Combustion aerosol particles from boreal forest fires were quantified to facilitate investigation of the potential effects of increased fire activity caused by global warming, by providing data inputs for global and regional climate modelling of the direct and indirect effects. Aerial sampling was carried out in smoke plumes from 1-ha prescribed burns in mature jack pine stands. The three sampled burns resulted in crown fires, with fuel consumption from 4.2 to 5.8 (kg)(m-2). Accumulation and coarse mode aerosol was quantified using a passive cavity aerosol spectrometer probe and cascade impactor. The number median diameter of particles in the smoke plume was 0.29 um, and the peak number and cross-sectional area density occurred around a particle size of 0.4 um. More than 99% of particles sized had diameters <1.2 um. Aerosol from flaming combustion was coarser than that from the smouldering phase, with number median diameters of 0.3 and 0.2 um, respectively.