The formation of combustion aerosols was studied in an 800 MWth suspension-fired power plant boiler, during combustion of pulverized wood pellets with and without addition of coal fly ash as alkali capture additive. The aerosol particles were sampled and characterized by a low-pressure cascade impactor (LPI), and elemental composition and particle morphology was studied by electron microscopy methods (SEM/EDS and TEM/EDS). During pulverized wood combustion, the mass-load of submicrometer particles (PM1) was in the range 44-47 mg/Nm(3), and the mass-based particle size distribution revealed a distinct submicrometer peak located around 0.2 mu m. This peak consisted mainly of aggregated ultrafine (<100 nm in size) primary particles rich in K, Cl, and S, probably formed by homogeneous and heterogeneous condensation of potassium chloride and sulfate from the gas phase. When coal fly ash was added as additive, in concentrations corresponding to a coal ash to wood ash mass-ratio of 2 or 4, respectively, the submicrometer peak was significantly smaller than for the experiments without coal fly ash addition, and the mass-load of PM, particles was reduced to 11-19 mg/Nm(3). The ultrafine, primary particles in this case appeared smaller (<60 nm in size) and more spherical, and the composition was changed from K-Cl-S rich toward Ca-P-Si rich, implying that they originated predominantly from the entrained coal fly ash. In condusion, coal fly ash proved effective in reducing the formation of combustion aerosols during pulverized wood combustion. The results imply a potential for optimizing/minimizing the coal fly ash addition at the present power plant. 2014