Citation:

Walker, J., M. Bell, D. Schwede, A. Cole, G. Beachley, G. Lear, AND W. Zhiyong. Aspects of inventory in total reactive nitrogen deposition estimates for North American critical load appplication. SCIENCE OF THE TOTAL ENVIRONMENT. Elsevier BV, AMSTERDAM, Netherlands, 690:1005-1018, (2019). https://doi.org/10.1016/j.scitotenv.2019.06.337

Impact/Purpose:

Determination of the amount of reactive nitrogen (Nr) deposition to an ecosystem in excess of the critical load requires an estimate of total deposition, which is typically derived from gridded chemical transport models (CTMs) or a combination of measurements and model output. The uncertainty of both the Nr deposition estimate and ecosystem response need to be understood to fully evaluate the risk to the ecosystem. Knowledge of these uncertainties is important in determining an appropriate policy action to manage ecosystem exposure.

Description:

In this paper we review the state of the science with respect to the sources of uncertainty in total Nr deposition budgets used for critical loads assessments in North America. In the absence of methods to rigorously quantify uncertainty in total Nr deposition, a simple weighted deposition uncertainty metric (WDUM) is introduced for consideration in critical load assessments. Maps of the WDUM as applied to NADP TDep total Nr deposition estimates show more uncertainty in areas where dry deposition makes a larger contribution to the budget, particularly for NH3 dry deposition in agricultural areas and NO2 dry deposition in urban areas. Absence of organic N from the total deposition budget is an important source of uncertainty over much of the U.S. Advancements in both measurements and models are needed to quantify and reduce uncertainty in Nr deposition budgets. Incorporation of measurements of bulk organic N in precipitation in national monitoring networks is a high priority, as is a more detailed treatment of organic N in CTMs. Additional monitoring of deposition in urban areas and improved representation of urban deposition in CTMs is needed for oxidized N. Expanded monitoring for NH3 in agricultural areas and development of methods for bias correcting modeled NH3 air concentrations in bidirectional flux frameworks is also needed. Improvement of dry deposition algorithms in CTMs, particularly for non-stomatal exchange pathways, is an ongoing high-priority endeavor. Establishment of long-term sites for Nr flux measurements across land use types and deposition regimes is key for such model improvements. Incorporation of land use specific deposition in measurement-model fusion procedures would reduce uncertainties associated with downscaling of grid-average fluxes to specific ecosystems. Finally, new methods are needed to quantify and aggregate different components of uncertainty within total deposition budgets.

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