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

Abstract

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 < 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.