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Title: The surface/atmosphere exchange of ammonia
Author: Sutton, Mark A.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1990
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Measurements of the exchange of gaseous ammonia (NH3) and particulate ammonium (NH4+ ) between the atmosphere and a range of vegetated surfaces were made using micrometeorological techniques. The aerodynamic gradient method was applied to estimate fluxes, and these interpreted using a resistance analogy and estimates of surface concentration. The results are used to develop an understanding of exchange processes with a view to estimating annual budgets for different surfaces. NH4+ was found to deposit very slowly so that the study focused on the exchange of NH3. Over natural and unfertilized vegetation, NH3 was generally found to deposit rapidly with near zero surface resistance (rc). Some exceptions were seen for dry vegetation (rc< 50 s m-1) and over vegetation with exposed calcareous soil (mean rc = 125 s m-1). Over fertilized agricultural vegetation both emission and deposition fluxes were recorded, with emission being favoured in warm dry conditions, and deposition in cool wet conditions. In wet conditions in summer rc was variable (0-130 s m-1), whereas in winter rc was small (< 30 s m-1). During frozen conditions an increased rc of up to 80 s m-1 was observed. In dry summer conditions emission of up to 24 ng NH3 m-2 s-1 was recorded. The estimation of the net 'compensation point' for exchange using surface concentration estimates is discussed, and shown to range over 0-7 μg m^-3 in the study here depending on surface type and environmental conditions. The different exchange patterns seen over unfertilized and fertilized vegetation are compared and described in terms of the net exchange with the surface resulting from exchange with leaf surfaces, stomata and the soil. Factors affecting these proceses are discussed and include environmental conditions (particularly temperature, wetness and humidity), the presence of acidic pollutants, and the status of the surface (including nitrogen status, stomatal opening, soil pH). Implications are drawn for the concentration dependence of exchange rates and approaches to modelling the atmospheric behaviour of NH_3. In order to estimate annual fluxes, atmospheric concentrations of NH_3 are needed. Background annual concentrations of NH_3 measured in this study at several sites in S. Scotland were in the range 0.4-1.1 μg m-3. Using these and other published data, annual fluxes over different surfaces are estimated. Over an example fertilized surface a small net NH3 emission is predicted (< 1 kg N ha-1 year-1), while over unfertilized surfaces larger deposition fluxes are expected (3-55 kg N ha-1 year-1 for the background sites considered). Comparison with other sources of atmospheric N shows that dry deposition of NH3 is frequently the dominant N input to unfertilized surfaces.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available