Micrometeorological methods

NO emissions did not exceed 2 ng Nm s and their measurement was only possible by chamber methods. The low NO emissions but high NjO emissions show that denitrification was the main source of NjO at this site. The discrepancies between the chamber and micrometeorological methods illustrated the need to define the flux-footprint of a micrometeorological measurement very carefully, and to use this information in the field to choose the locations in which chambers are placed. Without such an approach, the integration of results from chambers into estimates of field-scale emission remains an uncertain method. 

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... 

Additionally, micrometeorological methods have been applied to measure NH3 fluxes in coastal areas. For example, S0rensen Geemaert et al. (1998) measured the vertical NH3 profile above the seasurface in the Kattegat Strait (western Baltic Sea) and found a flux of NH3 to the seasurface. Recently, Biswas et al. (2005) applied a similar technique and could show that the Sundarban mangrove forest (northeastern Bay of Bengal) was a sink of atmospheric NH3. 

The dynamics of canopy-scale net fluxes of water and CO2 exchanged between vegetation and the atmosphere are routinely measured today with micrometeorological methods (e.g., with eddy covariance www.daac.ornl.gov/FLUXNET/ fluxnet.html). Combining these methods with isotopic measurements allows to partition a net flux into its gross flux components. The approach here is similar to that used on the global scale to... 

Baldocchi D. D., Hicks B. B., and Meyers T. P. (1988) Measuring biosphere-atmosphere exchanges of biologically related gases with micrometeorological methods. Ecology 69(5), 1331-1340. 

Denmead, O. T. (1995). Novel micrometeorological methods for measuring trace gas fluxes. Phil. Trans. Roy. Soc. Loud. A. 351,383-396. Denmead, O. X, Leuning, R., Griffith, D. W. X, and Meyer, C. P. (1999). Some recent developments in trace gas flux measurement techniques. In Approaches to Scaling of Trace Gas Fluxes in Ecosystems. (A. F. Bouwman, Ed.), pp. 69-84. Elsevier, Amsterdam. 

In short, micrometeorological methods generally meet the nondivergence criterion more frequently than do surface sampling methods satisfy representativeness for the surrounding area. 

Methods used in studies of NH3 loss at AGRI, Hurley, involve the micrometeorological mass balance method for studies in grazed swards and a system of wind tunnels for small field plots to which specific treatments have been applied (e.g., slurry or urine). In the mass balance method, NH3 loss is calculated from measurements of (i) wind speed to a height of 3 m (ii) wind direction and (iii) the NH3 concentration profile in air windward and leeward of a treated area. The method has been successfully applied in studies in which the distance between the windward and leeward sampling... 

Fig. I. Relationship between concurrent measurements of the rate of ammonia loss made using the wind tunnel and micrometeorological mass balance methods during an experiment in which the mean air speed through the tunnels was adjusted to maintain a value within 20% of the mean ambient wind speed. Rates of loss measured using the wind tunnels are the means of four replicates, the bars around each point indicating the 95% confidence limits (reproduced from ref. 12).

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. ... 

Direct methods require that fewer assumptions be made in analyzing the data but generally demand considerably more effort and relatively sophisticated instrumentation. A detailed treatment of micrometeorological measurement techniques is provided by Businger (1986). Erisman (1993a,b) and Baldocchi et al. (1988) di.scuss dry deposition measurement methods. 

Werle and Kormann (2001) report on a field campaign of CBU-emission measurements from rice paddy fields, simultaneously recording eddy correlation and closed-chamber data (the former by using a TDLAS system, the latter being based on the standard diffusion-chamber method). Simultaneously with the CEU-emission data, micrometeorological measurements were performed (e.g. recording wind direction and speed using a sonic anemometer). 

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