Evaluation measures used to characterize 4 × 4 SOM quality. Panel (a) presents the coefficient of variation for within-class similarity in bold and the standard error/mean in parentheses. Panel (b) presents a dendrogram of SOM classes labeled with xy coordinates. Panel (c) presents a Sammon map of SOM classes labeled with xy coordinates. Panel (d) presents a multidimensional scaling projection of daily multipollutant profiles and class profiles generated from SOM, k-means, and Ward’s clustering approaches.