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Principal Oceanographer, Applied Physics Laboratory
University of Washington
Seattle, WA 98105
asher@apl.washington.edu
Education
Ph.D. Environmental Science and Engineering, Oregon Graduate Institute of Science and Technology, 1987
B.A. Chemistry, Reed College, 1980
Employment
- 2005-present: Principal Oceanographer, Applied Physics Laboratory, University of Washington, Seattle
- 2001-present: Staff Scientist, Department of Environmental and Biomolecular Science, Oregon Graduate Institute of Science and Technology, Oregon Health and Science University, Beaverton, Oregon
- 1997-present: Senior Oceanographer, Applied Physics Laboratory, University of Washington, Seattle
- 1995-1997: Research Scientist, Joint Institute for the Atmosphere and the Ocean, University of Washington, Seattle
- 1990-1995: Senior Research Scientist, Pacific Northwest National Laboratory, Battelle Marine Science Laboratory, Sequim, Washington
- 1989-1990: NORCUS Post-Doctoral Research Associate, Pacific Northwest National Laboratory Battelle Marine Science Laboratory, Sequim, Washington
- 1987-1989: Post-Doctoral Research Associate, Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York
- 1982-1987: Graduate Student Research Assistant, Department of Environmental Science and Engineering, Oregon Graduate Institute, Beaverton, Oregon
- 1980-1982: Research Technician, Department of Environmental Science and Engineering, Oregon Graduate Institute, Beaverton, Oregon
Professional Societies
Member, American Geophysical Union
Member, American Chemical Society
Associate Editor, Atmospheric Chemistry and Physics
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Peer-reviewed Publications
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Asher, W.E., W. Luo, K.W. Robinson, K.W. Campo, D.A. Bender, J.S. Zogorski, and J.F. Pankow, 2006. "Application of a source apportionment model in consideration of volatile organic compounds in an urban stream," Environ. Toxicol. Chem., in press.
Asher, W.E., and J.F. Pankow, 2006. "Vapor pressure prediction for alkenoic and aromatic organic compounds by a UNIFAC-based group contribution method," Atmos. Env., 40, 3588-3600.
Erdakos G.B., W.E. Asher, J.H. Seinfeld, and J.F. Pankow, 2006. "Prediction of activity coefficients in liquid aerosol particles containing organic compounds, dissolved inorganic salts, and water – Part 1: Organic compounds and water by consideration of short- and long-range effects using X-UNIFAC.1," Atmos. Env., 40: 6410-6421.
Padmanabhan, S., S.C. Reising, W.E. Asher, L.A. Rose, and P.W. Gaiser, 2006. "Effects of foam on ocean surface microwave emission inferred from radiometric observations of reproducible breaking waves," IEEE Trans. Geosci. Remote Sens., 44: 569-583.
Pankow, J.F., W.E. Asher, and J.S. Zogorski, 2006. "Source apportionment modeling of volatile organic compounds (VOCs) in streams," Environ. Toxicol. Chem. 25: 921-932.
Aziz, M.A., S.C. Reising, W.E. Asher, L.A. Rose, P.W. Gaiser, and K.A. Horgan, 2005. "Effects of air-sea interaction parameters on ocean surface microwave emission at 10 and 37 Ghz," IEEE Trans. Geosci. Remote Sens., 43: 1763-1774.
Asher, W.E., A.T. Jessup, and M.A. Atmane, 2004. "Oceanic application of the active controlled flux technique for measuring air-sea transfer velocities of heat and gases," J. Geophys. Res., 109, C08S12, doi:10.1029/2003JC001862.
Atmane, M.A., W.E. Asher, and A.T. Jessup 2004. "On the use of the active infrared technique to infer heat and gas transfer velocities at the air-water free surface," J. Geophys. Res., 109, C08S14, doi:10.1029/2003JC1805.
McGillis, W.R., W.E. Asher, R. Wanninkhof, A.T. Jessup, and R.A. Feely, 2004. "Introduction to special section: Air-sea exchange," J. Geophys. Res. 109: C08S01.
Siddiqui, M.H.K., M.R. Loewen, W.E. Asher, and A.T. Jessup, 2004. "Coherent structures beneath wind waves and their influence on air-water gas transfer," J. Geophys. Res., 109, doi:10.1029/2002JC001559.
Zappa, C.J., W.E. Asher, A.T. Jessup, and J. Klinke, 2004. "Microbreaking and the enhancement of air-water gas transfer velocities," J. Geophys. Res., 109, C08S16, doi:10.1029/2003JC001897.
Asher, W.E., Jessup, A.T., and Atmane, M.A. 2004. "Oceanic application of the active controlled flux technique for measuring air-sea transfer velocities of heat and gases," J. Geophys. Res., in press.
Atmane, M.A., Asher, W.E., and Jessup, A.T. 2004. "On the use of the active infrared technique to infer heat and gas transfer velocities at the air-water free surface," J. Geophys. Res., in press.
Siddiqui, M.H.K., Loewen, M.R., Asher, W.E., and Jessup, A.T. 2004. "Coherent structures beneath wind waves and their influence on air-water gas transfer," J. Geophys. Res., 109, doi:10.1029/2002JC001559.
Zappa, C.J., Asher, W.E., Jessup, A.T., and Klinke, J. 2004. "Microbreaking and the enhancement of air-water gas transfer velocities," J. Geophys. Res., in press.
Chen, D., Tsang, L., Zhou, L., Reising, S.C., Asher, W.E., Rose, L.A., Ding, K.H., and Chen, C.T. 2003. "Microwave emission and scattering of foam based on Monte Carlo simulations of dense media," IEEE Trans. Geosci. Remote Sens., 41 (4): 782-790.
Pankow, J.F., Luo, W.T., Bender, D.A., Isabelle, L.M., Hollingsworth, J.S., Chen, C., Asher, W.E., and Zogorski, J.S. 2003. "Concentrations and co-occurrence correlations of 88 volatile organic compounds (VOCs) in the ambient air of 13 semi-rural to urban locations in the United States," Atmos. Environ., 37 (36): 5023-5046.
Asher, W.E., G.B. Erdakos, J.H. Seinfeld, and J.F. Pankow, 2002. "Estimating the Vapor Pressures of Multi-functional Oxygen-Containing Organic Compounds Using Group Contribution Methods," Atmos. Env., 36: 1483-1498.
Rose, L.A., W.E. Asher, S.C. Reising, P.W. Gaiser, K.M. St Germain, D.J. Dowgiallo, K.A. Horgan, G. Farquharson, and E.J. Knapp, 2002. "Radiometric Measurements of the Microwave Emissivity of Foam," IEEE Trans. Geosci. Rem. Sens., 40: 2619-2625.
Guo, J., L. Tsang, W. Asher, K.-H. Ding, and C.-T. Chen, 2001. "Applications of dense media radiative transfer for passive microwave remote sensing of foam covered ocean," IEEE Trans. Geosci. Rem. Sens., 39: 1019-1027.
Pankow, J. F., J. H. Seinfeld, W. E. Asher, and G. B. Erdakos, 2001. "Modeling the formation of secondary organic aerosol. 1. The application of theoretical principles to measurements obtained in the -Pinene-, b-Pinene-, Sabinene-, D3-Carene, and Cyclohexene-Ozone systems," Env. Sci. Tech., 35:1164-1172.
Seinfeld, J. H., G. B. Erdakos, W. E. Asher, and J. F. Pankow, 2001. "Modeling the formation of secondary organic aerosol. 2. The predicted effects of relative humidity on aerosol formation in the a-Pinene-, b-Pinene-, Sabinene- , D3-Carene-, and Cyclohexene-Ozone systems", Env. Sci. Tech, 35: 1806-1817.
Siddiqui, M.H. K., M. R. Loewen, C. Richardson, W. E. Asher, and A. T. Jessup, 2001. "Simultaneous Particle Image Velocimetry and Infrared
Imagery of Microscale Breaking Waves" Phys. Fluids, 13: 1891-1903.
Zappa, C. J., W. E. Asher, and A. T. Jessup, 2001. "Microscale wave breaking and air-water gas transfer", J. Geophys. Res., 106: 9385-9391.
Ho, D. T., W. E. Asher, P. Schlosser, L. Bliven, and E. Gordon, 2000. "On the mechanisms of rain-induced air-water gas transfer", J. Geophys. Res., 105 : 24,045-24,057.
Asher, W. E., Q. Wang, E. C. Monahan, and P. M. Smith, 1998. "Estimation of air-sea gas transfer velocities from apparent microwave brightness temperature", Mar. Tech. Soc. J. , 32: 32-40.
Asher, W. E., and R. Wanninkhof, 1998. "Transient tracers and air-sea gas exchange", J. Geophys. Res. , 103C: 15,939-15,958.
Asher, W. E., and R. Wanninkhof, 1998. "The effect of bubble-mediated gas transfer on purposeful dual-gaseous tracer experiments", J. Geophys. Res., 103: 10,555-10,560.
Asher, W.E., L.M. Karle and B.J. Higgins, 1997. "Differences in the parameterization of bubble-mediated air-water transfer in freshwater and seawater",J. Marine Res., 55: 813-845.
Asher, W.E., 1997. "The sea surface microlayer and its effect on global air/sea gas transfer", in The Sea Surface and Global Change, R. Duce and P. S. Liss, eds., Cambridge University Press, New York, pp. 251-285.
McKeown, W. and W. Asher, 1997. "A radiometric method to measure the concentration boundary layer thickness at an air-water interface", J. Atmos. Ocean. Tech., 14: 1494-1501.
Wanninkhof, R., G. Hitchcock, W. Wiseman, P. Ortner, W. Asher, D. Ho, P. Schlosser, M.-L. Dickson, M. Anderson, R. Masserini, K. Fanning, and J.-Z. Zhang, 1997. "Gas exchange, dispersion, and biological productivity on the West Florida Shelf: Results from a Lagrangian tracer study", Geophys. Res. Lett. , 24: 1767-1770.
Asher, W.E., L.M. Karle, B.J. Higgins, P.J. Farley, I.S. Leifer and E.C. Monahan, 1996. "The influence of bubble plumes on air/seawater gas transfer velocities", J. Geophys. Res.,101C : 12,027-12,042.
Asher, W.E. and P.J. Farley, 1995. "Phase-Doppler anemometer measurement of bubble concentrations in laboratory-simulated breaking waves", J. Geophys. Res., 100C: 7045-7056.
Korenowski, G.M., G.S. Frysinger and W.E. Asher, 1993. "Noninvasive probing of the ocean surface using laser-based nonlinear optical methods", Photogram. Eng. and Remote Sensing, 59: 363-369.
Wanninkhof, R.W., W.E. Asher, R. Weppernig, H. Chen, R. Schlosser, C. Langdon and R. Sambrotto, 1993. "Gas transfer experiment on Georges Bank using two volatile deliberate tracers", J. Geophys. Res., 98C:20,237-20,248.
Frysinger, G.S., W.E. Asher, G.M. Korenowski, W.R. Barger, M.A. Klusty, N.M. Frew and R.K. Nelson, 1992. "Study of ocean slicks by nonlinear laser processes 1. Second harmonic generation", J. Geophys. Res., 97C: 5253-5269.
Quinn, P.K., W.E. Asher and R.J. Charlson, 1992. "Equilibria of the marine multi-phase ammonia system", J. Atmos. Chem.,14: 11-30.
Asher, W.E. and J.F. Pankow, 1991. "Prediction of gas/water mass transport coefficients by a surface renewal model", Environ. Sci. Technol., 25: 1294-1300.
Asher, W. E., and J. F. Pankow, 1989. "Direct observation of concentration fluctuations close to a gas/liquid interface", Chem. Engrg. Sci., 44: 1451-1455.
Asher, W.E., G.S Frysinger and G.M. Korenowski, 1988. "Reflected optical second-harmonic generation in the study of naturally occurring organic films at the ocean surface" , J. Geophys. Res., 93C: 6955-6958.
Asher, W.E. and J.F. Pankow, 1986. "The interaction of mechanically generated turbulence and interfacial films with a liquid phase controlled gas/liquid transport process", Tellus, 38B: 305-318.
Pankow, J.F., L.M. Isabelle and W.E. Asher, 1984. "Trace organic compounds in rain. I. Sample design and analysis by adsorption/thermal description", Environ. Sci. Technol., 18: 310-318.
Pankow, J.F., W.E. Asher and L.M. Isabelle, 1983. "Reduction of gas chromatographic needle volatilization and septum bleed with active septum cooling", Anal. Chem., 55: 1451-1453.
Peer-reviewed Conference Proceedings
Asher, W. E., J. B. Edson, W. R. McGillis, R. Wanninkhof, D. T. Ho, and T. Litchendorf. "Fractional area whitecap coverage and air-sea gas transfer during GasEx-98", in Gas Transfer at Water Surfaces, M. A. Donelan, W. M. Drennan, E. S. Saltzman, and R. Wanninkhof, eds., American Geophysical Union, Washington D.C., 199-204.
Siddiqui, M.H. K., M. R. Loewen, C. Richardson, W. E. Asher, and A. T. Jessup, 2002. "Turbulence generated by microscale breaking waves and its influence on air-water gas transfer", in Gas Transfer at Water Surfaces, Geophysical Monograph 127, M. A. Donelan, W. M. Drennan, E. S. Saltzman, and R. Wanninkhof, eds., American Geophysical Union, Washington D.C., 11-17.
Zappa, C. J., W. E. Asher, and A. T. Jessup. "Microscale wave breaking and air-water gas transfer", in Gas Transfer at Water Surfaces, M. A. Donelan, W. M. Drennan, E. S. Saltzman, and R. Wanninkhof, eds., American Geophysical Union, Washington D.C., 23-30.
Asher, W. E., and R. Wanninkhof, 1995. "The effect of breaking waves on the analysis of dual-tracer gas exchange measurements", in Air-Water Gas Transfer, B. Jähne and E. C. Monahan, eds., Aeon Verlag, Hanau, pp. 517-528.
Asher, W. E., L. M. Karle, B. J. Higgins, P. J. Farley, I. S. Leifer, and E. C. Monahan, 1995. "The effect of bubble plume size on the parameterization of air/seawater gas transfer velocities", in Air-Water Gas Transfer, B. Jähne and E. C. Monahan, eds., Aeon Verlag, Hanau, pp. 227-238.
Asher, W. E., B. J. Higgins, L. M. Karle, P. J. Farley, C. R. Sherwood, W. W. Gardiner, R. Wanninkhof, H. Chen, T. Lantry, M. Steckley, E. Monahan, Q. Wang, and P. Smith, 1995. "Measurement of gas transfer, whitecap coverage, and brightness temperature in a surf pool: An overview of WABEX-93", in Air-Water Gas Transfer, B. Jähne and E. C. Monahan, eds.,Aeon Verlag, Hanau, pp. 205-216.
Leifer, I. S., W. E. Asher, and P. J. Farley, 1995. "Modeling bubble-plume mediated air-seawater gas transfer", in Air-Water Gas Transfer, B. Jähne and E. C. Monahan, eds., Aeon Verlag, Hanau, pp. 269-284.
Ogston, A. S., C. R. Sherwood, and W. E. Asher, 1995. "Estimation of turbulence dissipation rates and gas transfer velocities in a surf pool: Analysis of the results from WABEX-93", in Air-Water Gas Transfer , B. Jähne and E. C. Monahan, eds., Aeon Verlag, Hanau, pp. 255-268.
Wang, Q., E. C. Monahan, W. E. Asher, P. M. Smith, 1995. "Correlation of whitecaps and gas transfer velocity with microwave brightness temperature for plunging and spilling breaking waves", in Air-Water Gas Transfer, B. Jähne and E. C. Monahan, eds., Aeon Verlag, Hanau, pp. 217-226.
Wanninkhof, R., W. E. Asher, and E. C. Monahan, 1995. "The influence of bubbles on air-water gas exchange: Results from gas transfer experiments during WABEX-93", in Air-Water Gas Transfer, B. Jähne and E. C. Monahan, eds., Aeon Verlag, Hanau, pp. 239-254.
Asher, W. E., P. J. Farley, R. Wanninkhof, E. C. Monahan, and T. S. Bates, 1992. "Laboratory and field measurements concerning the correlation of fractional area foam coverage with air/sea gas transport", in Precipitation Scavenging and Atmosphere-Surface Exchange, Volume 2-The Semonin Volume: Atmosphere-Surface Exchange Processes, S. E. Schwartz and W. G. N. Slinn, eds., Hemisphere, Washington D.C., pp. 815-828.
Asher, W. E., E. C. Monahan, R. Wanninkhof, and T. S. Bates, 1991. "Correlation of fractional foam coverage with gas transport rates", in Air-Water Mass Transfer S. E. Wilhelms and J. S. Gulliver, eds., A.S.C.E., New York, pp. 536-548.
Asher, W. E., and J. F. Pankow, 1991. "The effect of surface films on concentration fluctuations close to a gas/liquid interface", in Air-Water Mass Transfer, S. E. Wilhelms and J. S. Gulliver, eds., A.S.C.E., New York, pp. 68-80.
Pankow, J. F., W. E. Asher, and E. J. List, 1984. "Carbon dioxide transfer at the air/water interface as a function of system turbulence", in Gas Transfer at Water Surfaces, W. Brutsaert and G. H. Jirka, eds., Reidel, Hingham, pp. 101-111.
Microwave Emission and Scattering of Foam Based on Monte Carlo Simulations of Dense Media
The foam-covered ocean surface is treated as densely packed air bubbles coated with thin layers of seawater. We apply Monte Carlo simulations of solutions of Maxwell's equations to calculate the absorption, scattering, and extinction coefficients at 10.8 and 36.5 GHz. These quantities are then used in dense-media radiative transfer theory to calculate the microwave emissivity. Numerical results of the model are illustrated As a function of foam parameters. Results of emissivities for both horizontal polarization and vertical polarizations at 10.8 and 36.5 GHz are compared with recent experimental measurements.
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Concentrations and Co-occurrence Correlations of 88 Volatile Organic Compounds (VOCs) in the Ambient Air of 13 Semi-rural to Urban Locations in the United States
The ambient air concentrations of 88 volatile organic compounds were determined in samples taken at 13 semi-rural to urban locations in Maine, Massachusetts, New Jersey, Pennsylvania, Ohio, Illinois, Louisiana, and California. The sampling periods ranged from 7 to 29 months, yielding a large data set with a total of 23,191 individual air concentration values, some of which were designated "ND" (not detected). For each compound at each sampling site, the air concentrations (c(a), ppbV) are reported in terms of means, medians, and means of the detected values. The analytical method utilized adsorption/thermal desorption with air-sampling cartridges. The analytes included numerous halogenated alkanes. halogenated alkenes, ethers, alcohols, nitriles, esters, ketones, aromatics, a disulfide, and a furan. At some sites, the air concentrations of the gasoline-related aromatic compounds and the gasoline additive methyl tertbutyl ether were seasonally dependent, with concentrations that maximized in the winter. For each site studied here, the concentrations of some compounds were highly correlated one with another [e.g., the BTEX group (benzene, toluene, ethylbenzene. and the xylenes)]. Other aromatic compounds were also all generally correlated with one another, while the concentrations of other compound pairs were not correlated (e.g., benzene was not correlated with CFC-12). The concentrations found for the BTEX group were generally lower than the values that have been previously reported for urbanized and industrialized areas of other nations.
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Coherent Structures Beneath Wind Waves and Their Influence on Air-Water Gas Transfer
Coherent structures generated beneath laboratory wind waves were investigated using particle image velocimetry. An algorithm was developed to detect these structures and to determine their size, vorticity, and kinetic energy. As the wind speed increased from 4.5 to 11.0 m s-1, the maximum vorticity of the coherent structures increased by 40%, their average size increased by 20%, their frequency of occurrence increased 400%, and the fraction of the water surface renewed by coherent structures increased from 0.12 to 0.33. Distributions of the total kinetic energy of the coherent structures as a function of size showed that the most energetic eddies occurred in the size range 0.8-1.6 cm in diameter. The near-surface flow could be divided into areas with one of two distinct characteristics: energetic regions occupied by coherent structures and quiescent regions largely devoid of coherent structures. A surface renewal model for air-water exchange was used to calculate the local transfer velocity in both types of regions. The model predicted that the gas transfer velocities in the energetic regions were 2.8 times larger than in the quies cent regions and that 60% of the total air-water gas flux occurred across the energetic regions at all wind speeds. In addition, the rate of turbulent kinetic energy dissipation was similar to 2.5 times higher in the energetic regions compared to the quiescent regions at all wind speeds.
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On the Use of the Active Infrared Technique to Infer Heat and Gas Transfer Velocities at the Air-Water Free Surface
A comparison study of the experimental and theoretical transfer velocities of heat ands gas transfer at a wavy air-water interface is undertaken using an active infrared technique and two gas tracers. Applying the surface renewal model formalism [Danckwerts, 1951], we find that the experimentally evaluated heat transfer velocity is roughly a factor of 2 higher than the transfer velocity of a gas with a low solubility in water when both are referenced to Sc = 600. Potential origins of such a discrepancy are investigated and we propose the use of the random eddy model [Harriott, 1962] to explain our results. The model is an extension of surface renewal to include the eddy approach distance as a new parameter. Numerical simulations of the random eddy model have been performed using a timescale evaluated from the Active Controlled Flux Technique (ACFT) and the characteristics of heat as well as the two gases used in the experiments (He and SF6). The simulation results show that the transfer velocities of two species referenced to the same Schmidt number by the surface renewal model are different and that their ratio depends on the average value of the approach distance and its distribution. The model as implemented in the present work also predicts changes in the Schmidt number exponent when the hydrodynamics conditions are varied.
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Microbreaking and the Enhancement of Air-Water Gas Transfer Velocities
The role of microscale wave breaking in controlling the air-water transfer of heat and gas is investigated in a laboratory wind-wave tank. The heat transfer velocity was measured using an active infrared technique and the gas transfer velocity was measured using bulk methods. Simultaneous, co-located infrared and wave slope imagery show that wave-related areas of thermal boundary layer disruption and renewal are the turbulent wakes of microscale breaking waves, or microbreakers. These signatures of disruption are associated with waves that have a steep forward face and a dimpled bore-like crest. The fractional area coverage, AB, of the surface affected by these renewal features is significant (0.25–0.40 for a cleaned surface at wind speeds ranging from 4.2 to 8.3 m s-1). Furthermore, the local air-water heat transfer velocity, kH, is linearly correlated with AB and this correlation is insensitive to the presence of surfactants and independent of fetch. The heat transfer velocity inside the wakes of microscale breaking waves was a factor of 3.4 larger than it was outside the wakes. The results show that up to 75% of the heat transfer across the air-water interface under moderate wind speeds is the direct result of microbreaking. The analogy between heat and gas transfer implies that the gas transfer velocity, kG, is similarly dependent on microbreaking. However, we found that when referenced to a common Schmidt number, kH was consistently greater than kG by a factor 2.5, implying that heat may not be appropriate as a proxy tracer for gas. The roughness features associated with microscale breaking waves are shown to contribute to the mean square slope, S2. Moreover, AB is found to correlate with S2, and this correlation provides a link that explains the correlation between air-water transfer velocities and S2. The correlation between kH and AB regardless of surfactant concentration, combined with the enhanced local transfer velocity and wave slope results, provides quantitative laboratory evidence that microbreaking is the dominant mechanism contributing to air-sea heat and gas transfer at low to moderate wind speeds.
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Oceanic Application of the Active Controlled Flux Technique for Measuring Air-Sea Transfer Velocities of Heat and Gases
Detailed understanding of the hydrodynamic mechanisms controlling the air-sea exchange of heat and gas requires a method for rapid measurement of the associated transfer velocities. The active controlled flux technique (ACFT), where the temperature decay of a small patch of water heated by an infrared laser is tracked using an infrared imager, has been proposed as a method for making these fast non-invasive measurements of the heat and gas transfer velocities. Here, we report on ACFT measurements of the transfer velocity of heat, kH, made in the ocean during the Fluxes, Air-sea Interactions and Remote Sensing (FAIRS) experiment (September/October, 2000) and GasEx-01 (January/February, 2001). The results for kH from both FAIRS and GasEx-01 compare favorably when plotted versus wind speed. However, when scaled to a Schmidt number of 660, the measured kH values were found to be a factor of two larger than gas transfer velocities measured during GasEx-01. The ACFT-derived kH values were combined with direct measurements of the bulk-skin oceanic temperature difference to calculate net air-sea heat fluxes during both experiments. Comparison of these values with heat fluxes determined by direct measurements of the latent, sensible, and radiative heat fluxes showed that the ACFT measurements are a factor of seven larger than the direct measurements. One possible theory explaining both the overprediction of the gas transfer velocities and the scale factor between the measured and calculated net heat fluxes is that air-sea exchange is best described by surface penetration rather than surface renewal.
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Estimating the Vapor Pressures of Multi-functional Oxygen-containing Organic Compounds Using Group Contribution Methods
A UNIFAC-based method for estimating the vapor pressure (pLo) values of oxygen-containing compounds of intermediate to low volatility has been developed as an aid in modeling the formation and behavior of organic aerosols. This UNIFAC-pLo method was constructed using a set of 76 compounds with experimentally-determined values. The compounds chosen are of intermediate-to-low volatility and contain multiple oxygen-containing functionalities. For test and development purposes, the 76 compounds were divided into a basis set of 43 compounds used to generate the coefficients required in the UNIFAC-pLo method and a second set of 30 compounds that was used to test the coefficients generated using the basis set. Both the basis and test set contained compounds that possessed similar structures and functional groups. For the 33 compounds in the test set, UNIFAC-pLo predicted the values to within a factor of 2 over the temperature range 290 K to 320 K. More importantly, UNIFAC-pLo did not show any correlation of prediction error with pLo, which implies that the UNIFAC-pLo method was equally likely to underpredict as overpredict regardless of volatility. The method also can be easily extended to other classes of compounds by construction and use of other basis sets of compounds.
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Radiometric Measurements of the Microwave Emissivity of Foam
Radiometric measurements of the microwave emissivity of foam were conducted during May 2000 at the Naval Research Laboratory's Chesapeake Bay Detachment using radiometers operating at 10.8 and 36.5 GHz. Horizontal and vertical polarization measurements were performed at 36.5 GHz; horizontal, vertical, + 45 , - 45 , left circular, and right circular polarization measurements were obtained at 10.8 GHz. These measurements were carried out over a range of incidence angles from 30 to 60 . Surface foam was generated by blowing compressed air through a matrix of gas permeable tubing supported by an aluminum frame and floats. Video micrographs of the foam were used to measure bubble size distribution and foam layer thickness. A video camera was boresighted with the radiometers to determine the beam-fill fraction of the foam generator. Results show emissivities that were greater than 0.9 and approximately constant in value over the range of incidence angles for vertically polarized radiation at both 10.8 and 36.5 GHz, while emissivities of horizontally polarized radiation showed a gradual decrease in value as incidence angle increased. Emissivities at + 45 , - 45 , left circular, and right circular polarizations were all very nearly equal to each other and were in turn approximately equal to the average values of the horizontal and vertical emissivities in each case.
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Applications of Dense Media Radiative Transfer Theory for Passive Microwave Remote Sensing of Foam Covered Ocean
The effect of the foam covered ocean surface on the passive microwave remote sensing measurements is studied based on the electromagnetic scattering theory. In formulating an electromagnetic scattering model, we treat the foam as densely packed sticky air bubbles coated with thin seawater coating. The layer of foam covers the ocean surface that have air bubbles. We then use dense media radiative transfer (DMRT) theory with Quasi-crystalline approximation (QCA) for densely distributed sticky moderate size particles to calculate the brightness temperatures of the foam-covered ocean surface. Results are illustrated for 19GHz and 37GHz and for both vertical and horizontal polarizations as a function of foam microstructure properties and foam layer thickness. Comparisons are also made with experimental measurements.
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Modeling the Formation of Secondary Organic Aerosol (SOA). The Application of Theoretical Principles to Measurements Obtained in the a-Pinene-, b-Pinene-, Sabinene- , D3-Carene, and Cyclohexene-Ozone Systems
Secondary organic aerosol (SOA) forms in the atmosphere when volatile parent compounds are oxidized to form low-volatility products which condense to yield organic particulate matter (PM). Under conditions of intense photochemical smog, from 40 to 80% of the particulate organic carbon can be secondary in origin. Because describing multicomponent condensation requires a compound-by-compound identification and quantification of the condensable compounds, the complexity of ambient SOA has made it difficult to test the ability of existing gas/particle (G/P) partitioning theory to predict SOA formation in urban air. This paper examines that ability using G/P data from past laboratory chamber experiments carried out at approximately 308 K with five parent hydrocarbons (HCs) (four monoterpenes and cyclohexene) in which significant fractions (61 to 100%) of the total mass of SOA formed from those hydrocarbons were identified and quantified by compound.
The model calculations were based on a matrix representation of the multiple component SOA G/P distribution process. The governing equations were solved by an iterative method. Input data for the model included: 1) DHC (mg m-3), the amount of reacted parent hydrocarbon; 2) the a values which give the total concentration T (gas + particle phase, mg m-3) values for each product i according to Ti = ai DHC; 3) estimates of the pure compound liquid vapor pressure pL0 values at 308 K for the products; and 4) UNIFAC parameters for estimating activity coefficients in the SOA phase for the product compounds as a function of SOA composition. The model predicts the total amount M o (mg m-3) of organic aerosol that will form from the reaction of DHC, the total aerosol yield Y (= M o/DHC), and the compound-by-compound yield values Yi.
An impediment in applying the model was the lack of literature data on pL0 values for the compounds of interest, or even on pL0 values for other, similarly low-volatility compounds. This was overcome in part by using the G/P data from the a-pinene and cyclohexene experiments to determine pL0 values for use (along with a set of 14 other independent polar compounds) in calculating UNIFAC vapor pressure parameters that were in turn used to estimate all of the needed pL0 values. The significant degree of resultant circularity in the calculations for a-pinene and cyclohexene required the good agreement that was found between the Yi values predicted by the model, and those measured experimentally for a-pinene and cyclohexene. However, the fact that model was able to do a good job of predicting the aerosol yield values from b-pinene, sabinene, and D3-carene suggests that given the correct input information, SOA formation can in fact be accurately modeled as a multicomponent condensation process.
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Modeling the Formation of Secondary Organic Aerosol (SOA). 2. The Predicted Effects of Relative Humidity on Aerosol Formation in the a-Pinene-,b-Pinene-,Sabinene-, D3- Carene-, and Cyclohexene-Ozone Systems
Atmospheric oxidation of volatile organic compounds can lead to the formation of secondary organic aerosol (SOA) through the gas/particle (G/P) partitioning of the oxidation products. Since water is ubiquitous in the atmosphere, the extent of the partitioning for any individual organic product depends not only on the amounts and properties of the partitioning organic compounds, but also on the amount of water present. Predicting the effects of water on the atmospheric G/P distributions of organic compounds is, therefore, central to understanding SOA formation. The goals of the current work are to gain understanding of how increases in RH affect: 1) overall SOA yields; 2) water uptake by SOA; 3) the behaviors of individual oxidation products; and 4) the fundamental physical properties of the SOA phase that govern the G/P distribution of each of the oxidation products. Part 1 of this series considered SOA formation from five parent hydrocarbons in the absence of water. This paper predicts how adding RH to those systems uniformly increases both the amount of condensed organic mass, and the amount of liquid water in the SOA phase. The effect of increasing RH is predicted to be stronger for SOA produced from cyclohexene as compared to SOA produced from four monoterpenes. This is likely a result of the greater general degree of oxidation (and hydrophilicity) of the cyclohexene products. Good agreement was obtained between predicted SOA yields and laboratory SOA yield data actually obtained in the presence of water. As RH increases, the compounds that play the largest roles in changing both the organic and water masses in the SOA phase are those with vapor pressures that are intermediate between those of essentially non-volatile and highly volatile species. RH-driven changes in the compound-dependent G/P partitioning coefficient Kp result from changes in both the average molecular weight MWom of the absorbing organic/water phase, and the compound-dependent activity coefficient z values. Adding water to the SOA phase by increasing the RH drives down MWom and thereby uniformly favors SOA condensation. The effect of RH on z values is compound specific, and depends on the hydrophilicity of the specific compound of interest; the more hydrophilic a compound, the more increasing RH will favor its condensation into the SOA phase. The results also indicate that it may be a useful first approximation to assume that z = 1 for many compounds making up SOA mixtures.
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Microscale Wave Breaking and Air-Water Gas Transfer
Laboratory results showing that the air--water gas transfer velocity, k, is correlated with mean square wave slope have been cited as evidence that a wave-related mechanism regulates k at low to moderate wind speeds [Jähne et al., 1987; Bock et al., 1999]. Csanady [1990] has modeled the effect of microscale wave breaking on air--water gas transfer with the result that k is proportional to the fractional surface area covered by surface renewal generated during the breaking process. In this report, we investigate the role of microscale wave breaking in gas transfer by determining the correlation between k and AB, the fractional area coverage of microscale breaking waves. Simultaneous, co-located infrared (IR) and wave slope imagery is used to verify that AB detected using IR techniques corresponds to the fraction of surface area covered by surface renewal in the wakes of microscale breaking waves. Using measurements of k and AB made at the University of Washington wind-wave tank at wind speeds from 4.6 to 10.7 m s-1, we show that k is linearly correlated with AB, regardless of the presence of surfactants. This result is consistent with Csanady's [1990] model and implies that microscale wave breaking is likely a fundamental physical mechanism contributing to gas transfer.
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On Mechanisms of Rain-induced Air-water Gas Exchange
Previous studies have shown that rain significantly enhances the rate of air-water gas exchange. However, even though an empirical correlation between the rain rate or kinetic energy flux (KEF) delivered to the water surface by rain and the gas transfer velocity has been established, the physical mechanisms underlying the gas exchange enhancement remain unexamined. During a series of experiments, the processes behind rain-induced air-water gas exchange were examined at NASA's Rain-Sea Interaction Facility (RSIF). Gas transfer velocities for helium (He), nitrous oxide (NO), and sulfur hexafluoride (SF6) were determined for 22 rain rates (13.6 to 115.2 mm h-1) and three drop sizes (2.3, 2.8, 4.2 mm). Bubbles generated by the raindrops were characterized using a video-microscope technique, and surface waves were characterized by a capacitance probe. Additionally, rain-generated turbulence was inferred from friction velocities u*w calculated from KEF. Together, these data suggest that rain-induced air-water gas exchange is mainly caused by turbulence-driven exchange processes, with bubbles contributing anywhere from 0 to 20%, depending on rain rate, drop size, and the solubility of the gas tracer. Furthermore, the data confirm that the previously selected variable KEF is the best correlate for rain-induced air-water gas exchange.
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Estimation of Air-Sea Gas Transfer Velocities from Apparent Microwave Brightness Temperature
Oceanic measurements of air-sea gas transfer velocities, wind speed, and fractional area whitecap coverage have shown that gas transfer rates are a function of both whitecap coverage and wind speed. The microwave emissivity of the sea surface is also known to be a function of these same environmental forcing functions. Therefore, it could be possible to estimate transfer velocities from microwave brightness temperatures measured by satellite-mounted or aircraft-borne radiometers. This hypothesis is supported by concurrent microwave radiometric and gas transfer data collected in a surf pool. In this paper, the average microwave emissivity of simulated breaking waves was measured at 19 GHz for horizontally and vertically polarized radiation. The emissivities were used in a simple model for the microwave response of the ocean surface to calculate brightness temperatures. Brightness temperatures could then be calculated for specific wind speeds and whitecap coverages corresponding to the physical conditions for which oceanic measurements of the air-sea gas transfer velocity are available. Comparing the increase in the calculated brightness temperatures with the measured increase in transfer velocity shows that currently available microwave radiometers have the necessary precision for in situ measurement of gas transfer velocities. However, the relations developed here must be verified by direct concurrent measurement of brightness temperature and gas transfer under oceanic conditions.
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Transient Tracers and Air-Sea Gas Exchange
This paper provides a review of the physics and chemistry associated with air-sea gas transfer of transient atmospheric trace gases and the available laboratory and field measurement techniques used to study air-water gas transfer. The mechanistic principals and their relation to the measurement techniques are used to show that the error associated with estimating air-sea transfer velocities of transient tracers from transfer velocities measured using proxy tracers can be significant if an incorrect dependence of the transfer velocity on molecular diffusivity is assumed. Bubble-mediated transfer processes are also demonstrated to have a significant effect on the parameterization of the transfer velocity.
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The Effect of Bubble-Mediated Gas Transfer on Purposeful Dual-Gaseous Tracer Experiments
For air-water gas exchange across unbroken surfaces, the only gas-dependent parameter affecting the transfer velocity is the molecular diffusivity of the transferring species. In contrast, bubble-mediated transfer processes can cause the transfer velocity to depend on both molecular diffusivity and aqueous-phase solubility. This can complicate the analysis of data from dual-gaseous tracer gas transfer experiments. Bubble effects also complicate the estimation of transfer velocities for other gases from the transfer velocity calculated using the dual-tracer data. Herein a method for incorporating the effects of bubble-mediated gas transfer processes on the transfer velocity is presented. This new procedure is used to analyze the data from two recent dual-tracer gas transfer experiments. Transfer velocities that include the effect of bubbles are calculated using the data from two previous oceanic dual-gaseous tracer experiments. Comparing these transfer velocities with transfer velocities calculated by neglecting the effect of bubbles shows that bubble-mediated transfer increased the transfer velocity of helium 3 by 5% at a wind speed of 10.6 m s-1. However, when using the transfer velocities for helium 3 to calculate transfer velocities for carbon dioxide under the same conditions, including the effect of bubbles decreases the transfer velocity of carbon dioxide by 18%. This shows that bubble-mediated transfer does not have a large effect on the analysis of dual-tracer data, but it is important in relating transfer velocities determined using helium 3 and sulfur hexafluoride to transfer velocities of more soluble gases at wind speeds above 10 m s-1.
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Differences in the Parameterization of Bubble-Mediated Air-Water Transfer in Freshwater and Seawater
Bubble populations and gas transfer velocities were measured in cleaned and surfactant-influenced freshwater and seawater. A nonlinear fitting technique was used to partition the total gas transfer velocity for a gas in each water type into a turbulence- and bubble-mediated fraction. This showed that the bubble-mediated transfer fraction was larger in cleaned freshwater than in cleaned seawater and that the difference was a function of diffusivity and solubility. This was explained by the fact that the bubble measurements showed that bubble plumes in cleaned freshwater had a higher concentration of large bubbles and a lower concentration of small bubbles than the plumes in cleaned seawater. The differences between the behavior of the bubble-mediated gas flux in cleaned freshwater and cleaned seawater show that caution should be used when intercomparing laboratory results from measurements made in different media. These differences also will make parameterizations of bubble-mediated gas exchange developed using freshwater laboratory data difficult to apply directly to oceanic conditions. It was found that adding a surfactant to seawater had minimal impact on the concentration of bubbles in the plumes. Because surfactants decrease the gas flux to the individual bubbles, the similarity in bubble population meant that the addition of surfactant to seawater decreased the bubble-mediated gas flux compared to the flux in cleaned seawater. In contrast, the addition of a surfactant to freshwater increased the concentration of bubbles by over an order of magnitude. This increase in bubble population was large enough to offset the decrease in the flux to the individual bubbles so that the net bubble-mediated gas flux in freshwater increased when surfactant was added. This difference in behavior of the bubble population and bubble-mediated transfer velocity between surfactant-influenced and cleaned waters further complicates interrelating laboratory measurements and applying laboratory results to the ocean.
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The Sea Surface Microlayer and its Effect on Global Air/Sea Gas Transfer
Laboratory measurements of air/water gas transfer rates for cleaned and film-covered water surfaces have shown that the presence of soluble and insoluble surfactants can inhibit air/water gas fluxes. Naturally occurring surface active material is known to concentrate in the marine surface microlayer and form films and slicks. It is reasonable that oceanic slicks and films may lower in situ gas transfer rates compared with air/sea gas exchange through a clean ocean surface. Here, a simple model of gas transfer through clean and surfactant-influenced water surfaces is used to develop parameterizations of liquid-phase, and gas-phase, rate-controlled gas transfer velocities through clean and surfactant-influenced ocean surfaces. The parameterization for liquid-phase, rate-controlled processes is used to estimate the effect of naturally occurring surface films on the net global flux of carbon dioxide. The gas-phase, rate-controlled relations are used to study the impact of films on the flux of ammonia from the central Pacific Ocean.
By relating the fractional area coverage of surfactant-influenced sea surface from a global map of net synthetic primary production, the model shows that surface films can increase or decrease the net global oceanic carbon dioxide flux, depending on the regional film coverage.
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A Radiometric Method to Measure the Concentration Boundary Layer Thinkness at an Air-Water Interface
An interferometric technique to measure the thickness of the concentration boundary layer at an air-water interface is presented. The technique uses heat as a proxy tracer for gas transfer by sensing the infrared (IR) radiation emitted by the water at wavelengths near 3.8 microns. The temperature gradient at the water surface is determined from the IR radiance by using the wavelength variation of the absorption coefficient of water in the wavelength region 3.3 microns to 4.1 microns. The variations in the absorption coefficient allow the emitted IR radiation to carry information about the surface and sub-surface water temperature. Interferometric measurements of the radiance variations as a function of optical wavelength can be related to the temperature gradient, or the change in water temperature with respect to depth. Previous laboratory measurements indicate that interfacial temperature gradients can be studied in great detail using this approach. Very near the water surface, the temperature gradient is linear and heat transport is mainly by molecular conduction. Beyond this molecular conduction zone, eddy diffusion dominates transfer and temperature is constant with depth. The thickness of this thermal boundary layer can be estimated directly from the depth where the measured gradient becomes constant or can be inferred by extrapolating the gradient to the known bulk temperature. The concentration boundary layer thickness can then be related to the thermal boundary layer depth by appropriately scaling the thermal and molecular diffusivities. The proposed experiment shows how this information could be used to remotely determine the air-sea gas transfer velocity and investigate the dependence of the concentration boundary layer thickness on both molecular diffusivity and physical forcing functions (i.e., wind speed, near-surface turbulence).
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Gas Exchange, Dispersion, and Biological Productivity on the West Florida Shelf: Results from a Lagrangian Tracer Study
A Lagrangian tracer study was performed on the west Florida shelf in April 1996 using deliberately injected trace gases. Although such studies have been performed previously, this work is the first where the deliberate tracers, in conjunction with carbon system parameters, are used to quantify changes in water column carbon inventories due to air-sea exchange and net community metabolism. The horizontal dispersion and the gas transfer velocity were determined over a period of 2 weeks from the change in both the concentrations and the concentration ratio of the two injected trace gases, sulfur hexafluoride (SF6) and helium-3 (He-3). The second moment of the patch grew to 1.6 x 10(3) km(2) over a period of 11 days. The gas transfer velocity, normalized to CO2 exchange at 20 degrees C, was 8.4 cm hr(-1) at an average wind speed, U-10, of 4.4 m s(-1) for the duration of the experiment, which is in good agreement with empirical estimates. Remineralization rates exceeded productivity, causing an increase in dissolved inorganic carbon of about 1 mu mol kg(-1) day(-1) in the water column. During this period of senescence, 80% of the increase in inorganic carbon is attributed to community remineralization and 20% due to invasion of atmospheric CO2.
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The Influence of Bubble Plumes on Air-Seawater Gas Transfer Velocities
Laboratory results have demonstrated that bubble plumes are a very efficient air-water gas transfer mechanism. Because breaking waves generate bubble plumes, it could be possible to correlate the air-sea gas transport velocity kL with whitecap coverage. This correlation would then allow kL to be predicted from measurements of apparent microwave brightness temperature through the increase in sea surface microwave emissivity associated with breaking waves. In order to develop this remote-sensing-based method for predicting air-sea gas fluxes, a whitecap simulation tank was used to measure evasive and invasive kL values for air-seawater transfer of carbon dioxide, oxygen, helium, sulfur hexafluoride, and dimethyl sulfide at cleaned and surfactant-influenced water surfaces. An empirical model has been developed that can predict kL from bubble plume coverage, diffusivity, and solubility. The observed dependence of kL on molecular diffusivity and aqueous-phase solubility agrees with the predictions of modeling studies of bubble-driven air-water gas transfer. It has also been shown that soluble surfactants can decrease kL even in the presence of breaking waves.
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Phase-Doppler Anemometer Measurement of Bubble Concentrations in Laboratory-Simulated Breaking Waves
Breaking waves and the bubble plumes they generate are thought to be an important air-sea gas transfer mechanism. In order to compare bubble populations generated by laboratory-simulated breaking waves and model bubble-mediated air-water gas transfer in a whitecap simulation tank (WST), it was necessary to measure bubble sizes and water velocities. The phase-Doppler anemometer (PDA) is well suited for this research because it provides simultaneous measurement of the size and velocity of a bubble. Bubble concentrations can be estimated from PDA data records with an accuracy of 15% (+/-1 sigma ) for bubbles with radii in the range of 50 to 160 microns and 40% (+/-1 sigma ) for larger bubbles. However, estimation of bubble concentrations requires knowledge of the instrument-scattering cross section as a function of bubble radius. A procedure for calibrating the scattering cross section of the PDA was developed and tested. The PDA was then used to measured size-segregated bubble concentrations in the WST as a function of depth, water temperature, total concentration of dissolved gas, and surfactant concentration. These measurements show that increases in dissolved gas concentration or decreases in water temperature increase bubble concentrations. Surfactants increased the concentration of small bubbles. Comparison of the WST bubble populations to measurements of oceanic bubbles showed that the two agree within the experimental uncertainty of the PDA.
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Noninvasive Probing of the Ocean Surface using Laser-Based Nonlinear Optical Methods
The laser-based nonlinear optical methods of second-harmonic generation and sum-frequency generation have been developed to study the chemical composition and concentration of natural surfactant materials present as slicks on the ocean surface. These noninvasive second-harmonic and sum-frequency generation methods produce signals which originate from only the top few molecular layers of the ocean surface, thereby producing an accurate picture of the ocean surface condition without interference from the bulk ocean chemistry. Chemical specificity of the methods is achieved by tuning the incident laser frequency to coincide with optical absorptions in the surface absorbed materials. We show that laser-based second-order nonlinear optical processes of SHG and SFG provide highly surface selective, noninvasive, in situ probes of the ocean surface. Although only preliminary experiments are reported in this paper, the probes provide important information about the nature of surfactants at the ocean surface and their behavior in response to dynamic forces at the sea/air interface. The future of the probes lies in their further development and use as in situ interfacial spectroscopic techniques.
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Gas Transfer Experiment on Georges Bank using Two Volatile Deliberate Tracers
A gas exchange experiment was performed on Georges Bank using deliberate tracers sulfur hexafluoride (SF6) and helium 3 (3He). The concentrations of the tracers were measured in the water column over a period of 10 days. During this time the patch grew from an 8-km-long injection streak to an area of about 500km2. The gas transfer velocity was determined from the change in the ratio of the tracers over time scaled to the ratio of the Schmidt numbers. A near-linear relationship between gas exchange and wind speed was observed based on four experimental points covering a wind speed range from 3 to 11 m/s. The results fall in the upper part of the range of gas transfer-wind speed relationships developed to date. Wind speeds during the experiment obtained from anemometers on the ship, on a free floating drifter, and on a fixed mooring showed significant differences. With the ability to measure gas transfer velocities over the ocean on timescales of several days, accurate wind speed/stress measurements are imperative to obtain a robust relationship between gas transfer and wind speed.
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Study of Ocean Slicks by Nonlinear Laser Processes. 1. Second-Harmonic Generation
Reflected optical second-harmonic generation (SHG) was applied to study of the air-sea interface. SHG signals were detected from the first several molecular layers of the ocean surface during the SAR X Band Ocean Nonlinearities (SAXON) Chesapeake Light Tower experiment in 1988, and during experiments on Nantucket Sound at the Woods Hole Oceanographic Institution in 1989. The SHG response of the ocean surface was observed to correlate with increased slick activity and surface tension measurements of surface water organic content. The SHG response was similar for naturally occurring slicks and for artificially created slicks of several known materials. The SHG signal intensity was also used to estimate the second-order nonlinear optical susceptibility of the ocean surface. It was determined that the SHG nonlinear laser technique is a useful noninvasive probe for in situ studies of ocean surface chemistry.
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Equilibria of the Marine Multi-phase Ammonia System
A lack of empirical data has made it difficult to ascertain whether ammonia is in equilibrium between the oceanic, atmospheric gas and atmospheric particle phases in the remote marine environment. Reported here are simultaneous measurements of the saturation concentration of ammonia relative to ammonia concentrations in ocean surface waters; total seawater ammonia; atmospheric gas phase ammonia; and atmospheric particulate-phase ammonium, non-seasalt sulfate, methanesulfonate, and nitrate. Sampling was performed in May of 1987 in the northeast Pacific Ocean environment and in April and May of 1988 in the central Pacific Ocean environment. These measurements were used to determine the degree to which ammonia approached equilibrium between the oceanic and atmospheric gas and aerosol particle phases. The experimental atmospheric gas phase ammonia concentrations were compared with calculated equilibrium concentrations assuming a Henry's law type of partitioning between the gas and condensed phases. The measured atmospheric gas phase and oceanic concentrations of ammonia indicate that ammonia is not in a Henry's law equilibrium across the air/sea interface. This disequilibrium is a result of the long air/sea exchange equilibration time relative to the lifetime of ammonia in the atmosphere.
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Prediction of Gas/Water Mass Transport Coefficients by a Surface Renewal Model
Aqueous phase carbon dioxide concentration fluctuation timescales measured at carbon dioxide/water interfaces were used in a surface renewal model to calculate the aqueous phase mass transport coefficient, kL, for a range of turbulence conditions for cleaned and film-covered water surfaces. The calculated kL values were compared to kL's measured in a separate set of experiments over the same range of turbulence and interfacial conditions. This test of a surface renewal model with directly measured input parameters has shown that kL may be accurately estimated by the model for clean interfaces and high turbulence intensities. The data also show that surface renewal models are not appropriate for use in calculation of mass fluxes at film-covered interfaces.
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Direct Observations of Concentration Fluctuations Close to a Gas/Liquid Interface
Previous measurement of near-surface concentration fluctuations has been done by Springer and Pigford (1970) and Luk and Lee (1986). However, in the case of the former experiment the fluctuations were observed indirectly and the latter an invasive sampling technique was used. Furthermore, neither of these previous studies used a gas-liquid interface which had been cleaned. This is an important point since Asher and Pankow (1986) have shown that stirred tank type systems such as used by Springer and Pigford (1970) and Luk and Lee (1986) are very sensitive to the presence of adventitious surface films. Therefore, systematic, non-invasive measurement of surface concentration fluctuation timescales for known turbulence and interfacial conditions has not been done.
Herein, modifications to the laser-inducded fluorescence (LIF) technique of Pankow et al. (1984) are described which enable the direct observation of carbon dioxide (CO2) concentration {[CO2] (mol cm-3)} fluctuations in the aqueous surface microlayer as a function of: (1) depth in the microlayer, (2) mechanically-generated aqueous-phase turbulence intensity, (3) length scale of the turbulence, (4) interfacial cleanliness. The non-invasive LIF technique used here allows study of [CO2] fluctuations in the aqueous surface microlayer with no disruption of the liquid phase turbulence of the gas-liquid interface.
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Reflected Optical Second-Harmonic Generation in the Study of Naturally Occurring Organic Films at the Ocean Surface
The nonlinear spectroscopic technique of second-harmonic generation was used to study the surface properties of bulk and microlayer seawater samples. It was found that the majority of the surface active components present in the seawater were present in the microlayer sample. Increases in film pressure correlated with increases in the second-harmonic generation signal for the microlayer samples studied. This indicates that the second-harmonic generation technique may be used to noninvasively study and determine the surface properties of seawater.
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The Interaction of Mechanically Generated Turbulence and Interfacial Films with a Liquid Phase Controlled Gas/Liquid Transport Process
Provides an examination of the interaction of mechanically-generated liquid phase turbulence of known length and velocity scales with a gas/liquid transport process which is rate controlled in the liquid phase. The effects of various liquid surface cleaning procedures and deliberately formed organic monolayers on the gas exchange process was also studied. A non-invasive laser-induced fluorescence (LIF) technique based on the pH-dependent fluorescence emissions of aqueous fluorescein dyes was used to study the process. This application of LIF to gas transport research allows the exchange process to be sampled with no disruption of the liquid phase turbulence or the gas/liquid interface.
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Trace Organic Compounds in Rain. 1. Sample Design and Analysis by Adsorption/Thermal Desorption (ATD)
The design and use of a rain sampler with a 0.89-m2 collection surface area are described. The sampler is controlled electronically, provides for the in situ filtration of the sample, and carries out the preconcentration of nonpolar organic compounds by means of cartridges of the sorbent Tenax-GC. Analytical results were obtained for 27 compounds by fused silica capillary column gas chromatography with detection by mass spectrometry for four rain events sampled 12 km southwest of Portland, OR, at the Oregon Graduate Center (OGC), and for five rain events sampled in southeast Portland.
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