Multivariate Receptor Modeling Workbook

June 2009

Front Matter

Training

Formation, Destruction, Transport (2 of 2)

•Concentrations of pollutants that are secondarily formed in the atmosphere

–are often highest downwind of the source of precursor compounds

–generally do not have steep concentration gradients near the original precursor emissions source

•Transport distance is determined by

–atmospheric chemistry (pollutant lifetimes and formation and removal processes)

–meteorology (air mass movement and precipitation)

–topography (mountains and valleys that affect air movement)

•Short-lived pollutants can only travel short distances from where they are emitted (10s to 100s of miles). Longer-lived pollutants can travel large distances from where they are formed or emitted (e.g., toxic metals in PM2.5) and may be more regionally homogenous.

•Some unreactive pollutants can remain in the atmosphere for months, years, or decades and spread across the Earth (e.g., carbon tetrachloride).


	Chemical or physical rates of formation determine how far the precursor pollutants travel before they begin forming secondary pollutants such as formaldehyde. Factors such as wind speed and temperature will also influence where these secondary pollutants are formed, relative to where they were originally emitted.
	The longer a pollutant stays in the atmosphere, the farther it can be transported. Some air toxics are removed quickly by chemical reactions (e.g., 1,3‑butadiene) or physical processes, (e.g., heavy larger particles deposit to the ground quickly).
	Other pollutants react more slowly and can travel large distances from where they are formed or emitted (e.g., toxic metals in PM_2.5). These pollutants may be more regionally homogenous.