The detailed evolution of primary and agglomerate particle growth during flame spray synthesis of 30 g/h of zirconia nanoparticles is monitored online by a differential mobility analyzer (DMA), an aerosol particle mass (APM) analyzer and a condensation particle counter (CPC) as well as thermophoretic sampling/microscopy. The differences between measured projected-area equivalent diameter, diameter of gyration and mobility diameter are elucidated. Particles were sampled from distinct positions in the flame at up to 1700 K and 1018 particles/m3 using a probe allowing continuous aerosol extraction and rapid dilution with quench air that effectively suppressed coagulation. Primary particle growth was complete at 10 cm above the burner at about 1500 K as the primary particle size did not change radially and axially downstream. As expected, the average agglomerate size increased with axial distance from the burner. Larger agglomerates, however, were observed at the edges of the aerosol plume attributed to prolonged residence time due to lower gas velocities there. Results were compared to off-line particle size characterization by nitrogen adsorption and thermophoretic sampling/transmission electron microscopy. (C) 2014 Elsevier Ltd. All rights reserved.