Soil and Environmental Analysis: Modern Instrumental Techniques - Chapter 11 ppsx

11 Measurement of Trace Gases, II: Micrometeorological Methods at the Plot-to-Landscape Scale John B. Moncrieff The University of Edinburgh, Edinburgh, Scotland I. INTRODUCTION The range of environmental concerns that face us and the realization that the issues are complex have produced a situation in which scientists from a number of disciplines increasingly find themselves collaborating to investigate some issue of soil–vegetation–atmosphere exchange. This chapter discusses the methods that can be used at the scale of the agricultural landscape. The method s belong to the general field of micrometeorology in that they have time and space scales that are on the order of tens of minutes and a few km 2 respectively. The space scale is influenced both by the length scales of atmospheric turbulence, which are a result of mechanical (surface friction) effects, and by thermal effects which influence atmospheric stability. The reporting time scale for surface fluxes of about 30 min is related to the need to make observations over a suitably long period, so that the majority of the spectra of flux-carrying eddies are sampled, yet not so long that natural diurnal variabilities in scalar concentrations, or forcing functions such as solar radiation, are included. One important difference between micrometeorological methods and other techniques to measure surface fluxes is that they are nondestructive in that they only sample the air as it advects past the sensor; they also are noncontact in that we merely sample passively as the air passes our instrumentation—they cannot alter the microclimate as chamber methods, TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. for instance, could do (see Chap. 10). Traditionally, micrometeorologists have sought to establish their measuring systems on landscapes that are as flat as possible and with homogeneous surface cover as far as possible upwind. The earliest micrometeorological experiments were made over surfaces such as extensive wheat fields or prairie grass. Measurements were made generally in good weather, partly to protect the type of instrumenta- tion then available. In the past decade, instrumentation has developed to the extent that routine flux measurements are possible in all weather and for long periods of time. There has been a proliferation of routine flux stations across the globe (although irregularly distributed in space), and this has inevitably led to many of the stations being in nonideal terrain, e.g., on the gentle slopes of hills. The challenge for micrometeorology in the next decade or so is to develop the theoretical issues that such an expansion of sites brings. Micrometeorological methods can be used to scale up to observations made by aircraft or interpolated from remote sensing platforms. They can also be used to check observations made on much smaller scales, e.g., chambers on leaves or soil. To that extent they are an integral part of the observation strategy that is being used to solve many of the current environmental problems facing us. It is the scaling issues that bring micrometeorologists and other observational and modeling scientists together. II. THEORY AND SCALES OF OBSERVATION A. Where the Observations Are Made All micrometeorological measurements are made within the atmospheric boundary layer (ABL, Fig. 1), defined by Lenschow (1995) as ‘‘the lower part of the atmosphere that interacts with the biosphere and is closely coupled to the surface by turbulent exchange processes.’’ The depth of the ABL depends on the degree of mechanical (caused by surface friction) and buoyant mixing (thermals rising from the warmed surface), and its depth can also be dictated by synoptic scale motions such as anticyclonic subsidence. The ABL is sometimes also called a convective boundary layer (CBL) when it is several kilometers deep as a result of the development of thermals rising during the day. At night, the ABL may be perhaps only a few tens of meters deep. The top of the ABL represents a fairly sharp boundary between the turbulent, chaotic motions of the ABL and the smoother, streamlined flow of the free atmosphere above. The rate of change in vertical profiles of temperature, water vapor, and carbon dioxide is highest near the active surface, in a region typically about one-tenth the 478 Moncrieff TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. depth of the ABL that is called the surface boundary layer (SBL). It is in this region that most conventional micrometeorological measurements are made The SBL can further be divided into two sublayers: inertial and roughness (Raupach and Thom, 1981). In the inertial sublayer, wind profiles in neutral stability conditions are logarithmic with height, and well-established scaling schemes apply (Kaimal and Finnegan, 1994). Fluxes are considered constant with height (or at least within 10% of their surface values). Close to surface vegetation lies an interfacial layer in which turbulence is enhanced over Figure 1 The atmospheric boundary layer and its sublayers. Measurement of Trace Gases, II 479 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. that in the inertial sublayer above by wake turbulence or thermal effects. The depth of this so-called roughness sublayer has been variously estimated to be three times the height of the vegetation (Kaimal and Finnegan, 1994) or sometimes three times the spacing between the vegetation elements (Raupach and Legg, 1984). The implication of the enhanced diffusivities in the roughness layer is that micrometeorological methods that rely on establishing eddy diffusivities are difficult to apply in this layer. By going beyond the roughness sublayer, however, concentration gradients can become small and difficult to measure over rough vegetation, and this poses further problems. The top of the surface layer is not physically as well defined as the top of the ABL. Although most tower-based flux measurements are made within the surface boundary layer, the evolving structure of the ABL over a day does present opportunities for other measurements using aircraft, tether- sondes, and ABL budget methods (Raupach et al., 1992). B. Flux Footprint Fluxes measured by micrometeorological sensors are effectively the integration of fluxes from a variety of sources and sinks in the landscape for a distance of several hundred meters upwind from the measuring point. The height at which the measurements are chosen to be made must be determined both by consideration of the frequency response of the instrumentation and also by the ‘‘fetch’’ or extent of the upwind area from which the signal comes. Eddies become progressively larger with height up to the depth of the planetary boundary layer, typically 1 km by day, and this means that instrumentation with a slower response can be used successfully at heights well above the vegetation. As the surface is approached, however, the spectrum of turbulence includes a l arger proportion of smaller eddies that actively exchange mass and momentum between the surface and the atmosphere. Instrumentation used in the eddy covariance method must therefore be capable of sampling high frequency eddies, typically up to 10 Hz. In principle one could use an eddy covariance system (see Sec. III.C) well above the canopy in order to avoid the problem of frequency response of analyzers. However, as we move up in height the area of flux integration becomes larger and the requirements of surface homogeneity become more and more stringent. In fact if the instruments are placed too high above the surface it is possible that they could extend above the boundary layer representative of the nearby vegeta tion and be measuring some component of fluxes from a different type of vegetation further upwind. A convenient rule of thumb suggests a fetch : height ratio of about 100 : 1; thus a fetch of 500 m would 480 Moncrieff TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. allow instrument s to be placed up to a height of about 5 m above the surface. The fetch:height ratio depends on atmospheric stability and surface roughness insofar as they influence the degree of mixing of internal boundary layers as they are advecte d over different types of surface (Mulhearn, 1977; Gash, 1986; Grelle and Lindroth, 1996). The footprint or source region defines the relative importance of sources upwind that contribute to the measured flux. This area can be regarded as contributing most of the flux measured, and its areal extent and position can be calculated from a knowledge of surface roughness, atmospheric stability, and wind speed and direction (e.g., Schuepp et al., 1990, Schmid and Oke, 1990). The approach is based on the same theory as underlies dispersion modeling of pollutants using the Gaussian plume approach. Such models can be used to calculate the relative contribution to the vertical flux at any measurement height coming from a point upwind. Figure 2 is an example of the distribution of the relative contribution to the vertical flux as a Figure 2 A ‘‘flux footprint’’ showing the relative contribution to the total measured flux contributed by sources at various distances upwind from the measuring point. (The simulation is available at http://www.ierm.ed.ac.uk/jbm/java/jflux/jf2.htm.) Measurement of Trace Gases, II 481 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. function of height of measurement. The data are normalized so that the flux at the distance of maximum source contribution appears as a peak in this representation. In this simulation, the peak source distribution is within about 50 m of the measurement point. The contribution from sources upwind decreases exponentially with distance from the tower. Calculation of the cumulative fraction shows that even at a distance of about 1500 m from the tower, only about 95% of the measured flux has been accounted for by sources within this distance or footprint. Both the peak and the cumulative fraction depend on measurement height and atmospheric stability. As the height of measurement increases, the peak of the flux footprint becomes more and more distan t from the point of measurement. Similarly, the peak contribution moves closer to the point of measurement as the atmosphere becomes more unstable. With increasing stability, the peak contribution moves further from the point of measurement. Many agricultural landscapes are characterized by relatively small- scale heterogeneity (small fields of a few hundred meters on a side) and thus any tower-based flux measuring system will ‘‘see’’ fluxes coming from different fields within the footprint. The flux footprint concept permits the integration of fluxes from such a landscape by permitting the calcula- tion of the relative source strengths in the area upwind of the flux tower. Soegaard et al. (2003) examined the CO 2 fluxes over several very different agricultural surfaces (winter wheat, winter barley, spring barley, maize, and grass) in conjunction with a footprint modeling exercise. Good agreement was found between the surface flux measured on a tower where it could be expected to integrate across the different land-use types and the modeled fluxes (based on 3-D footprint and biophysical models). Such an approach is likely to be of increasing value in the real world where homogeneous extensive landscapes for micrometeorological research are really quite rare. C. Measurements of Net Ecosystem Exchange Fluxes of gases measured by micrometeorological methods above the canopy are the net fluxes from the whole system. If the gases of interest are carbon dioxide and water vapor then the net fluxes are measures of the net ecosystem exchange of carbon (NEE) or total evaporation (ET), respec- tively, from the canopy, its elements and the ground surface (Fig. 3). The upper system boundary (USB) marks the level through which the vegetation exchanges carbon and water with the atmosphere. Direct micrometeoro- logical methods such as eddy covariance operate at this level. Below the 482 Moncrieff TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. USB, A l and R l are the net assimilatory and respiratory exchanges by the leaves. The subscripts w, s, and r refer to respiratory fluxes from wood, soil, and roots, respectively. Instrumentation placed above the upper system boundary (USB) measures the net exchange of material passing through that arbitrary level and of itself cannot distinguish the pathways by which that flux arrived at the sensor. Thus, the NEE is the measured flux (F c ) above the USB, plus the component that represents storage of carbon between the point of observation and the ground, i.e., F c + ÁS. (It is assumed that there are no advective fluxes in this representation; this assumption will be examined later.) Profiles of carbon dioxide concentration within the canopy, and up to the height at which the flux measurements are made, are used to measure changes in storage of carbon. NEE is the net sum of a number of component processes that take place within the stand. These include the gains of carbon in photosynthesis by the foliage of the trees, understorey, and mosses, and the losses from respiration by the above-ground foliage and wood as well as the below- ground roots, mycorrhizas, and heterotrophic microorganisms (the so-called ‘‘soil respiration’’). Of these the major components are the gains by photosynthesis and the losses through soil respiration. Figure 3 Fluxes above and within the canopy. Measurement of Trace Gases, II 483 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. III. PRINCIPLES The transport of gases, heat, and pollutants in the atmosphere is produced by the eddying motion of the atmosphere as air parcels are displaced from one level to another. Micrometeorological methods used to quantify this turbulent exchange can either sample the air as it flows past a sampling point for its vertical windsp eed and direction and its gas concentration directly (the eddy covariance or eddy accumulation methods), or they can be based on quantifying the rate of diffusion down concentration gradients (the aerodynamic and Bowen ratio methods). The direct method of eddy covariance involves sampling at one height only but with relatively sophisticated sensors and logging equipment. The methods ba sed on measuring gradients require measurements at two or more heights but use simpler sensors. The disadvantage of the gradient techniques is, however, that a number of empirical functions are required to account for thermal stratification of the atmosphere; additionally, the gradients in atmospheric properties become very small above vegetation canopies, and the aerodynamic technique in particular cannot be used inside plant canopies. All three techniques, when used above vegetation, require that steady-state conditions exist, i.e., that atmospheric conditions are not changing rapidly over the sampling period; they also all require extensive upwind areas of the vegetation, i.e., these methods cannot be used on isolated plots or small fields. If these conditions are met, it is assumed that the flux measured just above the vegetation is equal to that at the ground or plant surfaces and fluxes are constant with height up to a level dependent on the extent of upwind surface homogeneity and atmospheric mixing. The question of which method to use depends not only on the available resources but also crucially on the surface type over which the measurements are to be made (Moncrieff et al., 2000). For example, over very rough surfaces in an aerodynamic sense such as forests, gradients of scalars are small, and their measurement places extreme emphasis on the precision of sensors. Under these conditions, gradient techniques are problematic. On the other hand, as turbulence is enhanced over forests, the eddy covariance technique is made easier as the size of the fluctuations in vertical windspeed and other atmospheric properties is increased. Measure- ments over smooth surfaces such as water or ice are difficult using any of the techniques mentioned. Increasingly, eddy covariance-based systems are being integrated into global observing networks both to monitor carbon and water exchange on the global scale and to validate remotely sensed products such as fraction photosynthetically active radiation (FPAR) and normalized difference vegetation index (NDVI) both of which can be used to estimate 484 Moncrieff TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. leaf area index, which in turn can be used to model NEE (Running et al., 1998). A. Aerodynamic Gradient Method The vertical exchange of atmospheric entities such as momentum, temperature, water vapor, and CO 2 by turbulent transport is driven by and is proportional to their vertical concentration gradients. We can describe the transport process in a flux-gradient form which defines the constant of proportionality K known as an eddy diffusivity. In generic form, the flux density (F x , the amount of that entity transported vertically through unit area in unit time) of any scalar (X)is F x ¼ÀK x dX dz The eddy diffusivities for scalars such as temperature, water vapor, and CO 2 have been the subject of much experimentation over several decades, as their relative magnitudes depend on both surface and atmospheric features. In practice, the diffusivities are related to the eddy diffusivity for momentum, which can be established from profiles of wind speed. Wind profiles are used to calculate the friction velocity, a measure of the degree of atmospheric mixing (Thom, 1975) and from which we find K x ¼ ku à ðz À dÞ where k is von Karman’s constant (0.40), u * is the friction velocity (m s À1 ), z is height of measurement (m), and d is zero-plane displacement (m). The gradients of the scalar in question need to be precisely determined, and obtaining accurate gradients over rough vegetation can be difficult because of the substantial degree of atmospheric mixing over such canopies (McNeil and Shuttleworth, 1975). One of the concerns about the aerodynamic technique is that a number of empirical corrections are required to account for changes in atmospheric stability (which influences the shape of wind profiles). A number of semiempirical formulae have been extensively investigated and are well accepted (e.g., Webb, 1970; Paulson, 1970). The aerodynamic technique has been used extensively in the past to obtain fluxes of sen sible and latent heat over forests (e.g., Thom, 1975; Lindroth, 1984) and extended to other trace gas species over different surface types (e.g., Fowler and Unsworth, 1979; Sutton et al., 1993). It is not possible to use the aerodynamic technique in the roughness sublayer or Measurement of Trace Gases, II 485 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. within the canopy as the variation in sources and sinks for heat, water vapor, and carbon dioxide invalidates the underlying assumptions in the method (Raupach and Legg, 1984). B. Energy Balance/Bowen Ratio Method The energy balance at the surface is R n À G ÀS ¼ H þ LE where R n is net radiation absorbed by the vegetation, G is soil heat flux, and S is heat store d in the vegetation. The ratio of sensible (H) to latent heat (LE ) flux is known as the Bowen ratio (), and by writing the fluxes in their flux-gradient form, an equation can be found that permits either flux to be found from measurements of the gradient of temperature (T ) and humidity (e), irrespective of atmospheric stability: ¼ 1  dT de where  is the psychrometric constant (Monteith and Unsworth, 1990). Once  has been obtained, substituting for LE and H we find H ¼ R n À G ÀS 1 þ  À1 and LE ¼ R n À G ÀS 1 þ  This method can also be used to measure fluxes of other gases or pollutants by writing a more generalized form of the flux-gradient equation by combining H and LE as before to yield R n À G ÀS ¼ Kc p dT e dz where K is an eddy diffusivity assum ed equal for all entities (other than momentum) and T e is the equivalent temperature (T þ e/) (Monteith and 486 Moncrieff TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. [...]... to open and close the sampling valves is increased, effectively permitting the system to collect only the strongest updraughts and downdraughts that contribute to the flux The threshold is defined as a ‘‘hyperbolic hole’’ (H) and can be written as 0 0 w c H¼   w c where w0 and c0 are the turbulent fluctuations of vertical wind speed and CO2 concentration, respectively, and sw and sc are the standard... horizontal and vertical wind speeds Air is sucked down an inlet tube near the sonic head to a fast-responding infrared gas analyzer at the base of the tower The expanded schematic shows the gas path within the gas analyzer A mass flow controller and pressure transducer can be used to maintain a constant rate of flow down the sample tube (and hence constant lag of gas sample between the sonic head and the...), and by writing the fluxes in their flux-gradient form, an equation can be found that permits either flux to be found from measurements of the gradient of temperature (T ) and humidity (e), irrespective of atmospheric stability: ¼ 1 dT de where is the psychrometric constant (Monteith and Unsworth, 1990) Once has been obtained, substituting for LE and H we find H¼ Rn À G À S 1 þ À1 and LE ¼ Rn... the turbulent signals of vertical wind speed and CO2 measured over a forest We can split any of the turbulent signals into a mean and d fluctuating part, in the form of a Reynold’s decomposition Thus, for vertical wind speed, the instantaneous value (w) is a function of both the long-term mean value (w) and the instantaneous difference (w0 ) between that value and w, i.e., w ¼ w þ w0 We can write the signal... generalized form of the flux-gradient equation by combining H and LE as before to yield Rn À G À S ¼ Kcp dTe dz where K is an eddy diffusivity assumed equal for all entities (other than momentum) and Te is the equivalent temperature (T þ e/ ) (Monteith and TM Copyright n 2004 by Marcel Dekker, Inc All Rights Reserved Measurement of Trace Gases, II 487 Unsworth, 1990) If all pollutants and gases share this... be found using net radiometers, and soil heat flux plates, and from profiles of temperature within the canopy The technique is reliable in most conditions, but when the available energy becomes small, e.g., at night, or the gradients are small, as over rough vegetation, then large errors can occur Also the method involves measuring G and S, which poses problems In low-windspeed conditions, where the... energy [net radiation (Rn) minus heat taken up by the soil TM Copyright n 2004 by Marcel Dekker, Inc All Rights Reserved 492 Moncrieff Figure 7 Energy balance closure at Griffin Forest, Scotland, 1997–2001 (G)] balances the losses of energy through sensible (H) and latent heating (LE) of the air) Figure 7 shows results from the same forest as shown in Fig 6 and for the same period, but for energy balance... (Desjardins, 1977) Hicks and McMillen (1984) suggested, almost as an aside, that the simpler method of sampling air at a constant rate into either bag might work, and with fewer practical difficulties One bag will then contain air collected in updraughts and the concentration of CO2, say, will be cþ; the downdraught bag will have a CO2 concentration of cÀ The idea was taken up by Businger and Oncley (1990)... simultaneous fluxes of methane and nitrous oxide The vertical wind speed is measured by a sonic anemometer (center left of the diagram) and, dependent on whether an updraught or a downdraught has been measured, the appropriate sample valve is opened and air is sucked into a reservoir or passed into a gas analyzer Further details of the systems shown here can be found in Beverland et al., 1996 (The tunable... dried and scrubbed of carbon dioxide and water vapour The size of the storage correction can be small or negligible during the day, but in tall vegetation such as forests, it can be substantial at night A correction term for nonzero mean velocity or convergence arises at even seemingly ideal field sites for micrometeorology Atmospheric subsidence is common in highly convective conditions and in synoptic-scale . 11 Measurement of Trace Gases, II: Micrometeorological Methods at the Plot-to-Landscape Scale John B. Moncrieff The University of Edinburgh, Edinburgh, Scotland I. INTRODUCTION The range of environmental. different land-use types and the modeled fluxes (based on 3-D footprint and biophysical models). Such an approach is likely to be of increasing value in the real world where homogeneous extensive landscapes. below- ground roots, mycorrhizas, and heterotrophic microorganisms (the so-called ‘ soil respiration’’). Of these the major components are the gains by photosynthesis and the losses through soil