Characteristic parameters of black carbon aerosol (BC) emitted from jet engine were measured during ground tests and in-flight behind the same aircraft. Size distribution features were a primary BC mode at a modal diameter D approximate to 0.045 mu m, and a BC agglomeration mode at D < 0.2 mu m. The total BC number concentration at the engine exit was 2.9 x 10(7) cm(-3) with good agreement between model results and in-flight measured number concentrations of non volatile particles with D greater than or equal to 0.014 mu m. A comparison between total number concentration of BC particles and the non-volatile fraction of the total aerosol at the exit plane suggests that the non-volatile fraction of jet engine exhaust aerosol consists almost completely of BC. In-flight BC mass emission indices ranged from 0.11 to 0.15 g BC (kg fuel)(-1). The measured in-flight particle emission value was 1.75 +/- 0.15 x 10(15) kg(-1) with corresponding ground test values of 1.0-8.7 x 10(14) kg (-1). Both size distribution properties and mass emission indices can be scaled from ground test to in-flight conditions. Implications for atmospheric BC loading, BC and cirrus interaction and the potential of BC for perturbation of atmospheric chemistry are briefly outlined.