Atmospheric Modeling and Analysis Research
Sensitivity Studies: The Nitrogen Cycle
Biologically-available nitrogen is necessary to sustain life, but historically has been one of the most limited nutrients in the environment.
Nitrogen can be made biologically available by oxidizing (through combustion) or reducing (biologically or through the Haber-Bosch process) molecular nitrogen. In the past 150 years, the global rate of reactive nitrogen (oxidized + reduced nitrogen) production has more than doubled (Galloway et al. 2003). This excess reactive nitrogen then cascades along biogeochemical pathways in the environment leading to increases in tropospheric ozone and particulate matter, elevated nutrient deposition, soil and surface water acidification, eutrophication, and biodiversity loss. Increased levels of reactive nitrogen also contribute to global climate change.
The chemistry and physics of the atmosphere result in a complicated system with competing chemical and physical sources and sinks that impact the transformation and removal or reactive nitrogen. Regional scale air quality models, such as EPA's Community Multiscale Air Quality (CMAQ) model, are useful in studying this system. Modeled wet deposition can be evaluated against data, but evaluation of modeled surface fluxes is problematic due to the paucity of flux measurements.
EPA's air quality modeling scientists examined several uncertainties in the CMAQ air-surface exchange processes for unidirectional and bi-directional fluxes. Model sensitivity tests for relatively large changes (~50 percent) of key parameters in air-surface exchange formulations showed relatively minor changes (much less than a 5 percent) in total nitrogen deposition on the continental scale, due to feedbacks in the chemistry and rebalancing among removal pathways (Figure 1).
Bi-directional air-surface exchange parameter uncertainties regarding ammonia have greater impact because they change emissions (up to 15 percent change in nitrogen deposition). Nitrogen atmospheric removal budget uncertainties are less than expected due to offsetting system behavior and their assessment must be conducted with full regional chemical transport models, due to the complex chemical and physical interactions in the atmospheric system.
Figure 1. Ratio of sensitivity of bidirectional ammonia exchange base case for soil ammonium sensitivity (50% increase) increasing dry deposition of ammonia: (top left) ratio: ammonia emissions; (top right) ratio: dry Red-N; (middle left) ratio: dry total N; (middle right) ratio: wet Red-N; (bottom) ratio: total N deposition
Galloway, J.N., Aber, J.D., Erisman, J.W., Seitzinger, S.P., Howarth, R.W., Cowling, E.B., Cosby, B.J.: 2003, The nitrogen cascade, BioScience, 54, 341-356
- Dennis, R.L., Schwede, D., Bash, J.O., Pleim, J.E., Walker, J.T., Foley, K., Removal of gaseous and particulate nitrogen compounds from the atmosphere, submitted to Phil. Trans. R. Soc. B