Author
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Harper, Lowry |
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Submitted to: American Society of Agronomy Monograph Series
Publication Type: Book / Chapter Publication Acceptance Date: 9/15/2003 Publication Date: 6/1/2005 Citation: Harper, L.A. 2005. Ammonia: Measurement issues. In: Hatfield, J.L. and Baker, J.M., editors. Micrometeorological measurements in agricultural systems. Agronomy Monograph 47. American Society of Agronomy-Crop Science Society of America-Soil Science Society of America Annual Meetings. Madison, Wisconsin. p. 345-379. Interpretive Summary: The contribution of nitrogen compounds to the total deposition of pollutants from the atmosphere is becoming more important to assessing environmental impact. Atmospheric ammonia, because of its properties, is difficult to measure appropriately. Measurements may not be representative or accurate. The purpose of this chapter is to evaluate currently- available technologies and sensors for use in determining ammonia emissions. There is no single technique for all measurement situations and the researcher and/or monitoring agencies must be careful to use the most appropriate technology and sensors. In general, techniques that do not disturb the source are preferred as they do not modify the chemical and physical characteristics of the emissions. Currently, these techniques include methods called flux-gradient, eddy correlation, and relaxed eddy accumulation methods. Lagrangian dispersion analysis techniques have been useful and show future promise. Mathematical dispersion models require information that makes them specific for a site and this information generally is not available for agricultural situations. Tracer gas studies must be used with caution as the commonly used gases do not mimic the transport of ammonia. Closed chambers should not, in general, be used as they modify gas transport processes. The prospects for ammonia emissions reduction worldwide are limited. Therefore, representative and accurate measurements of ammonia emissions are needed to estimate and manage transport of ammonia in the environment. Technical Abstract: Ammonia is a difficult gas to measure without disturbing its transport characteristics. Its chemical properties dictate the use of transport technologies that do not interfere with the source temperature, concentration, pH, and turbulent or diffusive transport. Ammonia absorbs and desorbs with most surfaces and even with the sensors themselves. The purpose of this chapter is to review available atmospheric-exchange and transport technology and discuss strengths and weaknesses of the methodologies. Measurement of NH3 transport with closed chamber systems is generally not recommended. Non-interference techniques such as Micro- meteorological are to be preferred in principle. The flux-gradient, eddy correlation, and relaxed eddy accumulation techniques are non-interfering and integrate large spatial source areas but have relatively large fetch requirements. Mass-balance techniques are more physically straight-forward dand are useful for relatively small source areas. The Lagrangian dispersion analysis technique has been useful in evaluating source and sink strengths and absolute emissions have been shown to be comparable to other micrometeorological techniques. Dispersion models require experimental values close to the measurement situation as possible and should be used with caution as there is no exact model of turbulent dispersion. Gas tracer studies commonly use gases that are different in chemical and physical properties from NH3. This may bias emissions either low or high, depending on the tracer used. The prospects for worldwide emissions reduction are limited. Therefore, measurements of NH3 emissions, correctly and accurately, are needed to address linkages in the environment. |
