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
reaction with hydroxyl radical (OH), photolysis
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
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.
air toxics and their residence times are listed in the appendix to this