title: Using self-organizing maps to develop ambient air quality classifications: a time series example.
abstract: BACKGROUND
  Development of exposure metrics that capture features of the multipollutant environment are needed to investigate health effects of pollutant mixtures. This is a complex problem that requires development of new methodologies.
  
  
  OBJECTIVE
  Present a self-organizing map (SOM) framework for creating ambient air quality classifications that group days with similar multipollutant profiles.
  
  
  METHODS
  Eight years of day-level data from Atlanta, GA, for ten ambient air pollutants collected at a central monitor location were classified using SOM into a set of day types based on their day-level multipollutant profiles. We present strategies for using SOM to develop a multipollutant metric of air quality and compare results with more traditional techniques.
  
  
  RESULTS
  Our analysis found that 16 types of days reasonably describe the day-level multipollutant combinations that appear most frequently in our data. Multipollutant day types ranged from conditions when all pollutants measured low to days exhibiting relatively high concentrations for either primary or secondary pollutants or both. The temporal nature of class assignments indicated substantial heterogeneity in day type frequency distributions (~1%-14%), relatively short-term durations (<2 day persistence), and long-term and seasonal trends. Meteorological summaries revealed strong day type weather dependencies and pollutant concentration summaries provided interesting scenarios for further investigation. Comparison with traditional methods found SOM produced similar classifications with added insight regarding between-class relationships.
  
  
  CONCLUSIONS
  We find SOM to be an attractive framework for developing ambient air quality classification because the approach eases interpretation of results by allowing users to visualize classifications on an organized map. The presented approach provides an appealing tool for developing multipollutant metrics of air quality that can be used to support multipollutant health studies.
authors.affiliation: Department of Biostatistics and Bioinformatics, Rollins School of Public Health, Emory University, Atlanta, GA, USA. john.pearce@emory.edu.
authors.forename: John L
authors.initials: JL
authors.lastname: Pearce
authors.forename: Lance A
authors.initials: LA
authors.lastname: Waller
authors.forename: Howard H
authors.initials: HH
authors.lastname: Chang
authors.forename: Mitch
authors.initials: M
authors.lastname: Klein
authors.forename: James A
authors.initials: JA
authors.lastname: Mulholland
authors.forename: Jeremy A
authors.initials: JA
authors.lastname: Sarnat
authors.forename: Stefanie E
authors.initials: SE
authors.lastname: Sarnat
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Strickland
authors.forename: Paige E
authors.initials: PE
authors.lastname: Tolbert
mesh_terms: D000393:Air Pollutants / Q000032:analysis*; D004781:Environmental Exposure; D004784:Environmental Monitoring / Q000379:methods*; D016571:Neural Networks, Computer; D012621:Seasons; D013997:Time Factors; D014887:Weather
chemical_list: D000393:Air Pollutants