June 2009
Front Matter
Training
13
Why Analyze Ambient Air Toxics Data?
•Air toxics data analysis is needed to track progress in risk reduction.
•States collecting data have unique “local” perspectives on data quality, meteorology, and sources, and in articulating policy-relevant data analysis questions.
–Data anomalies at an individual site have little influence on the overall national-scale results.
–On a site-by-site basis, a fine level of detail is needed to understand the characteristics and trends observed. 
National-level analyses provide an overview of the air toxics program and build on the power of a large data set to find the central tendencies in the data.

Knowledge is needed of the nearby sources, operating schedules, facility upsets and closures, new emission sources, types of emissions, types of controls and scheduled implementation, data reporting and quality issues, changes in sampling and methodology, local meteorology, and other details to fully understand changes in ambient pollutant concentrations.