Air-sea gas transfer : its dependence on wind stress, small-scale roughness, and surface films

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): C08S17, doi:10.1029/2003JC002131.The influence of wind stress, small-scale waves, and surface films on air-sea gas exchange at low to moderate wind speeds (<10 m s−1) is examined. Coincident observations of wind stress, heat transfer velocity, surface wave slope, and surface film enrichments were made in coastal and offshore waters south of Cape Cod, New England, in July 1997 as part of the NSF-CoOP Coastal Air-Sea Chemical Fluxes study. Gas transfer velocities have been extrapolated from aqueous heat transfer velocities derived from infrared imagery and direct covariance and bulk heat flux estimates. Gas transfer velocity is found to follow a quadratic relationship with wind speed, which accounts for ~75–77% of the variance but which overpredicts transfer velocity in the presence of surface films. The dependence on wind stress as represented by the friction velocity is also nonlinear, reflecting a wave field-dependent transition between limiting transport regimes. In contrast, the dependence on mean square slope computed for the wave number range of 40–800 rad m−1 is found to be linear and in agreement with results from previous laboratory wind wave studies. The slope spectrum of the small-scale waves and the gas transfer velocity are attenuated in the presence of surface films. Observations over large-scale gradients of biological productivity and dissolved organic matter show that the reduction in slope and transfer velocity are more clearly correlated with surface film enrichments than with bulk organic matter concentrations. The mean square slope parameterization explains ~89–95% of the observed variance in the data and does not overpredict transfer velocities where films are present. While the specific relationships between gas transfer velocity and wind speed or mean square slope vary slightly with the choice of Schmidt number exponent used to scale the heat transfer velocities to gas transfer velocities, the correlation of heat or gas transfer velocity with mean square slope is consistently better than with wind speed.Funding for this work was provided by the NSF Coastal Ocean Processes (CoOP) program (OCE-9410534, OCE- 9711285 (WHOI) and OCE-9409222, OCE-9711391(URI)). Additional support for data analysis was provided by NASA (NAGW-2431, JPL Contract 961425). Partial funding was also provided by the German Science Foundation (DFG) through the DFG research unit FOR240: Image Sequence Analysis to Investigate Dynamic Processes

Dynamics of benzonitrile, propylene carbonate and butylene carbonate: the influence of molecular shape and flexibility on the dielectric relaxation behavior of dipolar aprotic liquids

The results of dielec. relaxation expts. on benzonitrile, propylene carbonate and butylene carbonate, performed between 228.15 K and 338.15 K in the frequency range 0.1 ≤ ν/GHz ≤ 89 are presented. The spectra of the three liqs. can be formally fitted with two relaxation processes. The long relaxation time, τ1(T), is equally well described by the Vogel-Fulcher-Tammann and the mode-coupling theory. However, the parameters differ from literature data for propylene carbonate based on the frequency of max. dielec. loss. This low-frequency dispersion step is attributed to the essentially isotropic rotational diffusion of the mol. dipole vector with the carbonates exhibiting some inter-mol. assocn. For butylene carbonate the fast relaxation process, τ2 ≈ 2 ps, partly arises from the rotation of the Et side chain. However, for all three liqs. the obsd. high-frequency contribution in the spectra reflects the transition from mol. dynamics governed by free rotation to long-time rotational diffusion

Ion association and solvation of perchlorate salts in N,N-dimethylformamide and N,N-dimethylacetamide

The results of dielec. relaxation expts. on solns. of LiClO4, NaClO4 and Bu4NClO4 in N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA), performed at 25 °C in the frequency range 0.2 ≤ ν/GHz ≤ 89 are presented. For all electrolytes, a solute relaxation process due to ion pairs is obsd. in addn. to the solvent relaxation. The alkali salts form predominantly solvent-shared ion pairs in DMF and DMA, whereas contact ion pairs are obsd. for Bu4NClO4. In all cases, the rate of ion-pair formation is nearly diffusion-controlled. Effective solvation nos. were deduced from the solvent dispersion amplitude. Data suggest that the anion-solvent interactions are weak, whereas the alkali ions form a well-defined primary solvation shell. Interestingly, irrotational bonding of solvent mols. by Bu4N+ is obsd., possibly due to steric interactions

Dielectric relaxation spectroscopy of electrolyte solutions. Recent developments and prospects

A review with 59 refs.; exemplified by recent temp.-dependent measurements from the authors' lab. it is shown that precise complex permittivity spectra obtained by the use of time domain and frequency domain methods in the mega- to gigahertz range yield specific information on solvent dynamics as well as on ion-solvation and ion-assocn. phenomena not available so far

Design and Evaluation of a Constraint-Based Head-Up Display for Helicopter Obstacle Avoidance During Forward Flight

This paper aims to reveal the effect of different display design principles in the helicopter domain. Two different obstacle avoidance support displays are evaluated during low-altitude, forward helicopter flight: a baseline Head-Up Display (HUD) is complemented either by a conventional advisory display, or a constraint-based display inspired by Ecological Interface Design. The latter has only been sparsely applied in the helicopter domain. It is hypothesized that the advisory display reduces workload, increases situation awareness, and improves performance measures in nominal obstacle avoidance situations, while the constraint-based display increases the resilience of the pilot-vehicle system towards unexpected, off-nominal situations. Twelve helicopter pilots with varying flight experience participated in an experiment in the SIMONA Research Simulator at Delft University of Technology. Contrary to expectations, the experiment revealed no significant effects of the displays on any of the dependent measures. However, there was a trend of decreasing pilot workload and increasing situation awareness when employing any of the support displays, compared to the baseline HUD. Pilots preferred the advisory display in nominal and the constraint-based display in off-nominal situations, reproducing similar findings from research in the fixed-wing domain. The relatively short time-frame and monotony of the control-task, an already cue-rich baseline HUD condition, and similarity between the displays possibly prohibited revealing larger differences between conditions. Future research will analyze the obstacle avoidance trajectories of this experiment, possibly revealing changes in control strategy caused by the displays, even when the lumped performance measures are similar. A follow-up experiment will focus on a longer task time-frame, more variable situations, and a truly ecological display to investigate the effect of applying Ecological Interface Design and different automation systems in the helicopter domain.Invited paperControl & Simulatio

Visualization of gas–liquid mass transfer and wake structure of rising bubbles using pH-sensitive PLIF

A planar laser-induced fluorescence (PLIF) technique for visualizing gas–liquid mass transfer and wake structure of rising gas bubbles is described. The method uses an aqueous solution of the pH-sensitive dye Naphthofluorescein and CO2 as a tracer gas. It features a high spatial resolution and frame rates of up to 500 Hz, providing the ability to capture cinematographic image sequences. By steering the laser beam with a set of two programmable scanning mirrors, sequences of three-dimensional LIF images can be recorded. The technique is applied to freely rising bubbles with diameters between 0.5 and 5 mm, which perform rectilinear, oscillatory or irregular motions. The resulting PLIF image sequences reveal the evolution of characteristic patterns in the near and far wake of the bubbles and prove the potential of the technique to provide new and detailed insights into the spatio-temporal dynamics of mass transfer of rising gas bubbles. The image sequences further allow the estimation of bubble size and rise velocity. The analysis of bubble rise velocities in the Naphthofluorescein solution indicates that surfactant-contaminated conditions are encountered

Human exome and mouse embryonic expression data implicate ZFHX3, TRPS1, and CHD7 in human esophageal atresia

Introduction: Esophageal atresia with or without tracheoesophageal fistula (EA/TEF) occurs approximately 1 in 3.500 live births representing the most common malformation of the upper digestive tract. Only half a century ago, EA/TEF was fatal among affected newborns suggesting that the steady birth prevalence might in parts be due to mutational de novo events in genes involved in foregut development. Methods: To identify mutational de novo events in EA/TEF patients, we surveyed the exome of 30 case-parent trios. Identified and confirmed de novo variants were prioritized using in silico prediction tools. To investigate the embryonic role of genes harboring prioritized de novo variants we performed targeted analysis of mouse transcriptome data of esophageal tissue obtained at the embryonic day (E) E8.5, E12.5, and postnatal. Results: In total we prioritized 14 novel de novo variants in 14 different genes (APOL2, EEF1D, CHD7, FANCB, GGT6, KIAA0556, NFX1, NPR2, PIGC, SLC5A2, TANC2, TRPS1, UBA3, and ZFHX3) and eight rare de novo variants in eight additional genes (CELSR1, CLP1, GPR133, HPS3, MTA3, PLEC, STAB1, and PPIP5K2). Through personal communication during the project, we identified an additional EA/TEF case-parent trio with a rare de novo variant in ZFHX3. In silico prediction analysis of the identified variants and comparative analysis of mouse transcriptome data of esophageal tissue obtained at E8.5, E12.5, and postnatal prioritized CHD7, TRPS1, and ZFHX3 as EA/TEF candidate genes. Re-sequencing of ZFHX3 in additional 192 EA/TEF patients did not identify further putative EA/TEF-associated variants. Conclusion: Our study suggests that rare mutational de novo events in genes involved in foregut development contribute to the development of EA/TEF