Gas bubbles bursting at the sea surface produce drops, which contribute to marine aerosols. The contamination or enrichment of water by surface‐active agents, of biological or anthropogenic origin, has long been recognized as affecting the bubble bursting processes and the spray composition. However, despite an improved understanding of the physics of a single bursting event, a quantitative understanding of the role of the physico‐chemical conditions on assemblies of bursting bubbles remains elusive. We present experiments on the drop production by millimetric, collective bursting bubbles, under varying surfactant concentration and bubble density. We demonstrate that the production of supermicron droplets (with radius larger than 35 μm) is non‐monotonic as the surfactant concentration increases. The bursting efficiency is optimal for short‐lived, sparsely distributed and non‐coalescing bubbles. We identify the combined role of contamination on the surface bubble arrangement and the modification of the jet drop production process in the bursting efficiency.
- Award ID(s):
- 1849762
- NSF-PAR ID:
- 10308652
- Date Published:
- Journal Name:
- Journal of Fluid Mechanics
- Volume:
- 917
- ISSN:
- 0022-1120
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Bubbles bursting at the ocean surface are an important source of ocean‐spray aerosol, with implications on radiative and cloud processes. Yet, very large uncertainties exist on the role of key physical controlling parameters, including wind speed, sea state and water temperature. We propose a mechanistic sea spray generation function that is based on the physics of bubble bursting. The number and mean droplet radius of jet and film drops is described by scaling laws derived from individual bubble bursting laboratory and numerical experiments, as a function of the bubble radius and the water physico‐chemical properties (viscosity, density and surface tension, all functions of temperature), with drops radii at production from 0.1 to 500 µm. Next, we integrate over the bubble size distribution entrained by breaking waves. Finally, the sea spray generation function is obtained by considering the volume flux of entrained bubbles due to breaking waves in the field constrained by the third moment of the breaking distribution (akin to the whitecap coverage). This mechanistic approach naturally integrates the role of wind and waves via the breaking distribution and entrained air flux, and a sensitivity to temperature via individual bubble bursting mechanisms. The resulting sea spray generation function has not been tuned or adjusted to match any existing data sets, in terms of magnitude of sea salt emissions and recently observed temperature dependencies. The remarkable coherence between the model and observations of sea salt emissions therefore strongly supports the mechanistic approach and the resulting sea spray generation function.
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Abstract Bubbles entrained by ocean waves rise to the surface and burst, creating a shower of droplets which contribute to sea spray aerosols. Submicron‐sized droplets, of which an estimated 60%–80% come from a bursting bubble film cap, play a key role in global climate atmospheric processes. However, many aspects of predicting the number and size of submicron drops emitted from a bursting bubble remain unknown. It is well‐documented that higher salinity increases submicron droplet production, which has been attributed to the role of salt in the suppression of bubble coalescence. We experimentally show that submicron drop production increases with salinity despite using a salt that does not affect bubble coalescence, indicating that salinity plays a role in the physics of submicron aerosol formation beyond coalescence. Laboratory experiments are conducted using sodium acetate solutions of salinity
S = 0.001–0.1 M with millimeter‐sized bubbles generated via a needle. Unlike previous studies, the measured droplet size distributions are converted to formation diameter, revealing that the peak aerosol formation diameter decreases with higher salinity. Applying this diameter conversion to past studies, we find the peak formation diameter exhibits a scaling ofD form ∼S −0.32across three orders of magnitude in salinity and for a variety of salts, bubble coalescence behaviors, and bubble generation mechanisms. This result suggests that salinity has a systematic effect on the length scale of the rupturing bubble film which generates the aerosols. Consequently, salinity likely impacts the submicron aerosol production in oceanic environments even if bubble coalescence is negligible. -
Croot, Peter (Ed.)Environmental context Saccharides contribute substantially to dissolved organic carbon in the ocean and are enriched at the ocean surface. In this study, we demonstrate that saccharides are more enriched in persistent whitecap foam compared to the sea surface. The maturation of bubbles at the air–water interface is thus expected to enhance the enrichment of organic matter at the ocean surface and ultimately in the sea spray aerosol that forms when bubbles burst at the ocean surface. Rationale Organic matter accumulates at the ocean surface. Herein, we provide the first quantitative assessment of the enrichment of dissolved saccharides in persistent whitecap foam and compare this enrichment to the sea surface microlayer (SSML) during a 9 day mesocosm experiment involving a phytoplankton bloom generated in a Marine Aerosol Reference Tank (MART). Methodology Free monosaccharides were quantified directly, total saccharides were determined following mild acid hydrolysis and the oligo/polysaccharide component was determined as the difference between total and free monosaccharides. Results Total saccharides contributed a significant fraction of dissolved organic carbon (DOC), accounting for 13% of DOC in seawater, 27% in SSML and 31% in foam. Median enrichment factors (EFs), calculated as the ratio of the concentrations of saccharides relative to sodium in SSML or foam to that of seawater, ranged from 1.7 to 6.4 in SSML and 2.1–12.1 in foam. Based on median EFs, xylitol, mannitol, glucose, galactose, mannose, xylose, fucose, rhamnose and ribose were more enriched in foam than SSML. Discussion The greatest EFs for saccharides coincided with high chlorophyll levels, indicating increasing ocean surface enrichment of saccharides during phytoplankton blooms. Higher enrichments of organic matter in sea foam over the SSML indicate that surface active organic compounds become increasingly enriched on persistent bubble film surfaces. These findings help to explain how marine organic matter becomes highly enriched in sea spray aerosol that is generated by bursting bubbles at the ocean surface.more » « less
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Abstract Ocean spray aerosol formed by bubble bursting are at the core of a broad range of atmospheric processes: they are efficient cloud condensation nuclei and carry a variety of chemical, biological, and biomass material from the surface of the ocean to the atmosphere. The origin and composition of these aerosols is sensibly controlled by the detailed fluid mechanics of bubble bursting. This perspective summarizes our present-day knowledge on how bursting bubbles at the surface of a liquid pool contribute to its fragmentation, namely to the formation of droplets stripped from the pool, and associated mechanisms. In particular, we describe bounds and yields for each distinct mechanism, and the way they are sensitive to the bubble production and environmental conditions. We also underline the consequences of each mechanism on some of the many air-sea interactions phenomena identified to date. Attention is specifically payed at delimiting the known from the unknown and the certitudes from the speculations.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1017/jfm.2021.272
