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-3 <br /> at 0.25% supersaturation and 400 <br />Background CCN concentrations were on the order of 200 to 600 cm <br />-3 <br />to 800 cm at 0.5% supersaturation. The CCN size distribution was typically broad with a mean diameter <br />around 4m (Figure 13a). When the aircraft approached location 1, the CCN concentration increased to <br />-3-3 <br />around 4000 cm with peaks reaching 10000 cm, indicating that the measurements were being made in <br />the urban pollution plume. At this point, the CCN size distributions were typically narrower with a mean <br />diameter around 3m (Figure 13b). An interesting feature of these measurements is that two distinct <br />peaks in CCN concentration are apparent (Figure 12). Brock et al. (2003) noted a similar structure in their <br />measurements, and attributed the left most peak to the emissions from a coal-fired power plant. <br />The DMA/TDMA system made measurements of the aerosol size distribution from 0.010m to 0.500 m. <br />These measurements indicate a very dramatic increase in aerosol concentration that coincides with the <br />increase in CCN concentration. The effect of Houston on aerosol number concentration and size <br />distribution can be clearly seen along the streamline intersecting points 3 and 7 in Figure 12. The <br />dramatic increase in number concentration between measurements at point 3, upwind of Houston, and <br />point 7, downwind of Houston, indicates that large numbers of small particles were added to the air mass <br />as it passed over the Houston area. The change in volume concentration between these points is much <br />less pronounced. This suggests that Houston emissions may have a significant impact on CCN <br />concentrations even when mass concentration measurements, which are more sensitive to larger <br />particles, are largely unaffected. <br />st <br />Similar observations were made on the 1 of October 2004 under similar surface wind conditions. Weak <br />southerly winds prevailed along the Gulf Coast but a strong onshore flow prevailed southwest of Houston. <br />A significant cumulus field developed south and west of Houston after 1700 UTC. To the east, no <br />significant cumulus field was present. In addition, the Lake Charles 1200 UTC and 0000 UTC soundings <br />were very stable with positive CAPE values of 1 J/kg and 82 J/kg respectively. The CIN values were 313 <br />J/kg and 433 J/kg for 1200 UTC and 0000 UTC respectively. The CCL height was 2653 m AGL and the <br />CCL temperature was 12.9C at Lake Charles for the 0000 UTC sounding. Trajectories indicated that <br />surface parcels in the Houston area originated over the central Gulf of Mexico. <br />CCN measurements were conducted north and south of the city of Houston to try and replicate <br />rd <br />measurements conducted on the 23 of August 2004. These results were replicated when it was <br />observed that CCN measurements (at a supersaturation of 0.5%) upwind of Houston were measured at <br />-3-3 <br />1500 cmwhile measurements downwind of the city peaked at 3500 cm. These measurements are <br />summarized in the plot in Figure 14. <br />37 <br />