniques for estimating marine aerosol surface area and the number of particles >0.5 μm, key quantities often used in INP parameterizations, were developed based on lidar and nephelometer measurements. An additional parameterization for marine INPs is proposed, which uses both wind speed and activation temperature, and reduces bias compared to the existing parameterization based solely on temperature. Marine boundary layer (MBL) and above cloud INP concentrations from the same SOCRATES flight support the hypothesis suggested by several modeling studies that marine INPs dominate at low altitudes, and mineral dust becomes increasingly important with height. Unexpectedly, enhanced INP and aerosol iron concentrations, but low iron solubilities, were observed for samples collected south of 60 °S during CAPRICORN-2. Antarctica is suggested as a potential source of both biological and inorganic INPs to the Southern Ocean marine boundary layer through the emission of mineral and soil dusts from ice-free areas. Similar high latitude dust sources in Iceland and Svalbard have been observed to contribute to INPs in the Arctic atmosphere, and are anticipated to increase in importance as the climate warms.
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    Simulations of Arctic mixed-phase clouds using a new aerosol-linked ice nuclei parameterization in a prognostic ice prediction scheme
    (Colorado State University. Libraries, 2013) Carpenter, James Michael, author; Kreidenweis, Sonia M., advisor; DeMott, Paul J., advisor; Randall, David A., committee member; Eykholt, Richard, committee member
    Despite the nearly universally-accepted notion that the Arctic is one of the most important areas to fully understand in the face of a changing global climate, observations from the region remain sparse, particularly of clouds and aerosol concentrations and sources. Low-level, mixed-phase clouds in the Arctic are capable of remarkable persistence, lasting for several days when our knowledge of the Wegener-Bergeron-Findeisen (WBF) process suggests that complete conversion to ice, or glaciation, should occur much faster, within a couple of hours. Multiple attempts at simulating these long-lived, mixed-phase clouds have been unable to accurately reproduce all cloud properties observed, with a major consequence being poor representation of radiative transfer, with important consequences for long-term climate simulations. Recent observational campaigns have sought to characterize ice-nucleating particles (IN) not just in the Arctic, but around the planet. A product of these campaigns, the DeMott IN parameterization (DeMott et al., 2010) seeks to provide a means for accurately implementing IN concentration calculations in a global model using minimal, readily-available proxy measurements or estimates of number concentrations of particles having diameters larger than 0.5 microns. In this study, the performance of this parameterization is tested in a cloud-resolving model capable of high resolution simulations of Arctic mixed-phase boundary layer stratus clouds. Three mixed-phase cloud case studies observed during the Indirect and Semi-Direct Aerosol Campaign (ISDAC) and Mixed-Phase Arctic Cloud Experiment (M-PACE) are simulated with varying complexity in their cloud microphysical packages. The goal is to test the new aerosol-linked parameterization as well as the sensitivity of the observed clouds to ice nuclei concentrations. In an effort to increase the realism of the aerosol-cloud interactions represented in the cloud-resolving model, a new, simple prognostic scheme for the activation of ice nuclei is incorporated. The new scheme imposes a finite budget on potential ice nuclei, which are depleted through ice activation and growth, and can potentially be replenished by sublimating ice crystals. Results are contrasted with simulations in which no depletion of IN is assumed. In this study, we found that while the DeMott IN parameterization successfully predicted available IN concentrations within observational error, the model was unable to predict sufficiently high pristine ice concentrations for one of the case studies. There were likely issues with the model or initialization in this case. For two of the case studies, the model performed exceptionally well, predicting accurate ice number concentrations as well as cloud droplet concentrations, leading to reasonable predictions of downwelling longwave radiation at the surface. In all cases, the model failed to predict reasonable cloud ice water contents. In the future, tests of ice crystal habits and growth rates may improve microphysical representation and predicted ice water contents. Replenishment of scavenged ice nuclei via surface fluxes and long-range transport can be included in the simulations to increase realism, but more observations are needed to accurately quantify these effects.
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    ItemOpen 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 member
    Ice 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.