Figure 7

Likely flow history of air masses during the spring and summer of 2001, 2002, and 2003 for Charlotte. Frequency clusters of 72-hour backward trajectories, using EDAS (Environmental Data Analysis System) wind fields for different air masses: (a) DM air mass, (b) DT air mass, (c) MT air mass, and (d) MM air mass. The number of times trajectories crossed each 1- × 1-km grid cell over the contiguous U.S., Canada, and Mexico was divided by the total number of trajectories crossing all the grid cells to give a percentage.. Color legend: turquoise = 5%, green = 10%, blue = 25%, yellow = 50%.