Surface-layer flux measurement

Methods based on spatial gradients in the direction of diffusion These include the standard gradient and Bowen-ratio methods of surface-layer flux measurement in micrometeorology. The key assumption is that terms I and II are small, so that the source term (j) (in this case identifiable with the flux at the surface) equals the flux f at a measurement height Z. ... 

In practice, instead of usually the electro-osmotic volume flow is measured. The simplest case concerns capillaries of fixed cross-section A that are so wide that the range close to the surface, where the liquid velocity drops from its bulk value in the core of the lumen to zero at the wall, is so small that the contribution of this surface layer to the flux may be ignored. Then 9 = Au. so that for this flow rate per unit of field strength... 

Means more direct than the measurement of new production exist for the estimation of particle fluxes into the deep. Moored or free-floating traps may be used to collect sinking particles (e.g., Martin et al, 1987). Alternatively, particle flux may be estimated from particle-reactive nuclides (e.g., Buesseler et al, 1992). Particle flux may also be estimated from the consumption of oxygen (associated with the decomposition of sinking POM) in waters below the surface layer (e.g., Jenkins, 1982). 

We can estimate the time scale in which the whole surface layer is affected by the primary ions. The lifetime of a surface may be simply estimated from the primary ion flux (Ip) and damage cross-section (er) generated by each impact. Ip is commonly measured in A cm-2 (1 A = 6.2 x 1018 charged particles per second). Assume that each primary ion generates a = 10-13 cm2. Then, 1013 primary ions cm-2 will affect the whole surface area of 1 cm2. It means that the lifetime of a surface with the flux density Ip= 1 pA cm-2 (= 6.2 x 1012 ions cm-2) is less than 1 second. Apparently, 1 p A cm-2 of flux density for primary ions is too high for static SIMS. Since it is commonly accepted for the static SIMS condition to limit the total amount of primary ions up to 1013 ions cm-2, for a 10-min duration of static SIMS examination a primary flux density of about 2.7 nA cm-2 is required to preserve the chemical structure of the surface top layer where the secondary ions are emitted. This flux is extremely low compared with that of dynamic SIMS, which requires a flux density of greater than 1 pA cm-2 to ensure a reasonable erosion rate of surface for depth profiling. 

Fig. 6.13 Effect of seasonal changes in physical characteristics of surface water, as measured by instruments on a surface air-sea interaction buoy, on particle flux to the deep ocean, recorded by sediment traps at 2 229 m during 1994-95. These moorings were deployed close to the US JGOFS Sta. S7 (Fig. 6.4, inset). Redrawn from Honjo etal. (1999) with the data for the mixed layer depth (MLD, taken as the depth at which temperature decreased by 1°Q from Dickey etal. (1998).

By definition the Monin-Obukhov length is the height at which the production of turbulence by both mechanical and buoyancy forces is equal. The parameter L, like the flux Richardson number, provides a measure of the stability of the surface layer. As we discussed, when Rf > 0 and therefore according to (16.69) L > 0 the atmosphere is stable. On the other hand, when the atmosphere is unstable, Rf < 0 and then L < 0. Because of the inverse relationship between Rf and L, an adiabatic atmosphere corresponds to very small (positive or negative) values of Rf and to very large (positive or negative) values of L. Typical values of L for different atmospheric stability conditions are given in Table 16.2. 

To infer a dry deposition rate from an eddy correlation measurement, a nondivergent vertical species flux should exist. Nondivergence essentially stipulates that quasi-one-dimensional transport exists. The nondivergence assumption is, in fact, equivalent to the constant-flux-layer assumption of the surface layer in practical terms, nondivergence is best satisfied in relatively flat topography for which a substantial fetch over the terrain exists. 

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. 

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