June 2009
Front Matter
Training
34
Residence Time
Overview
•Residence time is a pollutant-specific measure of the average lifetime of a molecule in the atmosphere.
•It is dependent on chemical and physical removal pathways that include
–Chemical:  reaction with hydroxyl radical (OH), photolysis
–Physical:  wet or dry deposition
•Why is it important to understand residence times?
–Residence times can provide insight into the spatial and temporal variability of air toxics.
–Longer residence times result in less spatial variability (e.g., carbon tetrachloride).
–Conversely, short residence times should result in steep gradients in concentrations near sources and temporal patterns that are dependent on emissions schedules.
•Residence times are not characterized well for all air toxics.  Some air toxics and their residence times are listed in the appendix to this section.