Tc. Strickland et al., A NATIONAL CRITICAL LOADS FRAMEWORK FOR ATMOSPHERIC DEPOSITION EFFECTS ASSESSMENT .1. METHOD SUMMARY, Environmental management, 17(3), 1993, pp. 329-334
The United States Environmental Protection Agency (EPA), with the assi
stance of the US Department of Energy (DOE) and the National Oceanogra
phic and Atmospheric Administration (NOAA) is examining the utility of
a critical loads approach for evaluating atmospheric pollutant effect
s on sensitive ecosystems. A critical load has been defined as, ''a qu
antitative estimate of an exposure to one or more pollutants below whi
ch significant harmful effects on specified sensitive elements of the
environment do not occur according to present knowledge.'' Working in
cooperation with the United Nations Economic Community for Europe's (U
N-ECE) Long Range Transboundary Air Pollution (LRTAP) Convention, the
EPA has developed a flexible, six-step approach for setting critical l
oads for a range of ecosystem types. The framework is based on regiona
l population characteristics of the ecosystem(s) of concern. The six s
teps of the approach are: (1) selection of ecosystem components, indic
ators, and characterization of the resource; (2) definition of functio
nal subregions; (3) characterization of deposition within each of the
subregions; (4) definition of an assessment end point; (5) selection a
nd application of models; and (6) mapping projected ecosystem response
s. The approach allows for variable ecosystem characteristics and data
availability. Specific recognition of data and model uncertainties is
an integral part of the process, and the use of multiple models to ob
tain ranges of critical loads estimates for each ecosystem component i
n a region is encouraged. Through this intercomparison process uncerta
inties in critical loads projections can be estimated.