•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.