Sea surface microlayer

The sea surface microlayer (SML) is the top 1000 micrometers (or 1 millimeter) of the ocean surface. It is the boundary layer where all exchange occurs between the atmosphere and the ocean.^[1] The chemical, physical, and biological properties of the SML differ greatly from the sub-surface water just a few centimeters beneath.^[2]

Overview of Properties

Organic compounds such as amino acids, carbohydrates, fatty acids, and phenols are highly enriched in the SML interface. Most of these come from biota in the sub-surface waters, which decay and become transported to the surface,^[3][4] though other sources exist also such as atmospheric deposition, coastal runoff, and anthropogenic nutrification.^[1] The relative concentration of these compounds is dependent on the nutrient sources as well as climate conditions such as wind speed and precipitation.^[4] These organic compounds on the surface create a "film," referred to as a "slick" when visible,^[2] which affects the physical and optical properties of the interface. These films occur because of the hydrophobic tendencies of many organic compounds, which causes them to protrude into the air-interface.^[1][5] The existence of organic surfactants on the ocean surface impedes wave formation for low wind speeds. For increasing concentrations of surfactant there is an increasing critical wind speed necessary to create ocean waves.^[1][2] Increased levels of organic compounds at the surface also hinders air-sea gas exchange at low wind speeds.^[6] One way in which particulates and organic compounds on the surface are transported into the atmosphere is the process called "bubble bursting".^[1][7] Bubbles generate the major portion of marine aerosols.^[6][8][9] They can be dispersed to heights of several meters, picking up whatever particles latch on to their surface. However, the major supplier of materials comes from the SML.^[3]

Health and Environment

Extensive research has shown that the SML contains elevated concentration of bacteria, viruses, toxic metals and organic pollutants as compared to the sub-surface water.^{[1][10][11][12][13]} These materials can be transferred from the sea-surface to the atmosphere in the form of wind-generated aqueous aerosols due to their high vapor tension and a process known as volatilisation.^[7] When airborne, these microbes can be transported long distances to coastal regions. If they hit land they can have detrimental effects on animals, vegetation and human health.^[14] Marine aerosols that contain viruses can travel hundreds of kilometers from their source and remain in liquid form as long as the humidity is high enough (over 70%).^[15][16][17] These aerosols are able to remain suspended in the atmosphere for about 31 days.^[3] Evidence suggests that bacteria can remain viable after being transported inland through aerosols. Some reached as far as 200 meters at 30 meters above sea level.^[18] A month-long study done by scientists in the Tyrrhenian Sea in 1999 revealed that signals of pollution from chemicals of petrogenic origin in the harbor of Livorno was the result of chemicals found in the SML.^[19] It was also noted that the process which transfers this material to the atmosphere causes further enrichment in both bacteria and viruses in comparison to either the SML or sub-surface waters (up to three orders of magnitude in some locations).^[18]

Measurement

Devices used to sample the concentrations of particulates and compounds of the SML include a glass fabric, metal mesh screens, and other hydrophobic surfaces. These are placed on a rotating cylinder which collects surface samples as it rotates on top of the ocean surface.^[20]

References

Physical oceanography
Waves	Airy wave theory Ballantine scale Benjamin–Feir instability Boussinesq approximation Breaking wave Clapotis Cnoidal wave Cross sea Dispersion Edge wave Equatorial waves Fetch Gravity wave Green's law Infragravity wave Internal wave Kelvin wave Kinematic wave Longshore drift Luke's variational principle Mild-slope equation Radiation stress Rogue wave Rossby wave Rossby-gravity waves Sea state Seiche Significant wave height Sneaker wave Soliton Stokes boundary layer Stokes drift Stokes wave Swell Trochoidal wave Tsunami megatsunami Undertow Ursell number Wave action Wave base Wave height Wave power Wave radar Wave setup Wave shoaling Wave turbulence Wave–current interaction Waves and shallow water one-dimensional Saint-Venant equations shallow water equations Wind wave model
Circulation	Atmospheric circulation Baroclinity Boundary current Coriolis effect Downwelling Eddy Ekman layer Ekman spiral Ekman transport El Niño Southern Oscillation General Circulation Model Geostrophic current Global Ocean Data Analysis Project Gulf Stream Halothermal circulation Humboldt Current Hydrothermal circulation Langmuir circulation Longshore drift Loop Current Maelstrom Modular ocean model Ocean dynamics Ocean gyre Princeton ocean model Rip current Subsurface currents Sverdrup balance Thermohaline circulation shutdown Upwelling Whirlpool World Ocean Circulation Experiment
Tides	Amphidromic point Earth tide Head of tide Internal tide Lunitidal interval Perigean spring tide Rip tide Rule of twelfths Slack water Tidal bore Tidal force Tidal power Tidal race Tidal range Tidal resonance Tide gauge Tideline
Landforms	Abyssal fan Abyssal plain Atoll Bathymetric chart Coastal geography Cold seep Continental margin Continental rise Continental shelf Contourite Guyot Hydrography Oceanic basin Oceanic plateau Oceanic trench Passive margin Seabed Seamount Submarine canyon Submarine volcano
Plate tectonics	Convergent boundary Divergent boundary Fracture zone Hydrothermal vent Marine geology Mid-ocean ridge Mohorovičić discontinuity Vine–Matthews–Morley hypothesis Oceanic crust Outer trench swell Ridge push Seafloor spreading Slab pull Slab suction Slab window Subduction Transform fault Volcanic arc
Ocean zones	Benthic Deep ocean water Deep sea Littoral Mesopelagic Oceanic Pelagic Photic Surf Swash
Sea level	Deep-ocean Assessment and Reporting of Tsunamis Future sea level Global Sea Level Observing System North West Shelf Operational Oceanographic System Sea-level curve Sea level rise World Geodetic System
Acoustics	Deep scattering layer Hydroacoustics Ocean acoustic tomography Sofar bomb SOFAR channel Underwater acoustics
Satellites	Jason-1 Jason-2 (Ocean Surface Topography Mission) Jason-3
Related	Argo Benthic lander Color of water DSV Alvin Marginal sea Marine energy Marine pollution Mooring National Oceanographic Data Center Ocean Ocean exploration Ocean observations Ocean reanalysis Ocean surface topography Ocean thermal energy conversion Oceanography Pelagic sediment Sea surface microlayer Sea surface temperature Seawater Science On a Sphere Thermocline Underwater glider Water column World Ocean Atlas
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