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Figure 12: Data collected by the SOAR Cheyenne II during a flight downwind and upwind of Houston. Calculated back <br />trajectories are represented by the five yellow lines that terminate along the blue leg of the flight track. The line colors used in the <br />top graph showing measured CCN concentration at 0.5% supersaturation reflect the position along the flight track shown on the <br />map. CCN concentrations immediately downwind of Houston were observed to be as much as 10 times higher than those <br />measured on the east, west, and south sides of the urban plume. The cause of the bimodality in concentration is not known, <br />although similar observations were made during the TexAQS study in 2000. Aerosol size distributions are shown in the seven <br />numbered graphs. The position of the aircraft during each of these measurements is identified by the corresponding numbers <br />along the flight track. Both number (black trace) and volume (red trace) distributions are shown in each graph. Total aerosol <br />concentration was substantially greater within the Houston plume than outside of it, while total aerosol volume concentration <br />differed by only about a factor of two. <br />Figure 13a: CCN size distributions generally flat from 2 Figure 13b: CCN size distributions in the urban plume <br />to 6 m away from the urban plume. CCN number with a mode around 3 m. CCN number concentration of <br />concentration of 565 cm at 0.25% supersaturation. 4566 cm at 0.25% supersaturation. <br />-3-3 <br />36 <br />