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Bill Asher

Senior Principal Oceanographer

Email

asherwe@apl.washington.edu

Phone

206-543-5942

Research Interests

Air-Sea Exchange, Bubbles, Remote Sensing

Biosketch

Dr. Asher's research experience includes modeling the formation of secondary organic aerosols, studying the physics and chemistry of air-water transfer, determining the physicochemical properties of the marine surface microlayer, and measuring the concentration of trace organic compounds in natural aquatic systems. His current research projects include developing thermodynamic models for predicting the formation of secondary organic aerosols, modeling the cycling and fate of volatile organic compounds in lakes and rivers, using infrared imaging to determine the relation of microscale wave breaking with air-water exchange processes, measuring the microwave emissivity of a foam-covered ocean surface, and characterizing spray droplets over the ocean surface at high wind speeds.

Education

B.A. Chemistry, Reed College, 1980

Ph.D. Environmental Science and Engineering, Oregon Graduate Institute of Science and Technology, 1987

Projects

Modeling the Cycle and Source Apportionment of Volatile Organic Compounds in Lakes and Rivers

A set of models to predict how changes in sources and environmental conditions will affect surface water concentrations of volatile organic compounds are being developed to aid regulatory decision makers.

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Two models have been developed under this project. The first is LakeVOC, which predicts the change in concentration of a volatile organic compound (VOC) in both the epilimnion and hypolimnion of lakes and reservoirs in response to changes in source input and environmental parameters. The second model is StreamVOC, which calculates source apportionment for a particular VOC in a river or stream with multiple discrete and distributed source regions.

Our objective is to develop a set of models for predicting how changes in sources and environmental conditions will affect surface water concentrations of volatile organic compounds. These models are designed to allow regulators to easily study the effects of policy, planning, mitigation, and operational strategies on achieving national water quality requirements.

Fluxes, Air-Sea Interaction, and Remote Sensing (FAIRS) Experiment

The transfer of momentum, heat, and gas across the air-sea boundary is characterized and quantified by measuring the underlying physical mechanisms with remote sensing instruments.

 

Publications

2000-present and while at APL-UW

Vibrational sum-frequency spectroscopy for trace chemical detection on surfaces at stand-off distances

Asher, W.E., and E. Willard-Schmoe, "Vibrational sum-frequency spectroscopy for trace chemical detection on surfaces at stand-off distances," Appl. Spectrosc., 67, 253-260, doi:10.1366/12-06792, 2013.

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1 Mar 2013

Vibrational sum-frequency spectroscopy (VSFS) has been used for some time as a laboratory-based surface chemical analytical tool. Here, theoretical considerations in applying the method as a remote-sensing probe for detecting trace levels of chemicals adsorbed on surfaces are presented. Additionally, a VSFS instrument is configured to operate at a stand-off distance of 2.2 m using near-nadir incidence angles. This system was used to measure VSFS spectra for films of pure 1-amino-4-nitrobenzene (p-nitroaniline, PNA) and pure 2-hydroxy-1,3,5-trinitrobenzene (picric acid, PA) adsorbed on polished T-6061 aluminum alloy. These spectra are used to investigate the effect of optical polarization on the sum-frequency response of these compounds at nadir optical geometries. Detection limits for each compound are also estimated and found to be 0.51 μg cm2 for PNA and 0.89 μg cm2 for PA. The implications of these results regarding remote sensing applications of VSFS are discussed.

Statistics of surface divergence and their relation to air–water gas transfer velocity

Asher, W.E., H. Kiang, C.J. Zappa, M.R. Loewen, M.A. Mukto, T.M. Litchendorf, and A.T. Jessup,"Statistics of surface divergence and their relation to air–water gas transfer velocity," J. Geophys. Res., 117, doi:10.1029/2001JC007390, 2012.

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24 May 2012

Air-sea gas fluxes are generally defined in terms of the air/water concentration difference of the gas and the gas transfer velocity, kL. Because it is difficult to measure kL in the ocean, it is often parameterized using more easily measured physical properties. Surface divergence theory suggests that infrared (IR) images of the water surface, which contain information concerning the movement of water very near the air-water interface, might be used to estimate kL. Therefore, a series of experiments testing whether IR imagery could provide a convenient means for estimating the surface divergence applicable to air-sea exchange were conducted in a synthetic jet array tank embedded in a wind tunnel. Gas transfer velocities were measured as a function of wind stress and mechanically generated turbulence; laser-induced fluorescence was used to measure the concentration of carbon dioxide in the top 300 µm of the water surface; IR imagery was used to measure the spatial and temporal distribution of the aqueous skin temperature; and particle image velocimetry was used to measure turbulence at a depth of 1 cm below the air-water interface. It is shown that an estimate of the surface divergence for both wind-shear driven turbulence and mechanically generated turbulence can be derived from the surface skin temperature. The estimates derived from the IR images are compared to velocity field divergences measured by the PIV and to independent estimates of the divergence made using the laser-induced fluorescence data. Divergence is shown to scale with kL values measured using gaseous tracers as predicted by conceptual models for both wind-driven and mechanically generated turbulence.

Aerial radiometric and video measurements of whitecap coverage.

Bobak, J.P., W.E. Asher, D.J. Dowgiallo, and M.D. Anguelova, "Aerial radiometric and video measurements of whitecap coverage." IEEE Trans. Geosci. Remote Sens., 49, 2183-2193, doi: 10.1109/TGRS.2010.2103565, 2011.

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1 Jun 2011

This paper presents the results of high-altitude microwave radiometric and video measurements in the presence of breaking waves made during the passage of Hurricane Dean on August 21, 2007, over the Gulf of Mexico. Previous measurements of foam fraction and radiometric brightness temperature have focused on the small scale, in which individual foam patches were of the same scale as the radiometer footprint. To work with data from spaceborne microwave radiometers, which have footprints on the scale of tens of kilometers, the knowledge of how the foam fraction sensitivity of brightness temperature scales when footprints increase from meters to kilometers is necessary. Video images of the sea surface recorded with a high-resolution monochrome digital camera were used to determine the foam fraction. Ocean-surface brightness temperature was measured with the Airborne Polarimetric Microwave Imaging Radiometer (APMIR) of the Naval Research Laboratory at frequencies of 6.6 [vertical and horizontal (VH) polarizations], 6.8 (VH), 7.2 (VH), and 10.7 GHz (V), with full polarimetric brightness temperatures measured at 19.35 and 37.0 GHz. Collocated nearly contemporaneous brightness temperatures were available from WindSat, Special Sensor Microwave Imager/Sounder, and Special Sensor Microwave/Imager satellite radiometer overpasses. Oceanographic and meteorological data were taken from buoys located along the flight track. There was good correlation between brightness temperatures measured with APMIR and satellite-borne radiometers with absolute differences largely within the expected uncertainty of the data. An analysis of the video imagery provided the fractional area coverage of the actively breaking waves on the ocean surface. The increase in brightness temperature from each of the microwave sensors was correlated with the whitecap coverage measured by the camera. The experiment not only serves as an important bridge between measurements made with spatial scales on the order o f tens of meters and data collected from satellites with spatial scales of tens of kilometers but also provides guidance for improving future field measurements on this topic.

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Silicon-on-sapphire integrated waveguides for the mid-infrared

Baehr-Jones, T., A. Spott, R. Ilic, A. Spott, B. Penkov, W. Asher, and M. Hochberg, "Silicon-on-sapphire integrated waveguides for the mid-infrared," Opt. Express, 18, 12127-12135, 2010.

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24 May 2010

Silicon waveguides are now widely used to guide radiation in the near-infrared, mainly in the wavelength range of 1.1 – 2.2 micrometers. While low-loss waveguides at longer wavelengths in silicon have been proposed, experimental realization has been elusive. Here we show that single-mode integrated silicon-on-sapphire waveguides can be used at mid-infrared wavelengths. We demonstrate waveguiding at 4.5 micrometers, or 2222.2 cm-1, with losses of 4.3 plus/minus 0.6 dB/cm. This result represents the first practical integrated waveguide system for the mid-infrared in silicon, and enables a range of new applications.

Application of a source apportionment model in consideration of volatile organic compounds in an urban stream

Asher, W.E., W. Luo, K.W. Campo, D.A. Bender, K.W. Robinson, J.S. Zogorski, and T.E. Pankow, "Application of a source apportionment model in consideration of volatile organic compounds in an urban stream," Environ. Toxicol. Chem., 26, 1606-1613, doi:10.1897/06-557R.1, 2007.

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9 Dec 2009

Position-dependent concentrations of trichloroethylene and methyl-tert-butyl ether are considered for a 2.81-km section of the Aberjona River in Massachusetts, USA. This river flows through Woburn and Winchester (Massachusetts, USA), an area that is highly urbanized, has a long history of industrial activities dating to the early 1800s, and has gained national attention because of contamination from chlorinated solvent compounds in Woburn wells G and H. The river study section is in Winchester and begins approximately five stream kilometers downstream from the Woburn wells superfund site. Approximately 300 toxic release sites are documented in the watershed upstream from the terminus of the study section. The inflow to the river study section is considered one source of contamination. Other sources are the atmosphere, a tributary flow, and groundwater flows entering the river; the latter are categorized according to stream zone (1, 2, 3, etc.). Loss processes considered include outflows to groundwater and water-to-atmosphere transfer of volatile compounds. For both trichloroethylene and methyl-tert-butyl ether, degradation is neglected over the timescale of interest. Source apportionment fractions with assigned values alphainflow, alpha1, alpha2, alpha3, etc. are tracked by a source apportionment model. The strengths of the groundwater and tributary sources serve as fitting parameters when minimizing a reduced least squares statistic between water concentrations measured during a synoptic study in July 2001 versus predictions from the model. The model fits provide strong evidence of substantial unknown groundwater sources of trichloroethylene and methyl-tert-butyl ether amounting to tens of grams per day of trichloroethylene and methyl-tert-butyl ether in the river along the study section. Modeling in a source apportionment manner can be useful to water quality managers allocating limited resources for remediation and source control.

Investigation of the physical scaling of sea spray spume droplet production

Fairall, C.W., M.L. Banner, W.L. Peirson, W. Asher, and R.P. Morison, "Investigation of the physical scaling of sea spray spume droplet production," J. Geophys. Res., 114, doi:10.1029/2008JC004918, 2009.

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2 Oct 2009

In this paper we report on a laboratory study, the Spray Production and Dynamics Experiment (SPANDEX), conducted at the University of New South Wales Water Research Laboratory in Australia. The goals of SPANDEX were to illuminate physical aspects of spume droplet production and dispersion; verify theoretical simplifications used to estimate the source function from ambient droplet concentration measurements; and examine the relationship between the implied source strength and forcing parameters such as wind speed, surface turbulent stress, and wave properties. Observations of droplet profiles give reasonable confirmation of the basic power law profile relationship that is commonly used to relate droplet concentrations to the surface source strength. This essentially confirms that, even in a wind tunnel, there is a near balance between droplet production and removal by gravitational settling. The observations also indicate considerable droplet mass may be present for sizes larger than 1.5 mm diameter. Phase Doppler Anemometry observations revealed significant mean horizontal and vertical slip velocities that were larger closer to the surface. The magnitude seems too large to be an acceleration time scale effect. Scaling of the droplet production surface source strength proved to be difficult. The wind speed forcing varied only 23% and the stress increased a factor of 2.2. Yet, the source strength increased by about a factor of 7. We related this to an estimate of surface wave energy flux through calculations of the standard deviation of small-scale water surface disturbance, a wave-stress parameterization, and numerical wave model simulations. This energy index only increased by a factor of 2.3 with the wind forcing. Nonetheless, a graph of spray mass surface flux versus surface disturbance energy is quasi-linear with a substantial threshold.

The effects of experimental uncertainty in parameterizing air-sea gas exchange using tracer experiment data

Asher, W.E., "The effects of experimental uncertainty in parameterizing air-sea gas exchange using tracer experiment data," Atmos. Chem. Phys., 9, 131-139, 2009.

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1 Sep 2009

It is not practical to measure air-sea gas fluxes in the open ocean for all conditions and areas of interest. Therefore, in many cases fluxes are estimated from measurements of air-phase and water-phase gas concentrations, a measured environmental forcing function such as wind speed, and a parameterization of the air-sea transfer velocity in terms of the environmental forcing function. One problem with this approach is that when direct measurements of the transfer velocity are plotted versus the most commonly used forcing function, wind speed, there is considerable scatter, leading to
a relatively large uncertainty in the flux. Because it is known that multiple processes can affect gas transfer, it is commonly assumed that this scatter is caused by single-forcing function parameterizations being incomplete in a physical sense. However, scatter in the experimental data can also result from experimental uncertainty (i.e., measurement error).

Here, results from field and laboratory results are used to estimate how experimental uncertainty contributes to the observed scatter in the measured fluxes and transfer velocities as a function of environmental forcing. The results show that experimental uncertainty could explain half of the observed scatter in field and laboratory measurements of air-sea gas transfer velocity.

Evidence for complete and partial surface renewal at an air-water interface

Jessup, A.T., W.E. Asher, M. Atmane, K. Phadnis, C.J. Zappa, and M.R. Loewen, "Evidence for complete and partial surface renewal at an air-water interface," Geophys. Res. Lett., 36, doi:10.1029/2009GL038986, 2009.

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27 Aug 2009

A wind-wave flume is used to determine the extent to which the thermal boundary layer (TBL) at a wind-forced air-water interface is completely renewed from below. We measure skin temperature, Tskin , radiometrically, temperature immediately below the TBL, Tsubskin , using a temperature profiler, and net heat flux using the gradient flux technique. The Tskin probability density function, p(Tskin), and surface renewal time scale were measured using passive and active infrared imaging techniques, respectively. We find that the mean percentile rank of Tsubskin in p(Tskin) is 99.90, implying that complete surface renewal occurs. This result suggests an alternative to radiometric measurement of Tskin through the simple combination of an infrared camera and an in situ temperature sensor. Comparison of the temperature difference across the TBL to the expected cooling implies that a significant portion of events only partially renew the TBL. This result should impact efforts to improve air-sea transfer models.

Visualizing near-surface concentration fluctuations using laser-induced fluorescence

Asher, W.E., and T.M. Litchendorf, "Visualizing near-surface concentration fluctuations using laser-induced fluorescence," Exp. Fluids, 46, 243-253, doi:10.1007/s00348-008-0554-9, 2009.

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1 Feb 2009

A method for observing near-surface fluctuations in pH caused by a water–air flux of carbon dioxide under conditions of ambient atmospheric carbon dioxide levels is developed and tested. Peaks in fluorescence intensity measured as a function of pH and turbulence are shown to be consistent with predictions from a chemical kinetics model of CO2 exchange. The square root of the frequency of the pH fluctuations scale linearly with independently measured bulk air–water gas transfer velocities in agreement with surface divergence models for air–water gas transfer. These data indicate that the method proposed here is tracking changes in near-surface CO2 concentrations. This laser-induced fluorescence method can be used to study the air–water exchange of CO2 in wind-wave tunnels without the need for elevated CO2 concentrations in the gas phase.

Advances in quantifying air-sea gas exchange and environmental forcing

Wanninkhof, R., W.E. Asher, D.T. Ho, C. Sweeney, and W.R. McGillis, "Advances in quantifying air-sea gas exchange and environmental forcing," Ann. Rev. Mar. Sci, 1, 213-244, doi:10.1146/annurev.marine.010908.163742, 2009.

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1 Jan 2009

The past decade has seen a substantial amount of research on air-sea gas exchange and its environmental controls. These studies have significantly advanced the understanding of processes that control gas transfer, led to higher quality field measurements, and improved estimates of the flux of climate-relevant gases between the ocean and atmosphere. This review discusses the fundamental principles of air-sea gas transfer and recent developments in gas transfer theory, parameterizations, and measurement techniques in the context of the exchange of carbon dioxide. However, much of this discussion is applicable to any sparingly soluble, non-reactive gas.

We show how the use of global variables of environmental forcing that have recently become available and gas exchange relationships that incorporate the main forcing factors will lead to improved estimates of global and regional air-sea gas fluxes based on better fundamental physical, chemical, and biological foundations.

Comparison of modeled and observed microwave emissivities of water surfaces in the presence of breaking waves and foam

Padmanabhan, S., S.C. Reising, W.E. Asher, V. Raizer, and P.W. Gaiser, "Comparison of modeled and observed microwave emissivities of water surfaces in the presence of breaking waves and foam," International Geoscience and Remote Sensing Symposium, IGARSS 2007, 23-28 June, Barcelona, Spain, 42-45 (IEEE: Piscataway, NJ, 2008).

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7 Jan 2008

Wind speed has been retrieved reliably from SSM/I microwave radiometric measurements since 1990 with a precision of better than 2 m/s. However, operational requirements of the National Polar-orbiting Operational Environmental Satellite System (NPOESS) dictate retrieval of the ocean surface wind vector, i.e. both speed and direction. This requires improved understanding and forward modeling of the physical processes governing ocean surface microwave emissivity. Although the microwave brightness temperature over the ocean exhibits a relatively strong dependence on wind speed, the dependence on wind direction with respect to the azimuthal angle of observation is at most a few Kelvin peak-to-peak. Although it is known that breaking waves and foam significantly affect the microwave emission of the ocean surface at 6.8, 10.8, 19 and 37 GHz, the effect of foam on the azimuthal dependence of sea surface emissivity is not well understood. This understanding is critical to improve forward models to achieve the accuracy required for ocean surface wind vector retrievals. Recently, electromagnetic modeling of the emissivity of foam generated by breaking waves has been improved by considering the vertical and horizontal heterogeneity of thick layers of foam at the ocean- atmosphere interface [1]. In this work, we compare foam emissivities both calculated using this model with those inferred from microwave brightness temperature measurements performed during the Polarimetric Observations of the Emissivity of Whitecaps Experiment (POEWEX'04).

SIMPOL.1: A simple group contribution method for predicting vapor pressures and enthalpies of vaporization of multifunctional organic compounds

Pankow, J.F., and W.E. Asher, "SIMPOL.1: A simple group contribution method for predicting vapor pressures and enthalpies of vaporization of multifunctional organic compounds," Atmos. Chem. Phys., 8, 2773-2796, 2008.

1 Jan 2008

SIMPOL.1:A simple group contribution method for predicting vapor pressures and enthalpies of vaporization of multifunctional organic compounds

Pankow, J.F., and W.E. Asher, "SIMPOL.1:A simple group contribution method for predicting vapor pressures and enthalpies of vaporization of multifunctional organic compounds," Atmos. Chem. Phys. Discuss., 7, 11839-11894, doi:10.5194/acpd-7-11839-2007, 2007.

1 Apr 2007

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

Erdakos G.B., W.E. Asher, J.H. Seinfeld, and J.F. Pankow, "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, doi:10.1016/j.atmosenv.2006.04.030, 2006.

1 Oct 2006

Vapor pressure prediction for alkenoic and aromatic organic compounds by a UNIFAC-based group contribution method

Asher, W.E., and J.F. Pankow, "Vapor pressure prediction for alkenoic and aromatic organic compounds by a UNIFAC-based group contribution method," Atmos. Environ., 40, 3588-3600, doi:10.1016/j.atmosenv.2005.12.004, 2006.

1 Jun 2006

Source apportionment modeling of volatile organic compounds (VOCs) in streams

Pankow, J.F., W.E. Asher, and J.S. Zogorski, "Source apportionment modeling of volatile organic compounds (VOCs) in streams," Environ. Toxicol. Chem. 25, 921-932, doi:10.1897/05-205R1.1, 2006.

1 Apr 2006

Effects of foam on ocean surface microwave emission inferred from radiometric observations of reproducible breaking waves

Padmanabhan, S., S.C. Reising, W.E. Asher, L.A. Rose, and P.W. Gaiser, "Effects of foam on ocean surface microwave emission inferred from radiometric observations of reproducible breaking waves," IEEE Trans. Geosci. Remote Sens., 44, 569-583, doi:10.1109/TGRS.2006.870234, 2006.

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1 Mar 2006

WindSat, the first satellite polarimetric microwave radiometer, and the NPOESS Conical Microwave Imager/Sounder both have as a key objective the retrieval of the ocean surface wind vector from radiometric brightness temperatures. Available observations and models to date show that the wind direction signal is only 1-3 K peak-to-peak at 19 and 37 GHz, much smaller than the wind speed signal. In order to obtain sufficient accuracy for reliable wind direction retrieval, uncertainties in geophysical modeling of the sea surface emission on the order of 0.2 K need to be removed. The surface roughness spectrum has been addressed by many studies, but the azimuthal signature of the microwave emission from breaking waves and foam has not been adequately addressed. Recently, a number of experiments have been conducted to quantify the increase in sea surface microwave emission due to foam. Measurements from the Floating Instrumentation Platform indicated that the increase in ocean surface emission due to breaking waves may depend on the incidence and azimuth angles of observation. The need to quantify this dependence motivated systematic measurement of the microwave emission from reproducible breaking waves as a function of incidence and azimuth angles. A number of empirical parameterizations of whitecap coverage with wind speed were used to estimate the increase in brightness temperatures measured by a satellite microwave radiometer due to wave breaking in the field of view. These results provide the first empirically based parameterization with wind speed of the effect of breaking waves and foam on satellite brightness temperatures at 10.8, 19, and 37 GHz.

Effects of air-sea interaction parameters on ocean surface microwave emission at 10 and 37 GHz

Aziz, M.A., S.C. Reising, W.E. Asher, L.A. Rose, P.W. Gaiser, and K.A. Horgan, "Effects of air-sea interaction parameters on ocean surface microwave emission at 10 and 37 GHz," IEEE Trans. Geosci. Remote Sens., 43, 1763-1774, DOI: 10.1109/TGRS.2005.848413, 2005

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30 Aug 2005

WindSat, the first polarimetric radiometer on orbit, launched in January 2003, provides the promise of passive ocean wind vector retrievals on a continuous basis, simultaneous with the retrieval of many other geophysical variables such as sea surface temperature, atmospheric water vapor, cloud liquid water, and sea ice extent and concentration. WindSat also serves as risk reduction for the upcoming National Polar-orbiting Operational Environmental Satellite System (NPOESS) Conical Scanning Microwave Imager/Sounder (CMIS). Since the dependence of microwave brightness temperatures on wind direction is small relative to that of other parameters such as wind speed, wind direction retrieval relies on increasingly accurate knowledge of the ocean surface microwave emission, which depends upon surface properties such as roughness and foam due to wave breaking. Coordinated near-surface measurements of ocean surface microwave emission and air-sea interaction parameters are needed to quantify the effects of the processes mentioned above in surface emission models to improve the accuracy of wind vector retrievals. Such coordinated observations were performed during the Fluxes, Air-Sea Interaction, and Remote Sensing (FAIRS) experiment conducted on the R/P Floating Instrument Platform (FLIP) in the northeastern Pacific Ocean during the Fall of 2000. X- and Ka-band partially polarimetric radiometers were mounted at the end of the port boom of R/P FLIP to measure ocean surface emission at incidence angles of 45°, 53°, and 65°. A bore-sighted video camera recorded the fractional area of foam in the field of view of the radiometers. Air-sea interaction parameters that were measured concurrently include wind speed, friction velocity, heat fluxes, and significant wave height. The measured dependence of ocean surface emissivity on wind speed and friction velocity is in good agreement with, and extends, earlier observations and empirical models based on satellite data. Concurrent radiometric measurements and fractional area foam coverage data strengthen the possibility of retrieval of sea surface foam coverage using airborne or spaceborne radiometry. The dependence of emissivity on atmospheric stability is shown to be much smaller than the dependence of em- issivity on wind speed. Analysis of emissivity dependence on atmospheric stability alone was inconclusive, due to the variation in atmospheric stability with wind speed. The effect of long-wave incidence angle modulation on sea surface emissivity for near-surface measurements was found to be negligible when emissivity measurements were averaged over tens to hundreds of long waves.

Microbreaking and the enhancement of air-water transfer velocity

Zappa, C.J., W.E. Asher, A.T. Jessup, J. Klinke, and S.R. Long, "Microbreaking and the enhancement of air-water transfer velocity," J. Geophys. Res., 109, 10.1029/2003JC001897, 2004.

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27 Aug 2004

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 local heat transfer velocity, kH , is measured using an active infrared technique and the tank-averaged gas transfer velocity, kG , is measured using conservative mass balances. Simultaneous, colocated 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. The fractional area coverage of microbreakers, AB , is found to be 0.10.4 in the wind speed range 4.2–9.3 m s-1 for cleaned and surfactant-influenced surfaces, and kH and kG are correlated with AB . The correlation of kH with AB is independent of fetch and the presence of surfactants, while that for kG with AB depends on surfactants. Additionally, AB is correlated with the mean square wave slope, S2, which has shown promise as a correlate for kG in previous studies. The ratio of kH measured inside and outside the microbreaker wakes is 3.4, demonstrating that at these wind speeds, up to 75% of the transfer is the direct result of microbreaking. These results provide quantitative evidence that microbreaking is the dominant mechanism contributing to air-water heat and gas transfer at low to moderate wind speeds.

Introduction to special edition: Air-sea exchange

McGillis, W.R., W.E. Asher, R. Wanninkhof, A.T. Jessup, and R.A. Feely, "Introduction to special edition: Air-sea exchange," J. Geophys. Res., 109, 10.1029/2004JC002605, 2004.

19 Aug 2004

Oceanic application of the active controlled flux technique for measuring air-sea transfer velocities of heat and gases

Asher, W.E., A.T. Jessup, and M.A. Atmane, "Oceanic application of the active controlled flux technique for measuring air-sea transfer velocities of heat and gases," J. Geophys. Res., 109, 10.1029/2003JC001862, 2004.

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13 Aug 2004

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 noninvasive 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.

On the use of the active infrared technique to infer heat and gas transfer velocities at the air-water free surface

Atmane, M.A., W.E. Asher, and A.T. Jessup, "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, 10.1029/2003JC001805, 2004.

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8 Jul 2004

A comparison study of the experimental and theoretical transfer velocities of heat and 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, 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.

Coherent structures beneath wind waves and their influence on air-water gas transfer

Siddiqui, M.H.K., M.R. Loewen, W.E. Asher, and A.T. Jessup, "Coherent structures beneath wind waves and their influence on air-water gas transfer," J. Geophys. Res., 109, C03024, 10.1029/2002JC001559, 2004.

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17 Mar 2004

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 quiescent 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 ~2.5 times higher in the energetic regions compared to the quiescent regions at all wind speeds.

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

Pankow, J.F., W.T. Luo, D.A. Bender, L.M. Isabelle, J.S. Hollingsworth, C. Chen, W.E. Asher, and J.S. Zogoorski, "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, 5023-5046, doi:10.1016/j.atmosenv.2003.08.006 , 2003.

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1 Nov 2003

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 (ca, 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 tert-butyl 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.

Microwave emission and scattering of foam based on Monte Carlo simulations of dense media

Chen, D., L. Tsang, L. Zhou, S.C. Reising, W.E. Asher, L.A. Rose, K.H. Ding, and C.T. Chen, "Microwave emission and scattering of foam based on Monte Carlo simulations of dense media," IEEE Trans. Geosci. Remote Sens., 41, 782-790, 10.1109/TGRS.2003.810711, 2003.

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30 Apr 2003

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 experimental measurements.

Radiometric measurements of the microwave emissivity of foam

Rose, L.A., W.E. Asher, S.C. Reising, P.W. Gaiser, K.W. St. Germain, D.J. Dowgiallo, K.A. Horgan, G. Farquharson, and E.J. Knapp, "Radiometric measurements of the microwave emissivity of foam," IEEE Trans. Geosci. Remote Sens., 40, 2619-2625, doi:10.1109/TGRS.2002.807006, 2002.

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1 Dec 2002

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.

Effect of microscale wave breaking on air-water gas transfer

Zappa, C.J., W.E. Asher, and A.T. Jessup, "Effect of microscale wave breaking on air-water gas transfer," in Gas Transfer at Water Surfaces, edited by M.A. Donelan, W.M. Drennan, E.S. Saltzman, and R. Wanninkhof, 23-30 (American Geophysical Union, Washington, D.C., 2002).

1 Jun 2002

Turbulence generated by microscale breaking waves and its influence on air-water gas transfer

Siddiqui, M.H.K., M.R. Loewen, C. Richardson, W.E. Asher, and A.T. Jessup, "Turbulence generated by microscale breaking waves and its influence on air-water gas transfer," in Gas Transfer at Water Surfaces, edited by M.A. Donelan, W.M. Drennan, E.S. Saltzman, and R. Wanninkhof, 11-16 (American Geophysical Union, Washington, D.C., 2002).

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1 Jun 2002

The results from a series of wind-wave flume experiments using simultaneous DPIV (digital particle image velocimetry) measurements and IR (infrared) imagery to investigate microscale breaking waves are presented. We show that the IR signatures of microscale breaking waves are produced by a series of strong vortices that form behind the leading edge of the breakers. These strong vortices disrupt the cool skin layer and generated a thin layer of enhanced turbulence immediately below the air-water interface. In addition we used CFT (controlled flux technique) to make measurements of the local heat transfer velocity and found that the transfer velocity was correlated with the near-surface vertical turbulent velocity. We conclude that near-surface turbulence generated by microscale wave breaking determines the transfer rate at low to moderate wind speeds.

Estimating the vapor pressures of multi-functional oxygen-containing organic compounds using group contribution methods

Asher, W.E., J.F. Pankow, G.B. Erdakos, and J.H. Seinfeld, "Estimating the vapor pressures of multi-functional oxygen-containing organic compounds using group contribution methods," Atmos. Environ., 36, 1483-1498, doi:10.1016/S1352-2310(01)00564-7, 2002.

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1 Mar 2002

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 pLo 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 33 compounds that was used to test the coefficients generated using the basis set. Both the basis and test sets contained compounds that possessed similar structures and functionalities. For the 33 compounds in the test set, on average UNIFAC-pLo predicted the pLo values to within a factor of 2 over the temperature range 290–320 K. Furthermore, the UNIFAC-pLo method did not show any correlation in prediction error with pLo so that it was equally likely to underpredict as overpredict pLo regardless of volatility. For comparison, three other vapor pressure estimation methods were applied to the test set of compounds. On average, these other methods all predicted the test set pLo values to within a factor of 3 over the temperature range 290–320 K. In contrast to the UNIFAC-pLo method, the prediction errors from the methods were found to be correlated with pLo so that the other methods overpredicted pLo as volatility decreased.

Fractional area whitecap coverage and air-sea gas transfer during GasEx-98

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, edited by M.A. Donelan, W.M. Drennan, E.S. Saltzman, and R. Wanninkhof, 199-204 (American Geophysical Union, Washington, D.C., 2002).

15 Jan 2002

Simultaneous particle image velocimetry and infrared imagery of microscale breaking waves

Siddiqui, M.H.K., M.R. Loewen, C. Richardson, W.E. Asher, and A.T. Jessup, "Simultaneous particle image velocimetry and infrared imagery of microscale breaking waves," Phys. Fluids, 13, 1891-1903, 2001.

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1 Jul 2001

We report the results from a laboratory investigation in which microscale breaking waves were detected using an infrared (IR) imager and two-dimensional (2-D) velocity fields were simultaneously measured using particle image velocimetry (PIV). In addition, the local heat transfer velocity was measured using the controlled flux technique. To the best of our knowledge these are the first measurements of the instantaneous 2-D velocity fields generated beneath microscale breaking waves. Careful measurements of the water surface profile enabled us to make accurate estimates of the near-surface velocities using PIV. Previous experiments have shown that behind the leading edge of a microscale breaker the cool skin layer is disrupted creating a thermal signature in the IR image [Jessup et al., J. Geophys. Res. 102, 23145 (1997)]. The simultaneously sampled IR images and PIV data enabled us to show that these disruptions or wakes are typically produced by a series of vortices that form behind the leading edge of the breaker. When the vortices are first formed they are very strong and coherent but as time passes, and they move from the crest region to the back face of the wave, they become weaker and less coherent. The near-surface vorticity was correlated with both the fractional area coverage of microscale breaking waves and the local heat transfer velocity. The strong correlations provide convincing evidence that the wakes produced by microscale breaking waves are regions of high near-surface vorticity that are in turn responsible for enhancing air–water heat transfer rates.

Microscale wave breaking and air-water gas transfer

Zappa, C.J., W.E. Asher, and A.T. Jessup, "Microscale wave breaking and air-water gas transfer," J. Geophys. Res., 106, 9385-9391, doi:10.1029/2000JC000262, 2001.

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15 May 2001

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, colocated 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.

Applications of dense media radiative transfer for passive microwave remote sensing of foam covered ocean

Guo, J., L. Tsang, W. Asher, K.-H. Ding, and C.-T. Chen, "Applications of dense media radiative transfer for passive microwave remote sensing of foam covered ocean," IEEE Trans. Geosci. Rem. Sens., 39, 1019-1027, 2001.

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1 May 2001

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, the authors treat the foam as densely packed sticky air bubbles coated with thin seawater coating. The layer of foam covers the ocean surface that has air bubbles. They 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 19 GHz and 37 GHz 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.

Modeling the formation of secondary organic aerosol. 2. The predicted effects of relative humidity on aerosol formation in the α-pinene/,β-pinene/, sabinene/,Δ3-carene/, and cyclohexene/Ozone systems

Seinfeld, J.H., G.B. Erdakos, W.E. Asher, and J.F. Pankow, "Modeling the formation of secondary organic aerosol. 2. The predicted effects of relative humidity on aerosol formation in the α-pinene/,β-pinene/, sabinene/,Δ3-carene/, and cyclohexene/Ozone systems," Env. Sci. Technol., 35, 1806-1817, 2001.

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1 May 2001

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 presence of inorganic components is not considered. 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 nonvolatile 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 zeta 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 zeta 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 zeta = 1 for many compounds making up SOA mixtures.

Modeling the formation of secondary organic aerosol. 1. The application of theoretical principles to measurements obtained in the α-Pinene/,β-Pinene/, Sabinene/,Δ3-Carene/, and Cyclohexene/Ozone systems

Pankow, J.F., J.H. Seinfeld, W.E. Asher, and G.B. Erdakos, "Modeling the formation of secondary organic aerosol. 1. The application of theoretical principles to measurements obtained in the α-Pinene/,β-Pinene/, Sabinene/,Δ3-Carene/, and Cyclohexene/Ozone systems," Env. Sci. Technol., 35, 1164-1172, 2001.

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14 Feb 2001

Secondary organic aerosol (SOA) forms in the atmosphere when volatile parent compounds are oxidized to form low-volatility products that 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 with five parent hydrocarbons (HCs) (four monoterpenes at 308 K and cyclohexene at 298 K) in which significant fractions (61–100%) of the total mass of SOA formed from those HCs were identified and quantified by compound. The model calculations were based on a matrix representation of the multicomponent, SOA G/P distribution process. The governing equations were solved by an iterative method. Input data for the model included (i) ΔHC (μm-3), the amount of reacted parent hydrocarbon; (ii) the α values that give the total concentration T (gas + particle phase, ng m-3) values for each product i according to Ti = 103 αiΔHC; (iii) estimates of the pure compound liquid vapor pressure values (at the reaction temperature) for the products; and (iv) UNIFAC parameters for estimating activity coefficients in the SOA phase for the products as a function of SOA composition. The model predicts the total amount Mo (&956;g m-3) of organic aerosol that will form from the reaction of ΔHC, the total aerosol yield Y (= Mo/ΔHC), and the compound-by-compound yield values Yi. An impediment in applying the model is the lack of literature data on poL values for the compounds of interest or even on poL values for other, similarly low-volatility compounds. This was overcome in part by using the G/P data from the α-pinene and cyclohexene experiments to determine poL 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 poL values. The significant degree of resultant circularity in the calculations for α-pinene and cyclohexene helped lead to the good agreement that was found between the Yi values predicted by the model, and those measured experimentally for those two compounds. However, the model was also able to predict the aerosol yield values from β-pinene, sabinene, and Δ3-carene, for which there was significatly less circularity in the calculations, thereby providing evidence supporting the idea that given the correct input information, SOA formation can in fact be accurately modeled as a multicomponent condensation process.

On the mechanisms of rain-induced air-water gas exchange

Ho, D.T., W.E. Asher, P. Schlosser, L. Bliven, and E. Gordon, "On the mechanisms of rain-induced air-water gas exchange," J. Geophys. Res., 105, 24,045-24,057, 2000.

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15 Oct 2000

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 (N2O), 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.

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center
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