Assessing uncertainty in total reactive nitrogen deposition estimates for North American critical load applications
Citation:
Walker, Johnt, M. Bell, D. Schwede, A. Cole, G. Beachley, G. Lear, AND Z. Wu. Assessing uncertainty in total reactive nitrogen deposition estimates for North American critical load applications. National Atmospheric Deposition Program 2018 Fall Science Symposium, Albany, New York, November 07 - 08, 2018.
Impact/Purpose:
Critical loads are used to quantify the amount of atmospheric deposition (load) that can be tolerated by ecosystems without significant harm or change occurring. Determination of the amount of deposition to the ecosystem in excess of the critical load (i.e. “exceedance”) requires an estimate of total deposition, which is typically derived from gridded chemical transport models (CTMs) or a combination of measurements and CTM output. Because the critical load exceedance is a metric used to inform policy decisions, uncertainty estimates for both the critical load and the exceedance itself are needed. However, estimates of uncertainty are not currently available for the reactive nitrogen (Nr) total deposition estimates most commonly used for North American ecosystem assessments. Assessment and prioritization of the sources of uncertainty in atmospheric deposition estimates, and improvements in measurements and models to reduce these uncertainties, are needed to better understand the effectiveness of critical load frameworks for nitrogen and the secondary NAAQS for NOx/SOx/PM.
Description:
Uncertainty in measurements of atmospheric deposition results from analytical uncertainty in the measurements themselves, from the completeness of the suite of species analyzed with respect to the significant forms of nitrogen that impact the deposition budget, and from ‘scaling-up’ measurements from field to ecosystem. Aspects of uncertainty in deposition budgets derived from CTMs or measurement-model fusion approaches include completeness of deposition budgets, uncertainties in input emissions and meteorology, representation of chemical reactions, deposition algorithms, fusion procedures, and spatial averaging of sub-grid processes. In this presentation, we explore the state of the science with respect to these and other sources of uncertainty in total Nr deposition budgets used for North American critical loads assessments and propose future research activities to address the underlying knowledge and data gaps.