Condensation nuclei measurements are examined in conjunction with measurements of reactive nitrogen species (NOy) to identify aircraft plumes in primary air traffic corridors over the North Atlantic. Several hundred plumes exhibiting ≥100 pptv enhancements in NOy mixing ratio were observed. The plumes were typically a few hundred meters wide, exhibited high NO/NOy ratios, and ranged in age from ~10 minutes to ~10 hours. Assuming the sampled aircraft emitted ~12 g NOx (as NO2) kg-1 fuel burned and that the loss of NOy to the particle phase was negligible, we calculate median aerosol emission indices in terms of number of particles kg-1 of fuel burned of ~120¿1015 for CN≥8 nm in size; ~50¿1015 for CN≥17 nm; and ~3¿1015 for the nonvolatile CN≥17 nm. Using published fuel burn statistics, background aerosol concentrations, and a 10 day particle lifetime, we conclude that present aviation sources enhance global averaged upper-tropospheric fine and nonvolatile aerosol number densities by ~6% and ~3%, respectively. ¿ 1999 American Geophysical Union