Project Overview

Summary:

Dr. Giacomo Valerio Iungo at the University of Texas at Dallas’s School of Engineering & Computer Science was awarded an RFP-VI grant at $334,344 to conduct the RFP-VI project titled, “Transport of Aerosolized Oil Droplets in Marine Atmospheric Boundary Layer: Coupling Wind LiDAR Measurements and Large-Eddy Simulations”.  The project consisted of 1 other institution (University of Houston); 1 principal investigator (Iungo), 1 co-PI (Dr. Di Yang), 4 Ph.D. students (Meng Li, Yajat Pandya, Bahzad Najafi, Ze Zhao), 1 Masters student (Daniele Volongo); 1 undergraduate student (Louay Al-Hamidi); several research technicians and administration personnel.

The proposed research aims to develop a numerical tool for thorough predictions of production of aerosolized oil droplets at the sea-air interface, their transport within the marine atmospheric boundary layer (MABL), and deposition over a coastal region. The scientific goals of the proposed project are to: 1) understand the role of MABL structure and wave motion on aerosol generation at the sea-air interface; 2) investigate effects of transitioning from the ocean to the coast on aerosol concentration and distribution; 3) develop eddy-diffusivity models for regional meteorology algorithms by avoiding the typical assumption of a flat and homogeneous ocean surface and including a realistic wave motion. This project has been conducted through two interrelated tasks: one LiDAR measurement campaign performed at the Galveston Island State Park, TX, to generate unprecedented simultaneous and co-located observations of wind speed and aerosol concentration, while high-fidelity wind-wave coupled large-eddy simulations (LES) have been performed to investigate aerosol dynamics, and finally reproduce the LiDAR observations.

A plume originated from oil spill rises following complex dynamics, which are affected by the multiphase plume composition, ocean currents and turbulence, and ultimately reaches the ocean surface generating aerosol. Plume gases are released in the atmosphere through bubble bursting, while oil droplets are aerosolized due to various mechanisms occurring at the sea-air interface that are affected by multiple parameters, such as wind stress, wind turbulence, and wave dynamics. Once suspended in the atmosphere, aerosolized oil droplets are entrained within turbulent eddies and transported by the MABL. The aerosol residence time varies from days down to a fraction of a second, depending on the size and composition of the aerosol particles, velocity, and turbulence in the MABL. Furthermore, when advected by wind, oil aerosol behaves in a more complicated fashion than passive scalars, which is a consequence of its non-negligible inertia and settling motion. The prediction of production, transport and deposition of aerosolized oil droplets for different wind and ocean conditions is still a great challenge due to the complex multiscale and multiphase nature resulting from the interaction between the MABL wind field and sea-surface waves. The proposed research has contributed to answering a number of key questions, including: 1) What are the dominant physical parameters, such as wave wavelength, wave phase velocity, wind shear, turbulence, relative direction between wind velocity and wave propagation, dominating the variability in aerosol flux at the sea-air interface? 2) What are the effects on aerosol distribution due to the transition from the ocean to the coast? 3) How significant is including in a transport model the wave motion and the characteristics of MABL structure? An important outcome of this research has been a numerical model for thorough aerosol-transport predictions, which has been formulated based on unprecedented simultaneous co-located LiDAR measurements of wind speed and aerosol concentration, while an in-depth understanding of the dominating physical mechanisms governing aerosol dynamics has been achieved with high-fidelity wind-wave coupled LES.

Accurate predictions of production, transport and deposition of aerosol from the ocean surface to a coastal region can be advantageously leveraged to investigate transport of pollutants, their deposition and settling over a coastal area. An improved aerosol prediction model is important for a broad range of scientific and technological pursuits, such as for planning effective projects for environmental restoration. It is possible now to explore under which atmospheric and wave conditions high pollutant concentrations can be observed, and which areas can mainly be affected. The developed numerical tool is highly valuable to estimate the environmental impact of oil spill on the air quality, which is essential information for medical research projects investigating respiratory diseases, such as asthma, for which enhanced morbidity has been observed in the Gulf area. Graduate students have been trained in an interdisciplinary fashion through LES and experiments performed with the UTD mobile LiDAR station, which is a unique facility for broadening participation of students from underrepresented groups and making them aware of important topics, such as oil spill in the Gulf of Mexico, environmental protection and climate change. The project has included outreach activities to connect the research with students in the Dallas-Fort Worth metroplex area.

Research Highlights

Dr. Iungo and Dr. Yang’s research to date, which included 2 outreach activities, resulted in 1 peer-reviewed publication, 7 scientific presentations at international conferences, and 3 datasets submitted to the GoMRI Information and Data Cooperative (GRIIDC), which are available to the public.  Significant outcomes of their research (all related to GoMRI Research Theme 1) are highlighted below.

The LiDAR experiment at the Galveston Island State Park, TX, has allowed us to collect a very reach dataset of simultaneous and co-located measurements of wind turbulence and aerosol concentrations. A variety of scans was executed encompassing range-height indicator (RHI) scans over vertical planes, planar-position indicator (PPI) scans over horizontal planes, and high-frequency vertical transects with a spatial resolution lower than 1 meter. These data have allowed us to investigate the variability of the aerodynamic roughness length of the marine atmospheric boundary layer with the wind shear, wave characteristics, and aerosol flux from the sea surface. We have shown how the aerodynamic roughness is affected by non-linear mechanisms involving the wind shear-stress contributing to the wave generation, the form drag induced by the wave motion, and the kinetic energy absorbed by the marine aerosol suspended within the lower layer of the MABL. A semi-empirical model has been developed for prediction of the MABL aerodynamic roughness and a manuscript on this topic is currently under preparation. Another work based on this dataset has involved the correlation and spatial distribution of turbulent coherent structures and the aerosol concentration. This analysis will unveil how the turbulent structures and the distribution of aerosol affect mutually leading to complex, yet predictable, spatio-temporal organization of the MABL.

Another LiDAR experiment was performed at the Surface Layer Turbulence and Environmental Science Test (SLTEST), which is part of the U.S. Dugway Proving Ground facility in Utah. For this experiment, we deployed the UTD mobile LiDAR station simultaneously to multiple sonic anemometers and aerosol particle counters. This dataset will be instrumental to quantify the accuracy of Doppler wind LiDARs in measuring aerosol concentration for various LiDAR settings, wind and atmospheric conditions.

During the project, several outreach activities have been performed at UT Dallas and at several public libraries to disseminate the project and the related environmental problematics involving under-represented youth and student population, such as for the yearly engineering day at UT Dallas.

The numerical modeling effort at UH has focused on applying a high-fidelity LES model to simulate the airborne transport of oil droplets in the MABL. The model used in this study couples the LES of wind turbulence with instantaneous sea-surface wave field modeled using a high-order spectral method. A finite-volume plume transport model is implemented in the LES model framework to track the evolution of the oil aerosol concentration field. This model is successfully applied to simulate the oil droplet transport in 10 m/s wind over a calm sea surface and over a surface covered by a swell wave train. Four different oil droplet diameters are considered, including 2.5 microns, 40 microns, 60 microns, and 100 microns. The simulation results show considerable suspension and downwind transport of aerosols equal to or smaller than 40 microns in diameter, while the oil droplets of 60 and 100 microns exhibit noticeable deposition back to the ocean within the simulation domain due to the settling effect of suspended oil droplet caused by the gravity. When swells are present, their highly organized wave motions induced strong distortion to the lower portion of the MABL and enhanced the vertical turbulence fluctuation, resulting in enhanced aerosol suspension and wave-phase-correlated streamwise variations. The LES model is also applied to simulate aerosol transport in wind over three-dimensional (3D) broadband wavefield. The preliminary simulation results suggest that the 3D features of the sea-surface wavefield can significantly increase the turbulence intensity of the MABL and enhance the vertical suspension as well as downwind transport of the oil droplets.

In summer 2019 an outreach activity was performed at UH. In collaboration with the Cullen College of Engineering at UH, the project team organized a lab tour and delivered a tutorial presentation to three teams of high school students from the BP STEM Academy. The visiting students observed the aerosol transport in the UH low-speed wind tunnel facility, watched visualization videos produced based on the LES results, and had interactive discussions with the project team members.

 Proposal Abstract - RFP-VI PI Giacomo Iungo