We examine concurrent measurements of CN (size >8 nm), NO, and NOy in the upper troposphere over the North Atlantic during the SONEX Experiment (Oct.--Nov., 1997). High CN and NOy observations are attributed largely to the enhancement in convective outflow. Using the ratio of NO/NOy as a chemical clock, we estimate that dilution of convective high-CN plumes is rapid (on a time scale of 10000 cm-3) plumes may be attributed to aircraft emissions. The contribution by aircraft emissions to upper tropospheric CN concentrations is estimated to be significantly higher than 7% because aircraft plumes dilute much faster than convective plumes and hence are sampled less frequently. ¿ 2000 American Geophysical Union