THE EVALUATION OF ACID DEPOSITION MODELS USING PRINCIPAL COMPONENT SPACES

An analytical technique involving principal component analysis is prop osed for use in the evaluation of acid deposition models. Relationship s among model predictions are compared to those among measured data, r ather than the more common one-to-one comparison of predictions to mea surements. A multidimensional comparison of principal component spaces , using general methodology developed by Krzanowski (J. Am. statist. A ss. 74, 703-707), is the basis of the technique. The technique is illu strated using concentrations measured by aircraft over the eastern Uni ted States of America. Model predictions are generated by the Regional Acid Deposition Model (RADM). A simple example provides motivation fo r the technique. More detailed applications are shown for systems of c oncentrations involved in the creation of sulfur and nitrogen depositi on, respectively. Orthogonal vectors describing the two-dimensional pr incipal component spaces are calculated for measurements and predictio ns, such that the angles between measured vector i and predicted vecto r i (i = 1, 2) are minimized. Using one of the two approaches that are presented (common scaling factors), the agreement between measured an d RADM-predicted spaces is generally good in the sulfur system, with a ngles of 4.5 and 11.4-degrees. Agreement is weaker in the nitrogen sys tem (17.2 and 27.7-degrees). Issues of interpretation and potential ex tensions of the technique's applicability are discussed.