Flux-gradient technique

Fluxes of HNO3 and HONO were measured by the flux-gradient technique, yielding 19 separate estimates of the fluxes. The mean deposition rate for HNO3 was 5.5 ng Nm s and all the observed fluxes were towards the ground. For... 

The system of Eq. (17.4) is a partial derivative hyperbolic system, whose numerical solution is achieved using shock-capturing numerical methods [10-13]. These methods are characterized by second-order accuracy and therefore they give rise to spurious oscillation when high gradients are present in the solution field. To suppress the occurrence of numerical instabilities, flux limiting techniques, or gradient limiters are used [8]. 

HONO the mean flux was an emission of 1 ng Nm s but this includes periods both of emission and deposition. On several occasions, no concentration gradients were detected. The direction of the flux was dependent on NOj concentration, with emission observed only when NOj concentration was less than 10 ppb. The process of HONO exchange appears to be regulated by the net result of small deposition flux to the surface and a surface chemistry production of HONO from NOj. Fluxes of PAN deposition were measured using a chamber technique " and were small (less than 0.5ng Nm s ). 

As predicted, the points next to the ends of the profile give unreliable (negative) estimates because the fluxes at the end points themselves become a substantial fraction of the fluxes in the neighborhood. In addition, noise in sections with rather flat gradients becomes a problem. More elaborated non-linear regression techniques should be used to handle this specific problem. Nevertheless, the profile between IS and SO/rm, where steep Ce variations are observed, shows evidence for substantial changes of the Off which seems to hint at much faster diffusion when this element is only in trace amounts in the apatite lattice, o... 

Gradient diffusion was assumed in the species-mass-conservation model of Shir and Shieh. Integration was carried out in the space between the ground and the mixing height with zero fluxes assumed at each boundary. A first-order decay of sulfur dioxide was the only chemical reaction, and it was suggested that this reaction is important only under low wind speed. Finite-difference numerical solutions for sulfur dioxide in the St. Louis, Missouri, area were obtained with a second-order central finite-difference scheme for horizontal terms and the Crank-Nicolson technique for the vertical-diffusion terms. The three-dimensional grid had 16,800 points on a 30 x 40 x 14 mesh. 

As noted earlier, air-velocity profiles during inhalation and exhalation are approximately uniform and partially developed or fully developed, depending on the airway generation, tidal volume, and respiration rate. Similarly, the concentration profiles of the pollutant in the airway lumen may be approximated by uniform partially developed or fully developed concentration profiles in rigid cylindrical tubes. In each airway, the simultaneous action of convection, axial diffusion, and radial diffusion determines a differential mass-balance equation. The gas-concentration profiles are obtained from this equation with appropriate boundary conditions. The flux or transfer rate of the gas to the mucus boundary and axially down the airway can be calculated from these concentration gradients. In a simpler approach, fixed velocity and concentration profiles are assumed, and separate mass balances can be written directly for convection, axial diffusion, and radial diffusion. The latter technique was applied by McJilton et al. 

In the case of rhodopsin membrane under calcium gradient, the technique shows a nonlinearity of fluxes with respect to imposed gradients. We also notice some permeability variations just after rhodopsin bleaching and at last a calcium adsorption consequent to lighting performed in situ. 

This is because of the effect of the total pressure gradient which develops which must be accounted for in the diffusion model. Clearly, although this may not play an important role in carefully designed experiments for determining diffusion coefficients using this technique, it has a considerable bearing on the use of such information where non-equimolal fluxes can arise, as with chemical reaction (6). 

Care is needed in applying the unsteady state pulse technique to a Wicke-Kallenbach cell in order to obtain values for effective diffusion coefficients. For sufficiently small concentrations, where the trace component is of higher diffusivity than the carrier, the commonly used isobaric model is adequate for defining the transport parameters if sufficiently short pulses are used. However, where adsorption of either carrier or trace component occurs or wheipe the trace is of lower diffusivity, then the induced total pressure gradients cause the fluxes to show unusual behaviour and may require analysis by a non-isobaric model. 

In more recent years, NMR spectroscopy has been developed as an in situ technique [55, 100, 203, 365, 419]. This is (indirectly) used for metabolic flux determination [39] (see also the contribution by Nielsen, this volume) and may require specifically designed reactors [151]. Scanning times he at least in the range of minutes to hours and cell concentrations needed are above 1010 cells l-1. Schuppenhauer et al. [381] have determined spatial gradients in a fluidized bed reactor of some relevant metabolites in a high density animal cell culture using localized 31P-NMR spectroscopy. Mass transfer resistance can also be directly observed using NMR spectroscopy [427]. 

To monitor CFTR activity in HBE cultures, Ussing chamber recording techniques are used (Fig. 7). Like the nasal potential difference recording technique used in vivo, the Ussing chamber technique is an extracellular recording of the membrane potential or short circuit current (Isc) due to ion flux through channels and transporters. To isolate CFTR activity, a basolateral to apical Cl" gradient is established and amiloride is added to block the ep-... 

Few such techniques are applicable in the case of trace gas exchange instead, micrometeorological methods have risen in popularity. In concept, such methods evaluate the flux across a plane above the surface rather than the deposition at the surface itself. Considerable care is necessary to ensure that the flux evaluated above the surface is the same as that at the surface. This constraint is the reason for the widely acknowledged micrometeorological requirements for uniform conditions, surface homogeneity, and terrain simplicity. The most common micrometeorological methods are eddy-correlation and the interpretation of gradients (2). Of these... 

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