Description:

Back trajectories from 20 receptor sites evenly distributed over the Eastern US. The trajectories extend for three days back in time from each receptor time during the three months June - August for the years 1991 - 1995. The receptor time, in Eastern Standard Time (EST), is located at the bottom of the movie, and has a four hour time increment. Each receptor airmass is tracked by three trajectories. The squares along each trajectory identify the location of the airmass every four hours along its pathway to the receptor.

Purpose:

These movies can be used as a resource to provide meteorological transport to aid in the interpretation of air quality data. The trajectories identify the pathway of airmasses in route to the receptors. They also identify whether the airmass is stagnating, recirculating, or part of a well defined flow field.

Method:

The back trajectories were calculated using the CAPITA Monte Carlo model, and 3-D Eulerian wind fields generated from the National Meteorological Centers Nested Grid Model. Three particles were released from each site at four hour time increments and tracked backwards for three days.

Interpretation:

Transport in the lower layers of the atmosphere is a random process resulting from a mean advecting flow and turbulent mixing. The mixing processes are generally very efficient during the day causing individual airmasses to lose their identity and become a part of a larger regional airmass. Consequently, a receptor airmass is made up of sub airmasses that may have had very different transport histories from each other. This phenomenon is simulated by using three particles to trace the transport route of each airmass. As shown in the movie, each particle follows its own pathway which at times is very different from the other particles comprising the airmass.