Evidence of convection as a major source of condensation nuclei in the northern midlatitude upper troposphere

We examine concurrent measurements of CN (size > 8 nm), NO, and NO y in the upper troposphere over the North Atlantic during the SONEX Experiment (Oct.–Nov., 1997). High CN and NO y , observations are attributed largely to the enhancement in convective outflow. Using the ratio of NO/NO y as a chemical clock, we estimate that dilution of convective high‐CN plumes is rapid (on a time scale of < 2 days) and accounts for a large fraction of elevated CN concentrations above the background. We estimate that less than 7% of observed high‐CN (> 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.