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

  1. 1 2 3 4 5 6 Liss, P.S., Duce, R.A., 1997. The Sea Surface and Global Change. Cambridge Univ. Press, Cambridge.
  2. 1 2 3 Zhang, Zhengbin et al. (2003). Studies on the sea surface microlayer II. The layer of sudden change of physical and chemical properties. Journal of Colloid and Interface Science. 264, 148-159.
  3. 1 2 3 Aller, J., Kuznetsova, M., Jahns, C., Kemp, P. (2005) The sea surface microlayer as a source of viral and bacterial enrichment in marine aerosols. Journal of aerosol science. Vol. 36, pp. 801-812.
  4. 1 2 Carlson, David J. (1983). Dissolved Organic Materials in Surface Microlayers: Temporal and Spatial Variability and Relation to Sea State. Limnology and Oceanography, 28.3. 415-431
  5. ↑ Carlson, David J. (1982). Surface microlayer phenolic enrichments indicate sea surface slicks. Nature. 296.1. 426-429.
  6. 1 2 Woodcock, A. (1953). Salt nuclei in marine air as a function of altitude and wind force. Journal of Meteorology, 10, 362–371.
  7. 1 2 Wallace Jr., G.T., Duce, R.A., 1978. Transport of particulateorganic matter by bubbles in marine waters. Limnol. Oceanogr. 23 Ž6., 1155–1167.
  8. ↑ Gustafsson, M. E. R., & Franzen, L. G. (2000). Inland transport of marine aerosols in southern Sweden. Atmospheric Environments, 34, 313–325.
  9. ↑ Grammatika, M., & Zimmerman,W. B. (2001). Microhydrodynamics offloatation process in the sea surface layer. Dynamics of Atmospheres and Ocean, 34, 327–348.
  10. ↑ Blanchard, D.C., 1983. The production, distribution and bacterial enrichment of the sea-salt aerosol. In: Liss, P.S., Slinn, W.G.N. ŽEds.., Air–Sea Exchange of Gases and Particles. D. Reidel Publishing Co., Dordrecht, Netherlands, pp. 407-444.
  11. ↑ Hoffmann, G.L., Duce, R.A., Walsh, P.R., Hoffmann, E.J., Ray, B.J., 1974. Residence time of some particulate trace metals in the oceanic surface microlayer: significance of atmospheric deposition. J. Rech. Atmos. 8, 745–759.
  12. ↑ Hunter, K.A., 1980. Process affecting particulate trace metals in the sea surface microlayer. Mar. Chem. 9, 49–70.
  13. ↑ Hardy, J.T., Word, J., 1986. Contamination of the water surface of Puget Sound. Puget Sound Notes, U.S. EPA. Region 10 Seattle, WA, pp. 3–6.
  14. ↑ WHO, 1998. Draft guidelines for safe recreational water environments: coastal and fresh waters, draft for consultation. World Health Organization, Geneva, EOSrDRAFTr98 14, pp. 207–299.
  15. ↑ Klassen, R. D., & Roberge, P. R. (1999). Aerosol transport modeling as an aid to understanding atmospheric corrosivity patterns. Materials & Design, 20, 159–168.
  16. ↑ Moorthy, K. K., Satheesh, S. K., & Krishna Murthy, B.V. (1998). Characteristics ofspectral optical depths and size distributions of aerosols over tropical oceanic regions. Journal of Atmospheric and Solar–Terrestrial Physics, 60, 981–992.
  17. ↑ Chow, J. C., Watson, J. G., Green, M. C., Lowenthal, D. H., Bates, B., Oslund, W., & Torre, G. (2000). Cross-border transport and spatial variability of suspended particles in Mexicali and California’s Imperial Valley. Atmospheric Environment, 34, 1833–1843.
  18. 1 2 Marks, R., Kruczalak, K., Jankowska, K., & Michalska, M. (2001). Bacteria and fungi in air over the GulfofGdansk and Baltic sea. Journal of Aerosol Science, 32, 237–250.
  19. ↑ Cincinelli A.; Stortini A.M.; Perugini M.; Checchini L.; Lepri L., 2001. Organic Pollutants in sea-surface microlayer and aerosol in the coastal environment Of Leghorn- (Tyrrhenian Sea). Marine Chemistry, Volume 76, Number 1, pp. 77-98(22)
  20. ↑ Harvey, George W. (1966). Microlayer Collection from the Sea Surface: A New Method and Initial Results. Limnology and Oceanography, 11.4. 608-613
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