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 required.  However, estimates of uncertainty are not currently available for the reactive nitrogen (Nr) total deposition estimates most commonly used for North American ecosystem assessments.  

Uncertainty in measured 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.