A 16-class representation of multipollutant air quality days observed in Atlanta, GA, during the years 2000 to 2007 identified with SOM analysis. Panel (a) discovered class profiles describing each multipollutant day type. Pollutant concentrations under each class are represented by the size of the pie and coordinate indices are in parentheses. Panel (b) resulting frequency counts for number of air quality days classified under each day type. Percentages are listed in parentheses. Panel (c) mean (range) number of days in which a day type persisted. (Lighter grey tones indicate lower values and darker shades indicate higher values).