Zhou G. Computational Study of the Source-Area Effect for Bubble Plumes in Stratified Environments. Journal of Hydraulic Engineering [Internet]. 2020;146 (6). Publisher's VersionAbstract
In this work, a large-eddy simulation of bubble plumes in linearly stratified environments is presented. The gas bubbles are treated as Lagrangian particles. The intrusion and peeling are clearly manifested in the computed flow fields. The results of about 50 simulations with different parameters reveal the importance of bubble source area for plumes on the laboratory scale. A new type of bubble plume with rapid and distinct peelings is observed which is favored by large source areas. With a proper normalization, the present data points collapse onto a single straight line after applying a virtual-source correction which reflects the source-area effect. These results provide a plausible explanation for the scatter of the previous experimental and computational data in literature. A simple relation between the trap height and the peel height is observed and its mechanism is discussed
Ram O, Katz J. Persistent thin water films encapsulate oil droplets crossing an oil-water interface. Chemical Engineering Journal [Internet]. 2020;387 (124075). Publisher's VersionAbstract
Understanding of oil-water interfacial phenomena is essential for predicting mixing and/or phase-separation in environmental and industrial systems. Time-resolved digital holography and planar laser-induced fluorescence are used for examining processes occurring after ascending buoyant oil droplets of varying viscosity cross a stratified oil-water interface. Previous studies have focused on the crossing process, and the current belief is that once this droplet becomes immersed in the oil, its content can mix with the bulk fluid. In contrast, we show that the droplets remain encapsulated by a stable continuous submicron thin water film, which prevent them from mixing. This film forms even in pure oil and water with minimal surfactant concentration and persists for periods that are three to four orders of magnitude longer than those of the crossing process. Observations following the film evolution reveal that segments located close to the interface appear to be attracted to the bulk water, causing the entire droplet to flatten slowly. The resulting reduction in the peripheral radius of curvature
eventually breaks up the film into suspended submicron droplets. The morphology of this flattening process varies with oil viscosity, and its duration increases from seconds to nearly one hour as the oil viscosity increases from one to fifty cSt. In processes involving multiple oil droplets crossing the interface, they form a separate persistent long-lasting layer containing a complex thin-film structure that does not mix with the bulk oil. In contrast, thin oil films do not form around a descending water droplet after it crosses the interface.
Park BS, Buskey EJ. The potential impact of bacterial communities exposed to crude oil and light on the growth of the harmful algal blooming species Karenia brevis (Dinophyceae) . Marine and Freshwater Research [Internet]. 2020. Publisher's VersionAbstract
Phytoplankton blooms have been occasionally observed to occur after oil spills, and changes in bacterial communities (BC) associated with phytoplankton are known to affect phytoplankton growth. In the present study, to examine the effects of BC exposed to crude oil on phytoplankton blooms, established free-living (FL) BC in Karenia brevis (Dinophyceae) culture were collected and then exposed to crude oil under light or dark conditions. These exposed FLBC were then added to K. brevis culture to investigate the effects on growth rate of this dinoflagellate. Enhanced growth of K. brevis was observed following addition of FL (24.7%) BC exposed to crude oil and light. Whereas BC grown with crude oil in the dark did not enhance growth, and BC without treatment showed a slight growth inhibition (13–15%) of K. brevis. In addition, the growth-promoting effect had a positive correlation with the inoculated bacterial density; the treatment with a higher (,1.5 times) density of FL (42.9%) BC that were exposed to crude oil and light showed an increase in the growth-promoting effect. Taken together, BC exposed to crude oil and light may play an important role in enhancement of K. brevis growth.
White A, Jalali M, Boufadel M, Sheng J. Bacteria forming drag-increasing streamers on a drop implicates complementary fates of rising deep-sea oil droplets. Scientific Reports [Internet]. 2020;10 (4305). Publisher's VersionAbstract
Competing time scales involved in rapid rising micro-droplets in comparison to substantially slower biodegradation processes at oil-water interfaces highlights a perplexing question: how do biotic processes occur and alter the fates of oil micro-droplets (<500 μm) in the 400 m thick Deepwater Horizon deep-sea plume? For instance, a 200 μm droplet traverses the plume in ~48 h, while known biodegradation processes require weeks to complete. Using a microfluidic platform allowing microcosm observations of a droplet passing through a bacterial suspension at ecologically relevant length and time scales, we discover that within minutes bacteria attach onto an oil droplet and extrude polymeric streamers that rapidly bundle into an elongated aggregate, drastically increasing drag that consequently slows droplet rising velocity. Results provide a key mechanism bridging competing scales and establish a potential pathway to biodegradation and sedimentations as well as substantially alter physical transport of droplets during a deep-sea oil spill with dispersant.
Brock L, Sheng J. Robust Fabrication of Polymeric Nanowire withAnodic Aluminum Oxide Templates. Micromachines [Internet]. 2019;11 (46). Publisher's VersionAbstract
Functionalization of a surface with biomimetic nano-/micro-scale roughness (wires) has attracted significant interests in surface science and engineering as well as has inspired many real-world applications including anti-fouling and superhydrophobic surfaces. Although methods relying on lithography include soft-lithography greatly increase our abilities in structuring hard surfaces with engineered nano-/micro-topologies mimicking real-world counterparts, such as lotus leaves, rose petals, and gecko toe pads, scalable tools enabling us to pattern polymeric substrates with the same structures are largely absent in literature. Here we present a robust and simple technique combining anodic aluminum oxide (AAO) templating and vacuum-assisted molding to fabricate nanowires over polymeric substrates. We have demonstrated the ecacy and robustness of the technique by successfully fabricating nanowires with large aspect ratios (>25) using several common soft materials including both cross-linking polymers and thermal plastics. Furthermore, a model is also developed to determine the length and molding time based on nanowires material properties (e.g., viscosity and interfacial tension) and operational parameters (e.g., pressure, vacuum, and AAO template dimension). Applying the technique, we have further demonstrated the confinement e ects on polymeric crosslinking processes and shown substantial lengthening of the curing time.
Chu S, Prosperetti A. Multiphase buoyant plumes with soluble drops or bubbles. Physical Review Fluids. 2019;4 (084306).Abstract

This paper presents the results of a scaling study of bubble and drop plumes in a stratified ambient. Use is made of a standard integral model of the top-hat type, which can be reduced to one of the Gaussian type by a simple transformation. The focus of the work is on the effects of the dissolving material on the plume dynamics. It is pointed out that, for a drop plume, the loss of buoyancy due to dissolution can be compensated by a lightening of the ambient liquid associated with the dissolved drop material, or even aggravated if the density of the solution is greater than that of the undissolved drops. For bubbles, these effects are compounded by the volume expansion due to the falling hydrostatic pressure. This process is not important in deep water, where the peel height is smaller than the water depth, but can be significant in shallow water, where the two may be comparable. With a focus on the analysis of a point-source, three important parameters are identified. The first one compares the drop/bubble dissolution rate with the rise time to the neutral height (the level at which the plume density equals the ambient density), the second one accounts for the effect of the dissolved material on the liquid density, and the third one is the drop/bubble rise velocity compared with the characteristic plume velocity.

Chandrala LD, Afshar-Mohajer N, Nishida K, Ronzhes Y, Sidhaye VK, Koehler K, Katz J. A Device for measuring the in-situ response of Human Bronchial Epithelial Cells to airborneenvironmental agents. Scientific Reports [Internet]. 2019;9 (7263). Publisher's VersionAbstract



Measuring the time evolution of response of Normal Human Bronchial Epithelial (NHBE) cells to aerosols is essential for understanding the pathogenesis of airway disease. This study introduces a novel Real-Time Examination of Cell Exposure (RTECE) system, which enables direct in situ assessment of functional responses of the cell culture during and following exposure to environmental agents. Included are cell morphology, migration, and specialized responses, such as ciliary beat frequency (CBF). Utilizing annular nozzles for aerosol injection and installing windows above and below the culture, the cells can be illuminated and examined during exposure. The performance of RTECE is compared to that of the commercial Vitrocell by exposing NHBE cells to cigarette smoke. Both systems show the same mass deposition and similar trends in smoke-induced changes to monolayer permeability, CBF and transepithelial resistance. In situ measurements performed during and after two exposures to smoke show that the CBF decreases gradually during both exposures, recovering after the first, but decreasing sharply after the second. Using Particle image velocimetry, the cell motions are monitored for twelve hours. Exposure to smoke increases the spatially-averaged cell velocity by an order of magnitude. The relative motion between cells peaks shortly after each exposure, but remains elevated and even increases further several hours later.

Chandrala_et_al _A_device_for_measuring_the_in-situ_response._may_2019.pdf
Uttieri M, Nihongi A, Hinow P, Motschman J, Jiang H, Alcaraz M, Strickler RJ. Copepod manipulation of oil droplet size distribution. Scientific Reports [Internet]. 2019;9 (547). Publisher's VersionAbstract
Oil spills are one of the most dangerous sources of pollution in aquatic ecosystems. Owing to their pivotal position in the food web, pelagic copepods can provide crucial intermediary transferring oil between trophic levels. In this study we show that the calanoid Paracartia granican actively modify the size-spectrum of oil droplets. Direct manipulation through the movement of the feeding appendages and egestion work in concert, splitting larger droplets (Ø = 16 µm) into smaller ones (Ø = 4–8 µm). The copepod-driven change in droplet size distribution can increase the availability of oil droplets to organisms feeding on smaller particles, sustaining the transfer of petrochemical compounds among different compartments. These results raise the curtain on complex small-scale interactions which can promote the understanding of oil spills fate in aquatic ecosystems.
Afshar-Mohajer N, Fox MA, Koehler K. The human health risk estimation of inhaled oil spill emissions with and without adding dispersant. Science of the Total Environment [Internet]. 2019;654 :924-932. Publisher's VersionAbstract

Airborne toxic compounds emitted from polluted seawater polluted after an oil spill raise health concerns when inhaled by humans or other species. Inhalation of these toxic compounds as volatile organic compounds (VOCs) or airborne fine particulate matter (PM) may cause serious pulmonary diseases, including lung cancer. Spraying chemical dispersants to enhance distribution of the crude oil into the water was employed extensively during the Deepwater Horizon spill. There is some evidence that dispersion of the crude oil decreased the emission rate of the VOCs but increased the emission rates of fine PM that may carry toxic compounds. In this study, the cancer risks and non-cancer hazards of the detected VOCs and particulates for spill-response workers were estimated with and without use of dispersant under action of breaking waves. A subchronic exposure scenario was modeled to address the inhalation health threat during initial phases of an oil spill response. A dosimetry model was used to estimate regional deposition of PM. Use of dispersant reduced benzene cancer risks from 57 to 37 excess life- time cancer cases per million for 1 h of daily exposure that continues for 3 months. Adding dispersant resulted in emissions reductions of the lighter VOCs (up to 30% lower). However, hazard quotients (HQs) of the non- carcinogenic VOCs even after dispersant addition were above 1 meaning there are serious concerns about expo- sure to these VOCs. Inhalation of airborne particles emitted from the slick containing dispersant increased the total mass of deposited particles in upper respiratory regions compared to the slick of crude oil only. This study showed the application of dispersant onto the pollution slick increased the total mass burden to the human respiratory system about 10 times, an exploratory HQ analysis is presented to evaluate the potential health risk.


Wei Z, Li C, Dalrymple RA, Derakhti M, Katz J. Chaos in breaking waves. Coastal Engineering [Internet]. 2018;(140) :272-291. Publisher's VersionAbstract
This study investigates the chaotic behavior of breaking waves by laboratory experiments and numerical modeling. Repeated laboratory runs with different initial velocity perturbations show that the wave profile before the wave breaks can be accurately reproduced, but the subsequent breaking process varies among runs, indicating the lack of repeatability of breaking waves in reality. Numerical simulations based on the Smoothed Particle Hydrodynamics method are further carried out to examine the repeatability of wave breaking process.
Consistent with the laboratory observation, multiple numerical simulations with variations in initial conditions present highly repeatable velocity field and free surface profile in the potential flow region but considerable variation at the breaking and post-breaking processes. Comparison also shows that 3D vortex structures induced by breaking waves are different among cases. Analysis of particle trajectory reveals that there is a similar trajectory thus a minor trajectory divergence among particles that are initially located at the pre-breaking region and the flume bottom, which are not directly impacted by the breaking process. However, a much more significant particle trajectory divergence is observed among particles that are initially located at the wave-splash region and the bore propagation region. The rate of divergence of particle trajectory under breaking waves is further examined by computing the Lyapunov exponent, a widely used indicator of chaos. This study reveals that different initial velocity perturbations lead to variations of near-surface velocity at the onset of wave breaking, which eventually cause the development of drastically different breaking wave jets and splashes. Therefore, the process of wave breaking, like many other dynamic processes in nature, exhibits a chaotic behavior.
Vaccari L, Molaei M, Leheny RL, Stebe KJ. Cargo carrying bacteria at interfaces. Soft Matter [Internet]. 2018;(14) :5643-5653. Publisher's VersionAbstract
The displacements of ensembles of colloids at the interface between oil and suspensions of the bacterium Pseudomonas aeruginosa PA14DpelA indicate enhanced colloid mobilities and apparently diffusive motion driven by interactions with the bacteria. However, inspection of individual trajectories of B500 particles reveals prolonged, directed displacements inconsistent with purely hydrodynamic interactions between swimming bacteria and colloids. Analysis of the properties of colloid paths indicates trajectories can be sorted into four distinct categories, including diffusive, persistent, curly, and mixed trajectory types. Non-diffusive trajectories are the norm, comprising 2/3 of the observed trajectories. Imaging of colloids in the interface reveals anisotropic assemblies formed by colloids decorated with one or more adhered bacteria that drive the colloids along these paths. The trajectories and enhanced transport result from individual colloids being moved as cargo by these adhered bacteria. The implications of these structures and open questions for interfacial transport are discussed and related to the active colloid literature. 
Seeley Evans M, Wang Q, Bacosa H, Rosenheim BE, Liu Z. Environmental petroleum pollution analysis using ramped pyrolysis-gas chromatography-mass spectrometry. Organic Geochemistry [Internet]. 2018;124 :180-189. Publisher's VersionAbstract
In response to the Deepwater Horizon oil spill, critical research has tracked the changes in petroleum hydrocarbons with environmental weathering. There are limitations, however, whereby single analytical techniques cannot always identify the wide breadth of petroleum and petroleum-derived compounds. We explore the analytical capabilities of ramped pyrolysis-gas chromatography–mass spectrometry (Py-GC–MS) to evaluate environmental samples of petroleum hydrocarbons from the Deepwater Horizon oil spill. We show that bulk flow Py-GC–MS can quantify the overall degree of petroleum hydrocarbon weathering. Furthermore, thermal slicing Py-GC–MS can quantify specific compounds in the “thermal desorption zone” (50–370 °C), as well as characterize pyrolyzed fragments from non-GC-amenable petroleum hydrocarbons (including oxygenated hydrocarbons) in the “cracking zone” (370–650 °C). Our data also suggest an increase in thermochemical stability, concentration of oxygenated products and complexity of high molecular weight and/or polar components with advanced weathering. This analysis not only elucidates weathering trends in Deepwater Horizon oil over several years, but also illustrates the analytical capacity of this method for future petroleum hydrocarbon investigations, filling a void in research connecting Py-GC–MS and environmentally weathered oil samples.
Almeda R, Cosgrove S, Buskey EJ. Oil Spills and Dispersants Can Cause the Initiation of Potentially Harmful Dinoflagellate Blooms (“Red Tides”). Environmental Science & Technology [Internet]. 2018. Publisher's VersionAbstract
ABSTRACT: After oil spills and dispersant applications the formation
of red tides or harmful algal blooms (HABs) has been observed, which
can cause additional negative impacts in areas affected by oil spills.
However, the link between oil spills and HABs is still unknown. Here,
we present experimental evidence that demonstrates a connection
between oil spills and HABs. We determined the effects of oil,
dispersant-treated oil, and dispersant alone on the structure of natural
plankton assemblages in the Northern Gulf of Mexico. In coastal
waters, large tintinnids and oligotrich ciliates, major grazers of phytoplankton, were negatively affected by the exposure to oil and dispersant, whereas bloom-forming dinoflagellates (Prorocentrum texanum, P. triestinum, and Scrippsiella trochoidea) notably increased their concentration. The removal of key grazers due to oil and dispersant disrupts the predator−prey controls (“top-down controls”) that normally function in plankton food webs. This disruption of grazing pressure opens a “loophole” that allows certain dinoflagellates with higher tolerance to oil and dispersants than their grazers to grow and form blooms when there are no growth limiting factors (e.g., nutrients). Therefore, oil spills and dispersants can act as disrupters of predator−prey controls in plankton food webs and as indirect inducers of potentially harmful dinoflagellate blooms.
Zhao L, Boufadel M, King T, Robinson B, Conmy R, Lee K. Impact of particle concentration and out-of-range sizes on the measurements of the LISST. Measurement Science and Technology [Internet]. 2018;29 (5). Publisher's VersionAbstract
The instrument LISST (laser in situ scattering and transmissiometry) has been widely used for measuring the size of oil droplets in relation to oil spills and sediment particles. Major concerns associated with using the instrument include the impact of high concentrations and/or out-of-range particle (droplet) sizes on the LISST reading. These were evaluated experimentally in this study using monosized microsphere particles. The key findings include: (1) When high particle concentration reduced the optical transmission (OT) to below 30%, the measured peak value tended to underestimate the true peak value, and the accuracy of the LISST decreased by ~8% to ~28%. The maximum concentration to reach the 30% OT was about 50% of the theoretical values, suggesting a lower concentration level should be considered during the instrument deployment. (2) The out-of-range sizes of particles affected the LISST measurements when the sizes were close to the LISST measurement range. Fine below-range sizes primarily affected the data in the lowest two bins of the LISST with  >75% of the volume at the smallest bin. Large out-of-range particles affected the sizes of the largest 8–10 bins only when very high concentration was present. The out-of-range particles slightly changed the size distribution of the in-range particles, but their concentration was conserved. An approach to interpret and quantify the effects of the out-of-range particles on the LISST measurement was proposed.
Gemmell BJ, Bacosa HP, Dickey BO, Gemmell CG, Alqasemi LR, Buskey EJ. Rapid alterations to marine microbiota communities following an oil spill. Ecotoxicology [Internet]. 2018 :1-12. Publisher's VersionAbstract
Field data from the first several days after an oil spill is rare but crucial for our understanding of a spill’s impact on marine microbiota given their short generation times. Field data collected within days of the Texas City “Y” oil spill showed that exposure to crude oil can rapidly imbalance populations of marine microbiota, which leads to the proliferation of more resistant organisms. Vibrionales bacteria were up to 48 times higher than background concentrations at the most impacted sites and populations of the dinoflagellate Prorocentrum texanum increased significantly as well. Laboratory microcosm experiments with a natural plankton community showed that P. texanum grew significantly faster under oiled conditions but monocultures of P. texanum did not. Additional laboratory experiments with natural communities from Tampa Bay, Florida showed similar results although a different species dominated, P. minimum. In both cases, tolerance to the presence of crude oil was enhanced by higher sensitivity of grazers led to a release from grazing pressure and allows Prorocentrum species to dominate after an oil spill. The results suggest careful monitoring for Vibrionales and Prorocentrum during future spills would be beneficial given the potential implications to human health.
Afshar-Mohajer N, Li C, Rule A, Katz J, Koehler K. A laboratory study of particulate and gaseous emissions from crude oil and crude oil-dispersant contaminated seawater due to breaking waves. Atmospheric Environment [Internet]. 2018;179 :177-186. Publisher's VersionAbstract
Crude oil spill incidents occur frequently causing a verity of occupational, ecological and environmental problems. Dispersants are applied to enhance the dispersion rate of crude oil slicks into the water column. In this study, the aerosol size distribution from 10 nm to 20 μm, total particle-bound aromatic hydrocarbons (pPAH) and volatile organic compounds (VOCs) are measured in a 6 x 0.3 x 0.6m tank as plunging breaking waves entrain oil slicks. The experiments are performed for seawater with slicks of crude oil, crude oil-dispersant mixture and dispersant only. The measurements investigate the effects of wave energy and slick properties on the temporal evolution of the emissions. The total number concentrations of particles originating from the oil dispersant
mixture are 1–2 orders of magnitude higher than those of crude oil across the entire nano-scale range, reaching 100x for 20 nm particles. Conversely, the differences in concentration are small in the micron range. The average concentrations of pPAH are variable but similar (150–270 ng/m3). The VOC concentrations for crude oil-dispersant mixtures are 2–3 times lower than those of crude oil, presumably due to the surfactant effect on mass diffusion. The drastic increase in ultrafine particle concentrations may raise concerns about effects of inhalation by cleanup workers and downstream communities though VOC emissions reduce. Findings through this study provide insight into how the spray of dispersant may change the ratio of airborne particulate matter and VOC emissions from seawater due to natural processes. 
Li C, Miller J, Wang J, Koley SS, Katz J. Size Distribution and Dispersion of Droplets Generated by Impingement of Breaking Waves on Oil Slicks. Journal of Geophysical Research [Internet]. 2017 :7938-7957. Publisher's VersionAbstract
This laboratory experimental study investigates the temporal evolution of the size distribution of subsurface oil droplets generated as breaking waves entrain oil slicks. The measurements are performed for varying wave energy, as well as large variations in oil viscosity and oil-water interfacial tension, the latter achieved by premixing the oil with dispersant. In situ measurements using digital inline holography at two magnifications are applied for measuring the droplet sizes and Particle Image Velocimetry (PIV) for determining the temporal evolution of turbulence after wave breaking. All early (2–10 s) size distributions have two distinct size ranges with different slopes. For low dispersant to oil ratios (DOR), the transition between them could be predicted based on a turbulent Weber (We) number in the 2–4 range, suggesting that turbulence plays an important role. For smaller droplets, all the number size distributions have power of about 22.1, and for larger droplets, the power decreases well below 23. The measured steepening of the size distribution over time is predicted by a simple model involving buoyant rise and turbulence dispersion. Conversely, for DOR 1:100 and 1:25 oils, the diameter of slope transition decreases from1 mm to 46 and 14 mm, respectively, much faster than the We-based prediction, and the size distribution steepens with increasing DOR. Furthermore, the concentration of micron-sized droplets of DOR 1:25 oil increases for the first 10 min after entrainment. These phenomena are presumably caused by the observed formation and breakup oil microthreads associated with tip streaming.
Niepa THR, Vaccari L, Leheny R, Goulian M, Lee D, Stebe K. Films of Bacteria at Interfaces (FBI): Remodeling of Fluid Interfaces by Pseudomonas aeruginosa. Scientific Reports [Internet]. 2017;7. Publisher's VersionAbstract
Bacteria at fluid interfaces endure physical and chemical stresses unique to these highly asymmetric environments. The responses of Pseudomonas aeruginosa PAO1 and PA14 to a hexadecane-water interface are compared. PAO1 cells form elastic films of bacteria, excreted polysaccharides and proteins, whereas PA14 cells move actively without forming an elastic film. Studies of PAO1 mutants show that, unlike solid-supported biofilms, elastic interfacial film formation occurs in the absence of flagella, pili, or certain polysaccharides. Highly induced genes identified in transcriptional profiling include those for putative enzymes and a carbohydrate metabolism enzyme, alkB2; this latter gene is not upregulated in PA14 cells. Notably, PAO1 mutants lacking the alkB2 gene fail to form an elastic layer. Rather, they form an active film like that formed by PA14. These findings demonstrate that genetic expression is altered by interfacial confinement, and suggest that the ability to metabolize alkanes may play a role in elastic film formation at oil-water interfaces.
Brakstad OG, Almas I, Krause DF. Biotransformation of natural gas and oil compounds associated with marine oil discharges. Chemosphere [Internet]. 2017;182 :555-558. Publisher's VersionAbstract
Field data from the Deepwater Horizon (DWH) oil spill in the Gulf of Mexico (GoM) suggested that oxidation of gas compounds stimulated biodegradation of oil compounds in the deep sea plume. We performed experiments with local seawater from a Norwegian fjord to examine if the presence of dissolved gas compounds (methane, ethane and propane) affected biodegradation of volatile oil compounds, and if oil compounds likewise affected gas compound oxidation. The results from the experiment showed comparable oil compound biotransformation rates in seawater at 5 °C between seawater with and without soluble gases. Gas oxidation was not affected by the presence of volatile oil compounds. Contrary to DWH deep sea plume data, propane oxidation was not faster than methane oxidation. These data may reflect variations between biodegradation of oil and gas in seawater environments with different history of oil and gas exposure.
Chu S, Prosperetti A. Bubble plumes in a stratified environment: Source parameters, intrusion height and neutral height. Physical Review Fluids [Internet]. 2017;2 (10). Publisher's VersionAbstract
A cross-sectionally averaged model is used to study several aspects of the physics of a bubble plume rising in a stratified quiescent liquid. Scaling analyses for the peel height, at which the plume momentum vanishes, and the neutral height, at which its average density equals the ambient density, are presented. Contrary to a widespread practice in the literature, it is argued that the neutral height cannot be identified with the experimentally reported intrusion height. Recognizing this difference provides an explanation of the reason why the intrusion height is found so frequently to be much above predictions and brings the theoretical results in line with observations. The mathematical model depends on three dimensionless parameters, some of which are related to the inlet conditions at the plume source. Their influence on the peel and neutral heights is illustrated by means of physical considerations, scaling analyses, and numerical results.