Eddy correlation, requirements

Substances can be removed from the atmosphere by dry deposition to surfaces. A method for obtaining the parameters of dry deposition uses eddy correlation flux measurements that require chemical sensors with very fast... 

The requirements for chemical sensors suitable for use in eddy correlation direct measurements of surface fluxes are examined. The resolution of chemical sensors is examined and defined in terms of surface flux and commonly measured micrometeorological parameters. Aspects of the design and operation of sensor systems are considered. In particular, the effects of the inlet ductingy the sensing volume, and the signal processing on the ability to measure surface fluxes were analyzed. 

MAJOR limitation TO research on surface-exchange and flux measurements is the lack of sensitive, reliable, and fast-response chemical species sensors that can be used for eddy correlation flux measurement. Therefore we recommend that continued effort and resources be expended in developing chemical species sensors with the responsiveness and sensitivity required for direct eddy correlation flux measurements. This recommendation (I) was assigned the first priority in the report of the recent Global Tropospheric Chemistry workshop jointly convened by the National Science Foundation, the National Aeronautics and Space Administration, and the National Oceanic and Atmospheric Administration. The authors of the report recognized that the limited availability of fast, accurate chemical sensors is a major measurement challenge in the field of atmospheric chemistry. 

Businger and Delany (12) examined this problem and derived a relationship that defines the chemical resolution required for sensors used for eddy correlation measurements. Their approach was to specify the standard deviation of the chemical concentration ac (the root mean square of the chemical fluctuation c ) in terms of the surface chemical flux, Fc, and readily measured micrometeorological parameters. 

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... 

Eddy correlation measurements require fast-response instrumentation to resolve the turbulent fluctuations that contribute primarily to the vertical flux. These requirements are particularly severe under stable conditions where response times on the order of 0.2 s or less may be required. In practice, it is often possible to use somewhat slower instruments and apply various corrections to the computed fluxes as compensation. The eddy correlation technique has been used in aircraft (Pearson and Steadman 1980 Lenschow et al. 1982) as well as with tower-mounted instruments. 

To ensure a constant-flux layer, one can simply move the measurement height closer to the surface. For the eddy correlation method, however, the response time of the instrument must be faster as the measurement height approaches the surface, because high-frequency turbulent eddies then contribute proportionally more to the concentration fluxes than at higher levels. On the other hand, fluxes measured very close to the surface may be less representative of those over the entire area for which the measurement is intended. For the gradient method, the requirement that z/zo 3> 1 (based on the requirements of similarity theory) constrains the minimum measurement height. Under very stable conditions, when turbulence may be intermittent, turbulent fluxes may become very small, and the constant-flux layer may be very shallow. Under conditions such as these, it can be quite difficult to determine the aerodynamic resistance term ra. 

A key issue in the research of our ecosystem and in atmospheric studies today is the ability to quantify even small concentrations of trace gases, and follow their evolution from a source to their final destination. For biosphere-atmosphere or air-sea exchange, trace gas flux measurements based on the eddy correlation technique in addition to high temporal resolution (sometimes less than 1 s) is required. Of the many... 

The original eddy motion which sets up the chain of events leading to eruptions may be caused by forced flow of the bulk phases, density differences due to concentration or temperature gradients (B12), or earlier eruptions. Strong eruptions occur when a critical concentration driving force or a critical interfacial tension depression is exceeded (03, S8, S9). At lower concentration differences ripples may result (E4), eruptions may occur only over part of the interface (S8) with the jets taking some time to form (T9), or no interfacial motion at all may occur. Attempts to correlate the minimum driving force required for spontaneous interfacial motions have met with little success. 

A general technique for determining fluxes is to measure the correlation of fluctuations of concentration with those of vertical wind speed, which are due to vertical eddies in the atmosphere (see reference 80 and references cited therein). If a species is emitted from the surface, for example, it is somewhat more concentrated in air parcels moving upward than in those moving downward. To effect this approach requires sensors with at least 1 Hz, and preferably 10 Hz, time response. Presently, such instrumentation is unavailable, except for NO and NOy measurements. Developing such techniques for N20, NO (or NO ), NH3, and HN03 would be desirable. 

Oroskar and Turian (48) developed a critical deposition velocity correlation based on balancing energy required to suspend particles with energy dissipated by an appropriate fraction, F, of turbulent eddies present in the flow. They found F to be usually very close to unity (>0.95) and therefore its inclusion, especially when raised to a fractional power, has essentially no influence on correlation predictions. Their equation appears in the following form ... 

The above emulsification methods (perhaps except the Couette flow technique) have as a common feature that the final DSD is primarily determined by the interaction of turbulent eddies with interfaces. Note, however, that turbulence is hard to control and to maintain consistently throughout the whole reactor volume. From a practical point of view, it is almost impossible to predict the DSD after a scale-up based on laboratory-scale experiments. Emulsification techniques based on other principles are necessary to overcome these drawbacks. An alternative technique is the so-called membrane emulsification method where the liquid forming the disperse phase is pressed through a porous membrane. The other side of the membrane where the droplets are formed is in contact with the continuous phase. This concept is simple and it is assumed to be superior to the above techniques (35). The basic relationship of membrane emulsification (equation (8.10)) correlates the trans-membrane pressure required to start the drop-wise flow through the pores (ft) with the average pore diameter of the membrane (Dm) with being the contact angle of the mixture with the wall of the pore ... 

As Dryden (4) emphasizes, turbulence is a lack of uniformity in the flow conditions, characterized bylin rF gularfluctuation of the fliiid Ioc-ity at any point from instant to instant. Two factors are ordinarily required to express the degree of turbulence in quantitative fashion, the intensity and the scale. Intensity is defined as the root-mean-square fluctuation of velocity at anjTpoint. The scale relates the fluctuations at different points within the fluid at the same instant and has been quantitatively defined by Taylor (28) as the area under a curve of the correlation between the velocity fluctuation at two points taken perpendicular to the line joining the points plotted against the distance between the points. The scale may be taken as the size of an eddy. 

The effect of treating the particles as point particles (see also Balachandar, 2009) is that the detailed flow between the particles in response to the presence and motion of the particles remains unresolved (see, e.g., Derksen, 2003). As a result, a correlation is required in order to take the fluid—particle interaction into account. The Euler—Lagrange approach is most often used for simulating dilute gas—solid and liquid—sohd flows where the particle size is smaller than the smallest turbulent length scale (eddy) considered in the flow simulation of the carrier phase (Balachandar and Eaton, 2010). 

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