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Callaghan, AH, Deane GB, Stokes MD.  2017.  On the imprint of surfactant-driven stabilization of laboratory breaking wave foam with comparison to oceanic whitecaps. Journal of Geophysical Research-Oceans. 122:6110-6128.   10.1002/2017jc012809   AbstractWebsite

Surfactants are ubiquitous in the global oceans: they help form the materially-distinct sea surface microlayer (SML) across which global ocean-atmosphere exchanges take place, and they reside on the surfaces of bubbles and whitecap foam cells prolonging their lifetime thus altering ocean albedo. Despite their importance, the occurrence, spatial distribution, and composition of surfactants within the upper ocean and the SML remains under-characterized during conditions of vigorous wave breaking when in-situ sampling methods are difficult to implement. Additionally, no quantitative framework exists to evaluate the importance of surfactant activity on ocean whitecap foam coverage estimates. Here we use individual laboratory breaking waves generated in filtered seawater and seawater with added soluble surfactant to identify the imprint of surfactant activity in whitecap foam evolution. The data show a distinct surfactant imprint in the decay phase of foam evolution. The area-time-integral of foam evolution is used to develop a time-varying stabilization function, phi(t) and a stabilization factor, circle dot, which can be used to identify and quantify the extent of this surfactant imprint for individual breaking waves. The approach is then applied to wind-driven oceanic whitecaps, and the laboratory and ocean H distributions overlap. It is proposed that whitecap foam evolution may be used to determine the occurrence and extent of oceanic surfactant activity to complement traditional in-situ techniques and extend measurement capabilities to more severe sea states occurring at wind speeds in excess of about 10m/s. The analysis procedure also provides a framework to assess surfactant-driven variability within and between whitecap coverage data sets. Plain Language Summary The foam patches made by breaking waves, also known as "whitecaps'', are an important source of marine sea spray, which impacts weather and climate through the formation of cloud drops and ice. Sea spray chemistry depends on the chemistry of the whitecap that makes it. This chemistry is poorly understood, especially during storms when whitecaps are most prevalent but chemistry measurements are also the most difficult. In this article, we show that foam chemistry affects the persistence of laboratory whitecaps: the more surfactant a whitecap contains, the longer it persists. This effect has enabled us to develop a remote sensing tool to detect the presence of chemistry in whitecaps by analyzing a time-series of photographs of the foam. We have applied the technique to an existing set of whitecap images, and get reasonable values for implied surfactant concentrations in the ocean but validation of the technique in the field will have to await simultaneous measurement of whitecaps and sea surface chemistry. If validated, the new remote sensing tool will provide the first large-scale observations of ocean surface chemistry and its variation in space and time on wind-driven seas.

Wang, XF, Deane GB, Moore KA, Ryder OS, Stokes MD, Beall CM, Collins DB, Santander MV, Burrows SM, Sultana CM, Prather KA.  2017.  The role of jet and film drops in controlling the mixing state of submicron sea spray aerosol particles. Proceedings of the National Academy of Sciences of the United States of America. 114:6978-6983.   10.1073/pnas.1702420114   AbstractWebsite

The oceans represent a significant global source of atmospheric aerosols. Sea spray aerosol (SSA) particles comprise sea salts and organic species in varying proportions. In addition to size, the overall composition of SSA particles determines how effectively they can form cloud droplets and ice crystals. Thus, understanding the factors controlling SSA composition is critical to predicting aerosol impacts on clouds and climate. It is often assumed that submicrometer SSAs are mainly formed by film drops produced from bursting bubble-cap films, which become enriched with hydrophobic organic species contained within the sea surface microlayer. In contrast, jet drops formed from the base of bursting bubbles are postulated to mainly produce larger supermicrometer particles from bulk seawater, which comprises largely salts and water-soluble organic species. However, here we demonstrate that jet drops produce up to 43% of total submicrometer SSA number concentrations, and that the fraction of SSA produced by jet drops can be modulated by marine biological activity. We show that the chemical composition, organic volume fraction, and ice nucleating ability of submicrometer particles from jet drops differ from those formed from film drops. Thus, the chemical composition of a substantial fraction of submicrometer particles will not be controlled by the composition of the sea surface microlayer, a major assumption in previous studies. This finding has significant ramifications for understanding the factors controlling the mixing state of submicrometer SSA particles and must be taken into consideration when predicting SSA impacts on clouds and climate.

Hines, PC, Murphy SM, Abraham DA, Deane GB.  2017.  The dependence of signal coherence on sea-surface roughness for high and low duty cycle sonars in a shallow-water channel. Ieee Journal of Oceanic Engineering. 42:298-318.   10.1109/joe.2016.2609019   AbstractWebsite

It is anticipated that high duty cycle (HDC) sonars will typically maintain the same bandwidth as the pulsed active sonars (PASs) that they might replace. This will significantly increase their time-bandwidth product, but may not produce the increased gain anticipated, if there are coherence limitations of the acoustic channel. To compare performance of HDC with conventional PAS in the littorals, a set of experiments was conducted as part of the Target and Reverberation Experiment in spring 2013 (TREX13). This paper presents the results of an examination of short-range single surface-reflection echoes, and longer range target echoes from an air hose. The Pearson product-moment correlation coefficient (Pearson's R) was used to confirm significance of the results. Measurements showed that for an 18-s HDC pulse, the mean (coherent) component of the specular arrival decreased by as much as 5 dB as root mean square (rms) surface roughness increased, whereas the 0.5-s PAS pulse echoes showed no correlation with roughness. The standard deviations of the mean levels were used to examine the incoherent (scattered) component of the specular arrivals. The incoherent component of the specular arrival increased with the product of the surface correlation length and the square of the rms roughness, for both HDC and PAS, with the PAS data having a 1-dB higher standard deviation. A normal mode propagation model and a rough surface scattering model used in conjunction with a simple model that accounts for coherence loss from the matched filter were successfully used to interpret the results.

Glowacki, O, Moskalik M, Deane GB.  2016.  The impact of glacier meltwater on the underwater noise field in a glacial bay. Journal of Geophysical Research-Oceans. 121:8455-8470.   10.1002/2016jc012355   AbstractWebsite

Ambient noise oceanography is proving to be an efficient and effective tool for the study of ice-ocean interactions in the bays of marine-terminating glaciers. However, obtaining quantitative estimates of ice melting or calving processes from ambient noise requires an understanding of how sound propagation through the bay attenuates and filters the noise spectrum. Measurements of the vertical structure in sound speed in the vicinity of the Hans Glacier in Hornsund Fjord, Spitsbergen, made with O(130) CTD casts between May and November 2015, reveal high-gradient, upward-refracting sound speed profiles created by cold, fresh meltwater during summer months. Simultaneous recordings of underwater ambient noise made at depths of 1, 10, and 20 m in combination with propagation model calculations using the model Bellhop illustrate the dominant role these surface ducts play in shaping the underwater soundscape. The surface ducts lead to a higher intensity and greater variability of acoustic energy in the near-surface layer covered by glacially modified waters relative to deeper waters, indicating deeper zones as most appropriate for interseasonal acoustic monitoring of the glacial melt. Surface waveguides in Hornsund are relatively shallow and trap sound above O(1 kHz). Deeper waveguides observed elsewhere will also trap low-frequency sounds, such as those generated by calving events for example. Finally, the ambient noise field in Hornsund is shown to be strongly dependent on the distribution of ice throughout the bay, stressing the importance of performing complementary environmental measurements when interpreting the results of acoustic surveys.

Callaghan, AH, Deane GB, Stokes MD.  2016.  Laboratory air-entraining breaking waves: Imaging visible foam signatures to estimate energy dissipation. Geophysical Research Letters. 43:11320-11328.   10.1002/2016gl071226   AbstractWebsite

Oceanic air-entraining breaking waves fundamentally influence weather and climate through bubble-mediated ocean-atmosphere exchanges, and influence marine engineering design by impacting statistics of wave heights, crest heights, and wave loading. However, estimating individual breaking wave energy dissipation in the field remains a fundamental problem. Using laboratory experiments, we introduce a new method to estimate energy dissipation by individual breaking waves using above-water images of evolving foam. The data show the volume of the breaking wave two-phase flow integrated in time during active breaking scales linearly with wave energy dissipated. To determine the volume time-integral, above-water images of surface foam provide the breaking wave timescale and horizontal extent of the submerged bubble plume, and the foam decay time provides an estimate of the bubble plume penetration depth. We anticipate that this novel remote sensing method will improve predictions of air-sea exchanges, validate models of wave energy dissipation, and inform ocean engineering design.

Deane, GB.  2016.  The performance of high-frequency Doppler sonars in actively breaking wave crests. Ieee Journal of Oceanic Engineering. 41:1028-1034.   10.1109/joe.2016.2521247   AbstractWebsite

Breaking ocean waves influence wave dynamics, momentum transfer, air-sea exchange, ocean albedo, and ambient noise generation, all of which are impacted by the transient, two-phase flow in a whitecap. Lasting O(1s) or so, actively breaking whitecaps contain air fractions up to 0.6, bubbles ranging in size O(10-1000) mu m and turbulent dissipation rates O(1) Strong fluid turbulence, high air fractions, large bubbles, and short duration make active whitecaps a challenging process to study. This paper presents a model for the performance of high-frequency Doppler sonar (0.5-2 MHz) when used to probe the interior of actively breaking whitecaps. The results suggest that the ability of high-frequency sonars to penetrate the interior of bubble plumes in whitecaps becomes limited for air fractions greater than 0.03-0.06 and plumes become completely impenetrable for air fractions greater than 0.08-0.17. This severely limits their usefulness as a tool to probe the interior of breaking waves. Moreover, the bias introduced by the terminal rise velocity of large bubbles interacting with fluid turbulence within the wave crest will need to be accounted for when interpreting any backscatter signals that are returned from the plume interior. At this time, in situ methods such as optical fiber probes, conductivity cells, and cameras remain the best option for field studies of the interior of breaking oceanic waves.

Stokes, MD, Deane G, Collins DB, Cappa C, Bertram T, Dommer A, Schill S, Forestieri S, Survilo M.  2016.  A miniature Marine Aerosol Reference Tank (miniMART) as a compact breaking wave analogue. Atmospheric Measurement Techniques. 9:4257-4267.   10.5194/amt-9-4257-2016   AbstractWebsite

In order to understand the processes governing the production of marine aerosols, repeatable, controlled methods for their generation are required. A new system, the miniature Marine Aerosol Reference Tank (miniMART), has been designed after the success of the original MART system, to approximate a small oceanic spilling breaker by producing an evolving bubble plume and surface foam patch. The smaller tank utilizes an intermittently plunging jet of water produced by a rotating water wheel, into an approximately 6 L reservoir to simulate bubble plume and foam formation and generate aerosols. This system produces bubble plumes characteristic of small whitecaps without the large external pump inherent in the original MART design. Without the pump it is possible to easily culture delicate planktonic and microbial communities in the bulk water during experiments while continuously producing aerosols for study. However, due to the reduced volume and smaller plunging jet, the absolute numbers of particles generated are approximately an order of magnitude less than in the original MART design.

Walstead, SP, Deane GB.  2016.  Determination of ocean surface wave shape from forward scattered sound. Journal of the Acoustical Society of America. 140:787-797.   10.1121/1.4960478   AbstractWebsite

Forward scattered sound from the ocean surface is inverted for wave shape during three periods: low wind, mix of wind and swell, and stormy. Derived wave profiles are spatially limited to a Fresnel region at or near the nominal surface specular reflection point. In some cases, the surface wave profiles exhibit unrealistic temporal and spatial properties. To remedy this, the spatial gradient of inverted waves is constrained to a maximum slope of 0.88. Under this global constraint, only surface waves during low wind conditions result in a modeled surface multipath that accurately matches data. The power spectral density of the inverted surface wave field saturates around a frequency of 8 Hz while upward looking SONAR saturates at 1 Hz. Each shows a high frequency spectral slope of -4 that is in agreement with various empirical ocean wave spectra. The improved high frequency resolution provided by the scattering inversion indicates that it is possible to remotely gain information about high frequency components of ocean waves. The inability of the inversion algorithm to determine physically realistic surface waves in periods of high wind indicates that bubbles and out of plane scattering become important in those operating scenarios. (C) 2016 Acoustical Society of America.

Walstead, SP, Deane GB.  2016.  Intensity statistics of very high frequency sound scattered from wind-driven waves. Journal of the Acoustical Society of America. 139:2784-2796.   10.1121/1.4948449   AbstractWebsite

The interaction of vhf 100-1000 kHz underwater sound with the ocean surface is explored. The bistatic forward scatter of 300 kHz sound is measured in a wind driven wave channel. Fluctuations in arrival amplitude are described by the scintillation index (SI) which is a measure of arrival intensity variance. SI initially increases with wind speed but eventually saturates to a value of 0.5 when the root-mean-square (rms) roughness is 0.5 mm. An adjusted scintillation index (SI*) is suggested that accounts for the multiple arrivals and properly saturates to a value of 1. Fluctuations in arrival time do not saturate and increase proportionately to the dominant surface wave component. Forward scattering is modeled at frequencies ranging from 50 to 2000 kHz using the Helmholtz-Kirchhoff integral with surface wave realizations derived from wave gauge data. The amplitude and temporal statistics of the simulated scattering agree well with measured data. Intensity saturation occurs at lower wind speeds for higher frequency sound. Both measured and modeled vhf sound is characterized by many surface arrivals at saturation. Doppler shifts associated with wave motion are expected to vary rapidly for vhf sound however further analysis is required. (C) 2016 Acoustical Society of America.

DeMott, PJ, Hill TCJ, McCluskey CS, Prather KA, Collins DB, Sullivan RC, Ruppel MJ, Mason RH, Irish VE, Lee T, Hwang CY, Rhee TS, Snider JR, McMeeking GR, Dhaniyala S, Lewis ER, Wentzell JJB, Abbatt J, Lee C, Sultana CM, Ault AP, Axson JL, Martinez MD, Venero I, Santos-Figueroa G, Stokes MD, Deane GB, Mayol-Bracero OL, Grassian VH, Bertram TH, Bertram AK, Moffett BF, Franc GD.  2016.  Sea spray aerosol as a unique source of ice nucleating particles. Proceedings of the National Academy of Sciences of the United States of America. 113:5797-5803.   10.1073/pnas.1514034112   AbstractWebsite

Ice nucleating particles (INPs) are vital for ice initiation in, and precipitation from, mixed-phase clouds. A source of INPs from oceans within sea spray aerosol (SSA) emissions has been suggested in previous studies but remained unconfirmed. Here, we show that INPs are emitted using real wave breaking in a laboratory flume to produce SSA. The number concentrations of INPs from laboratory-generated SSA, when normalized to typical total aerosol number concentrations in the marine boundary layer, agree well with measurements from diverse regions over the oceans. Data in the present study are also in accord with previously published INP measurements made over remote ocean regions. INP number concentrations active within liquid water droplets increase exponentially in number with a decrease in temperature below 0 degrees C, averaging an order of magnitude increase per 5 degrees C interval. The plausibility of a strong increase in SSA INP emissions in association with phytoplankton blooms is also shown in laboratory simulations. Nevertheless, INP number concentrations, or active site densities approximated using "dry" geometric SSA surface areas, are a few orders of magnitude lower than corresponding concentrations or site densities in the surface boundary layer over continental regions. These findings have important implications for cloud radiative forcing and precipitation within low-level and midlevel marine clouds unaffected by continental INP sources, such as may occur over the Southern Ocean.

Deane, GB, Stokes MD, Callaghan AH.  2016.  The saturation of fluid turbulence in breaking laboratory waves and implications for whitecaps. Journal of Physical Oceanography. 46:975-992.   10.1175/jpo-d-14-0187.1   AbstractWebsite

Measurements of energy dissipated in breaking laboratory waves, averaged over time and space and directly visualized with a bioluminescent technique, are presented. These data show that the energy dissipated in the crest of the breaking waves is constrained: average turbulence intensity within the crest saturates at around 0.5-1.2 W kg(-1), whereas breaking crest volume scales with wave energy lost. These results are consistent with laboratory and field observations of the Hinze scale, which is the radius of the largest bubble entrained within a breaking crest that is stabilized against turbulent fragmentation. The Hinze scale depends on turbulence intensity but lies in the restricted range 0.7-1.7 mm over more than two orders of magnitude variation in underlying unbroken wave energy. The results have important implications for understanding the energetics of breaking waves in the field, the injection of turbulence into the upper ocean, and air-sea exchange processes in wind-driven seas.

Deane, GB, Stokes DM, Latz MI.  2016.  Bubble stimulation efficiency of dinoflagellate bioluminescence. Luminescence. 31:270-280.   10.1002/bio.2957   Abstract

Dinoflagellate bioluminescence, a common source of bioluminescence in coastal waters, is stimulated by flow agitation. Although bubbles are anecdotally known to be stimulatory, the process has never been experimentally investigated. This study quantified the flash response of the bioluminescent dinoflagellate Lingulodinium polyedrum to stimulation by bubbles rising through still seawater. Cells were stimulated by isolated bubbles of 0.3–3 mm radii rising at their terminal velocity, and also by bubble clouds containing bubbles of 0.06–10 mm radii for different air flow rates. Stimulation efficiency, the proportion of cells producing a flash within the volume of water swept out by a rising bubble, decreased with decreasing bubble radius for radii less than approximately 1 mm. Bubbles smaller than a critical radius in the range 0.275–0.325 mm did not stimulate a flash response. The fraction of cells stimulated by bubble clouds was proportional to the volume of air in the bubble cloud, with lower stimulation levels observed for clouds with smaller bubbles. An empirical model for bubble cloud stimulation based on the isolated bubble observations successfully reproduced the observed stimulation by bubble clouds for low air flow rates. High air flow rates stimulated more light emission than expected, presumably because of additional fluid shear stress associated with collective buoyancy effects generated by the high air fraction bubble cloud. These results are relevant to bioluminescence stimulation by bubbles in two-phase flows, such as in ship wakes, breaking waves, and sparged bioreactors. Copyright © 2015 John Wiley & Sons, Ltd.

Glowacki, O, Deane GB, Moskalik M, Tegowski J, Blondel P.  2015.  Two-element acoustic array gives insight into ice-ocean interactions in Hornsund Fjord, Spitsbergen. Polish Polar Research. 36:355-367.   10.1515/popore-2015-0025   AbstractWebsite

Glacierized fjords are dynamic regions, with variable oceanographic conditions and complex ice-ocean interactions, which are still poorly understood. Recent studies have shown that passive underwater acoustics offers new promising tools in this branch of polar research. Here, we present results from two field campaigns, conducted in summer 2013 and spring 2014. Several recordings with a bespoke two-hydrophone acoustic buoy were made in different parts of Hornsund Fjord, Spitsbergen in the vicinity of tidewater glaciers to study the directionality of underwater ambient noise. Representative segments of the data are used to illustrate the analyses, and determine the directions of sound sources by using the time differences of arrivals between two horizontally aligned, broadband hydrophones. The results reveal that low frequency noise (<3 kHz) is radiated mostly from the ice cliffs, while high-frequency (>3 kHz) noise directionality strongly depends on the distribution of floating glacial ice throughout the fjord. Changing rates of iceberg production as seen for example in field photographs and logs are, in turn, most likely linked to signal amplitudes for relevant directions. These findings demonstrate the potential offered by passive acoustics to study the dynamics of individual tidewater glaciers.

Lee, C, Sultana CM, Collins DB, Santander MV, Axson JL, Malfatti F, Cornwell GC, Grandquist JR, Deane GB, Stokes MD, Azam F, Grassian VH, Prather KA.  2015.  Advancing model systems for fundamental laboratory studies of sea spray aerosol using the microbial loop. Journal of Physical Chemistry A. 119:8860-8870.   10.1021/acs.jpca.5b03488   AbstractWebsite

Sea spray aerosol (SSA) particles represent one of the most abundant surfaces available for heterogeneous reactions to occur upon and thus profoundly alter the composition of the troposphere. In an effort to better understand tropospheric heterogeneous reaction processes, fundamental laboratory studies must be able to accurately reproduce the chemical complexity of SSA. Here we describe a new approach that uses microbial processes to control the composition of seawater and SSA particle composition. By inducing a phytoplankton bloom, we are able to create dynamic ecosystem interactions between marine microorganisms, which serve to alter the organic mixtures present in seawater. Using this controlled approach, changes in seawater composition become reflected in the chemical composition of SSA particles 4 to 10 d after the peak in chlorophyll-a. This approach for producing and varying the chemical complexity of a dominant tropospheric aerosol provides the foundation for further investigations of the physical and chemical properties of realistic SSA particles under controlled conditions.

Glowacki, O, Deane GB, Moskalik M, Blondel P, Tegowski J, Blaszczyk M.  2015.  Underwater acoustic signatures of glacier calving. Geophysical Research Letters. :2014GL062859.   10.1002/2014GL062859   Abstract

Climate-driven ice-water interactions in the contact zone between marine-terminating glaciers and the ocean surface show a dynamic and complex nature. Tidewater glaciers lose volume through the poorly understood process of calving. A detailed description of the mechanisms controlling the course of calving is essential for the reliable estimation and prediction of mass loss from glaciers. Here we present the potential of hydroacoustic methods to investigate different modes of ice detachments. High-frequency underwater ambient noise recordings are combined with synchronized, high-resolution, time-lapse photography of the Hans Glacier cliff in Hornsund Fjord, Spitsbergen, to identify three types of calving events: typical subaerial, sliding subaerial, and submarine. A quantitative analysis of the data reveals a robust correlation between ice impact energy and acoustic emission at frequencies below 200 Hz for subaerial calving. We suggest that relatively inexpensive acoustic methods can be successfully used to provide quantitative descriptions of the various calving types.

Deane, GB, Glowacki O, Tegowski J, Moskalik M, Blondel P.  2014.  Directionality of the ambient noise field in an Arctic, glacial bay. Journal of the Acoustical Society of America. 136:EL350-EL356.   10.1121/1.4897354   AbstractWebsite

The directionality of ambient noise in an Arctic tidewater glacier bay was measured using two horizontally spaced, broadband hydrophones. Segments of noise were divided into two frequency bands and analyzed for arrival angle. These data show that different classes of source radiate noise in distinct spectral bands and are spatially diverse. A previously unidentified source, the interaction of surface gravity waves with underside of ice ledges at the periphery of icebergs, is described. The generation of noise by ice-wave interaction suggests that surface waves should be measured if ambient noise is to be used to monitor ice dynamics in glacial fjords. (C) 2014 Acoustical Society of America

Callaghan, AH, Stokes MD, Deane GB.  2014.  The effect of water temperature on air entrainment, bubble plumes, and surface foam in a laboratory breaking-wave analog. Journal of Geophysical Research-Oceans. 119:7463-7482.   10.1002/2014jc010351   AbstractWebsite

Air-entraining breaking waves form oceanic whitecaps and play a key role in climate regulation through air-sea bubble-mediated gas transfer, and sea spray aerosol production. The effect of varying sea surface temperature on air entrainment, subsurface bubble plume dynamics, and surface foam evolution intrinsic to oceanic whitecaps has not been well studied. By using a breaking wave analog in the laboratory over a range of water temperatures (T-w=5 degrees C to T-w=30 degrees C) and different source waters, we have examined changes in air entrainment, subsurface bubble plumes, and surface foam evolution over the course of a breaking event. For filtered seawater, air entrainment was estimated to increase by 6% between T-w=6 degrees C and T-w=30 degrees C, driven by increases of about 43% in the measured surface roughness of the plunging water sheet. After active air entrainment, the rate of loss of air through bubble degassing was more rapid at colder water temperatures within the first 0.5 s of plume evolution. Thereafter, the trend reversed and bubbles degassed more quickly in warmer water. The largest observed temperature-dependent differences in subsurface bubble distributions occurred at radii greater than about 700 m. Temperature-dependent trends observed in the subsurface bubble plume were mirrored in the temporal evolution of the surface whitecap foam area demonstrating the intrinsic link between surface whitecap foam and the subsurface bubble plume. Differences in foam and plume characteristics due to different water sources were greater than the temperature dependencies for the filtered seawater examined. Key Points Entrainment increases with increasing water temperature Integrated foam area increases with increasing water temperature Subsurface bubble population evolution exhibits a temperature dependence

Collins, DB, Zhao DF, Ruppel MJ, Laskina O, Grandquist JR, Modini RL, Stokes MD, Russell LM, Bertram TH, Grassian VH, Deane GB, Prather KA.  2014.  Direct aerosol chemical composition measurements to evaluate the physicochemical differences between controlled sea spray aerosol generation schemes. Atmospheric Measurement Techniques. 7:3667-3683.   10.5194/amt-7-3667-2014   AbstractWebsite

Controlled laboratory studies of the physical and chemical properties of sea spray aerosol (SSA) must be underpinned by a physically and chemically accurate representation of the bubble-mediated production of nascent SSA particles. Bubble bursting is sensitive to the physicochemical properties of seawater. For a sample of seawater, any important differences in the SSA production mechanism are projected into the composition of the aerosol particles produced. Using direct chemical measurements of SSA at the single-particle level, this study presents an intercomparison of three laboratory-based, bubble-mediated SSA production schemes: gas forced through submerged sintered glass filters ("frits"), a pulsed plunging-waterfall apparatus, and breaking waves in a wave channel filled with natural seawater. The size-resolved chemical composition of SSA particles produced by breaking waves is more similar to particles produced by the plunging waterfall than those produced by sintered glass filters. Aerosol generated by disintegrating foam produced by sintered glass filters contained a larger fraction of organic-enriched particles and a different size-resolved elemental composition, especially in the 0.8-2 mu m dry diameter range. Interestingly, chemical differences between the methods only emerged when the particles were chemically analyzed at the single-particle level as a function of size; averaging the elemental composition of all particles across all sizes masked the differences between the SSA samples. When dried, SSA generated by the sintered glass filters had the highest fraction of particles with spherical morphology compared to the more cubic structure expected for pure NaCl particles produced when the particle contains relatively little organic carbon. In addition to an intercomparison of three SSA production methods, the role of the episodic or "pulsed" nature of the waterfall method on SSA composition was undertaken. In organic-enriched seawater, the continuous operation of the plunging waterfall resulted in the accumulation of surface foam and an over-expression of organic matter in SSA particles compared to those produced by a pulsed plunging waterfall. Throughout this set of experiments, comparative differences in the SSA number size distribution were coincident with differences in aerosol particle composition, indicating that the production mechanism of SSA exerts important controls on both the physical and chemical properties of the resulting aerosol with respect to both the internal and external mixing state of particles. This study provides insight into the inextricable physicochemical differences between each of the bubble-mediated SSA generation mechanisms tested and the aerosol particles that they produce, and also serves as a guideline for future laboratory studies of SSA particles.

Walstead, SP, Deane GB.  2014.  Reconstructing surface wave profiles from reflected acoustic pulses using multiple receivers. Journal of the Acoustical Society of America. 136:604-613.   10.1121/1.4887449   AbstractWebsite

Surface wave shapes are determined by analyzing underwater reflected acoustic signals collected at multiple receivers. The transmitted signals are of nominal frequency 300 kHz and are reflected off surface gravity waves that are paddle-generated in a wave tank. An inverse processing algorithm reconstructs 50 surface wave shapes over a length span of 2.10 m. The inverse scheme uses a broad-band forward scattering model based on Kirchhoff's diffraction formula to determine wave shapes. The surface reconstruction algorithm is self-starting in that source and receiver geometry and initial estimates of wave shape are determined from the same acoustic signals used in the inverse processing. A high speed camera provides ground-truth measurements of the surface wave field for comparison with the acoustically derived surface waves. Within Fresnel zone regions the statistical confidence of the inversely optimized surface profile exceeds that of the camera profile. Reconstructed surfaces are accurate to a resolution of about a quarter-wavelength of the acoustic pulse only within Fresnel zones associated with each source and receiver pair. Multiple isolated Fresnel zones from multiple receivers extend the spatial extent of accurate surface reconstruction while overlapping Fresnel zones increase confidence in the optimized profiles there. (C) 2014 Acoustical Society of America.

Modini, RL, Russell LM, Deane GB, Stokes MD.  2013.  Effect of soluble surfactant on bubble persistence and bubble-produced aerosol particles. Journal of Geophysical Research-Atmospheres. 118:1388-1400.   10.1002/jgrd.50186   AbstractWebsite

The effect of soluble surfactant on the persistence of salt-water bubbles and their ability to produce aerosol particles upon bursting was investigated. Ensembles of individual, millimetric bubbles were produced in NaCl solutions of varying surfactant concentration. Aerosol production efficiency-a fundamental property of single bubbles defined as the number of particles produced per bubble film cap area-decreased by 79% to 98% following the addition of surfactant and increase in solution film pressure from 1-2 to 7-27 mN m(-1). The generated particle size distributions (0.01-10 mu m dry diameter) contained up to three modes and did not change much for film pressures up to 13.8 mN m(-1). The persistence of the bubbles at the water surface and the thickness of their film caps were investigated with high-speed videography. Addition of soluble surfactant increased average bubble persistence providing more time for the bubbles to drain and thin out with the aid of marginal regeneration flows. Bubble film cap thicknesses ranged from around 1 mu m for relatively clean, short-lived bubbles to less than 0.1 mu m for surfactant-stabilized, persistent bubbles. The suppression of aerosol production from the surfactant-stabilized bubbles may have resulted from the dramatic thinning of their caps or reduced surface forces at high film pressure. Previously reported Sea Spray Aerosol source functions were compared to measured aerosol production efficiencies and found to be significantly greater in magnitude, suggesting that there is a source of particles from whitecaps that was not captured in these single-bubble experiments.

Jin, K, Klima JC, Deane G, Dale Stokes M, Latz MI.  2013.  Pharmacological investigation of the bioluminescence signaling pathway of the dinoflagellate Lingulodinium polyedrum: evidence for the role of stretch-activated ion channels. Journal of Phycology. 49:733-745.   10.1111/jpy.12084   AbstractWebsite

Dinoflagellate bioluminescence serves as a whole-cell reporter of mechanical stress, which activates a signaling pathway that appears to involve the opening of voltage-sensitive ion channels and release of calcium from intracellular stores. However, little else is known about the initial signaling events that facilitate the transduction of mechanical stimuli. In the present study using the red tide dinoflagellate Lingulodinium polyedrum (Stein) Dodge, two forms of dinoflagellate bioluminescence, mechanically stimulated and spontaneous flashes, were used as reporter systems to pharmacological treatments that targeted various predicted signaling events at the plasma membrane level of the signaling pathway. Pretreatment with 200 μM Gadolinium III (Gd3+), a nonspecific blocker of stretch-activated and some voltage-gated ion channels, resulted in strong inhibition of both forms of bioluminescence. Pretreatment with 50 μM nifedipine, an inhibitor of L-type voltage-gated Ca2+ channels that inhibits mechanically stimulated bioluminescence, did not inhibit spontaneous bioluminescence. Treatment with 1 mM benzyl alcohol, a membrane fluidizer, was very effective in stimulating bioluminescence. Benzyl alcohol-stimulated bioluminescence was inhibited by Gd3+ but not by nifedipine, suggesting that its role is through stretch activation via a change in plasma membrane fluidity. These results are consistent with the presence of stretch-activated and voltage-gated ion channels in the bioluminescence mechanotransduction signaling pathway, with spontaneous flashing associated with a stretch-activated component at the plasma membrane.

Callaghan, AH, Deane GB, Stokes MD.  2013.  Two regimes of laboratory whitecap foam decay: Bubble-plume controlled and surfactant stabilized. Journal of Physical Oceanography. 43:1114-1126.   10.1175/jpo-d-12-0148.1   AbstractWebsite

A laboratory experiment to quantify whitecap foam decay time in the presence or absence of surface active material is presented. The investigation was carried out in the glass seawater channel at the Hydraulics Facility of Scripps Institution of Oceanography. Whitecaps were generated with focused, breaking wave packets in filtered seawater pumped from La Jolla Shores Beach with and without the addition of the surfactant Triton X-100. Concentrations of Triton X-100 (204 mu g L-1) were chosen to correspond to ocean conditions of medium productivity. Whitecap foam and subsurface bubble-plume decay times were determined from digital images for a range of wave scales and wave slopes. The experiment showed that foam lifetime is variable and controlled by subsurface bubble-plume-degassing times, which are a function of wave scale and breaking wave slope. This is true whether or not surfactants are present. However, in the presence of surfactants, whitecap foam is stabilized and persists for roughly a factor of 3 times its clean seawater value. The range of foam decay times observed in the laboratory study lie within the range of values observed in an oceanic dataset obtained off Martha's Vineyard in 2008.

Prather, KA, Bertram TH, Grassian VH, Deane GB, Stokes MD, DeMott PJ, Aluwihare LI, Palenik BP, Azam F, Seinfeld JH, Moffet RC, Molina MJ, Cappa CD, Geiger FM, Roberts GC, Russell LM, Ault AP, Baltrusaitis J, Collins DB, Corrigan CE, Cuadra-Rodriguez LA, Ebben CJ, Forestieri SD, Guasco TL, Hersey SP, Kim MJ, Lambert WF, Modini RL, Mui W, Pedler BE, Ruppel MJ, Ryder OS, Schoepp NG, Sullivan RC, Zhao DF.  2013.  Bringing the ocean into the laboratory to probe the chemical complexity of sea spray aerosol. Proceedings of the National Academy of Sciences of the United States of America. 110:7550-7555.   10.1073/pnas.1300262110   AbstractWebsite

The production, size, and chemical composition of sea spray aerosol (SSA) particles strongly depend on seawater chemistry, which is controlled by physical, chemical, and biological processes. Despite decades of studies in marine environments, a direct relationship has yet to be established between ocean biology and the physicochemical properties of SSA. The ability to establish such relationships is hindered by the fact that SSA measurements are typically dominated by overwhelming background aerosol concentrations even in remote marine environments. Herein, we describe a newly developed approach for reproducing the chemical complexity of SSA in a laboratory setting, comprising a unique ocean-atmosphere facility equipped with actual breaking waves. A mesocosm experiment was performed in natural seawater, using controlled phytoplankton and heterotrophic bacteria concentrations, which showed SSA size and chemical mixing state are acutely sensitive to the aerosol production mechanism, as well as to the type of biological species present. The largest reduction in the hygroscopicity of SSA occurred as heterotrophic bacteria concentrations increased, whereas phytoplankton and chlorophyll-a concentrations decreased, directly corresponding to a change in mixing state in the smallest (60-180 nm) size range. Using this newly developed approach to generate realistic SSA, systematic studies can now be performed to advance our fundamental understanding of the impact of ocean biology on SSA chemical mixing state, heterogeneous reactivity, and the resulting climate-relevant properties.

Walstead, SP, Deane GB.  2013.  Reconstructing surface wave profiles from reflected acoustic pulses. Journal of the Acoustical Society of America. 133:2597-2611.   10.1121/1.4795791   AbstractWebsite

Surface wave shapes are determined by analyzing underwater reflected acoustic signals. The acoustic signals (of nominal frequency 200 kHz) are forward scattered from the underside of surface waves that are generated in a wave tank and scaled to model smooth ocean swell. An inverse processing algorithm is designed and implemented to reconstruct the surface displacement profiles of the waves over one complete period. The inverse processing uses the surface scattered pulses collected at the receiver, an initial wave profile (two are considered), and a broadband forward scattering model based on Kirchhoff's diffraction formula to iteratively adjust the surface until it is considered optimized or reconstructed. Two physical length scales over which information can be known about the surface are confirmed. An outer length scale, the Fresnel zone surrounding each specular reflection point, is the only region where optimized surfaces resulting from each initial profile converge within a resolution set by the inner length scale, a quarter-wavelength of the acoustic pulse. The statistical confidence of each optimized surface is also highest within a Fresnel zone. Future design considerations are suggested such as an array of receivers that increases the region of surface reconstruction by a factor of 2 to 3. (C) 2013 Acoustical Society of America.

Stokes, MD, Deane GB, Prather K, Bertram TH, Ruppel MJ, Ryder OS, Brady JM, Zhao D.  2013.  A Marine Aerosol Reference Tank system as a breaking wave analogue for the production of foam and sea-spray aerosols. Atmospheric Measurement Techniques. 6:1085-1094.   10.5194/amt-6-1085-2013   AbstractWebsite

In order to better understand the processes governing the production of marine aerosols a repeatable, controlled method for their generation is required. The Marine Aerosol Reference Tank (MART) has been designed to closely approximate oceanic conditions by producing an evolving bubble plume and surface foam patch. The tank utilizes an intermittently plunging sheet of water and large volume tank reservoir to simulate turbulence, plume and foam formation, and the water flow is monitored volumetrically and acoustically to ensure the repeatability of conditions.