Description and Overview

The critical load of a pollutant is classically defined as a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge (Nilsson and Grennfelt 1998). This definition implies some sort of measurable threshold value of ecosystem condition (termed the critical limit) that is demonstrably influenced by the pollutant(s) in question. The critical load is the pollutant deposition load, which can be shown to perturb a system such that the critical limit condition is attained. In the context of the adverse impacts of S and N deposition, many different critical limits have been proposed based on values of the Ca/Al ratio of soil water, the Ca/Al ratio of soil exchange sites, soil percent base saturation, soil water nitrate concentrations, and soil and plant C/N ratios. The relationship between pollutant loading and the value of the ecosystem condition being evaluated can be established empirically or by mechanistic modeling. For mechanistic modeling, both steady-state and dynamic-process models have been employed. Important modeling considerations are appropriate data availability at the spatial scale desired and the existence or lack of data that can be used for dynamic model calibration and evaluation.