Date of Award
Doctor of Philosophy
Joshua S. Fu
Forrest Hoffman M., John Schwartz S., Xinhua Yin
The increasing nitrogen (N) emissions caused by human activities lead to elevations of N deposition in the ecosystems. And excessive N deposition is associated with a host of environmental issues. This study investigates several issues about N deposition. First, we map the global distribution of N deposition with ensemble results of several global climate models. The model predictions of N wet deposition generally agree well with site observations over North America, Europe and East Asia, but model underestimation of NH₄⁺ wet deposition exists over all three regions. And more studies are required for measurement-poor regions, some of which happen to be the most heavily polluted regions (i.e. China and India). Then, we investigate the two predominant drivers of future N deposition: anthropogenic emissions and climate changes. We examine the effectiveness of emission control on reducing N deposition over United States via modelling approach. NH₃ emission abatement is not likely to cause significant reduction of reduced forms of nitrogen (NHx̳) deposition, owing to the current atmospheric level of gas-phase NH₃. Long-range transport of air pollutants, as a potential impact of climate changes, increases the N burden on low emission intensity regions (i.e. Russia), coastal regions and Open Ocean, especially on those in the downwind regions of intensive emission sources. The impacts of excessive N deposition on terrestrial ecosystems are assessed by critical loads (CL), which is a threshold to show the natural capability on bearing N deposition. We collect several CLs from literature, and most of them were developed on regional scale (i.e. United States, Europe and China). In the practice of applying these datasets to access the exceedance of CLs, we find large uncertainties related to the land type classification, which may challenge the interpolation of results. We conduct a preliminary study on the deposition of phosphorus (P). The switches of nutrient limitation patterns from N-limited to P-limited by many ecosystems, due to inputs of N by human activities, draw the public attention on P deposition. But the lack of long-term measurement data and uncertainty on major parameters in the numerical simulations limit our understanding on the P budget.
Tan, Jiani, "Global Sulfur and Nitrogen Depositions – Distribution, Source Contribution, Critical Loads and Phosphorus Deposition. " PhD diss., University of Tennessee, 2019.