The objectives of the present work were to investigate the aerosol formation mechanisms during full-scale suspension firing of wood, and, to evaluate the effect of coal fly ash addition on the formation of aerosols under different ash load conditions. Tests with suspension firing of 100 % wood pellets, with and without injection of coal fly ash as additive, were carried out at the 800 MWth multifuel boiler at Avedøre Power Plant. An extractive sampling system consisting of a gas ejector-diluter connected to a 10-stage Berner type low pressure cascade impactor (aerodynamic diameter range of 0.03 – 12.7 μm) was used to sample aerosols in the flue gas, in the top of the boiler before the SCR (Tfluegas ~350 oC). The collected aerosols were subsequently characterized with respect to particle size distribution, morphology, and chemical composition. The mass-based size distribution of the aerosols revealed that the formation of submicron particles was increased significantly when no coal ash was injected, as compared to the reference experiments with addition of coal fly ash. PM1 for the experiments without coal ash addition was in the range 44 – 47 mg/Nm3, while it was only 11 – 19 mg/Nm3 for the experiments with coal fly ash addition. This indicates that the coal fly ash is effective in capturing volatile alkalis released from the wood during combustion, thus suppressing the homogeneous nucleation of alkali-salts. SEM/EDS and TEM/EDS analysis revealed that the large condensation peak from pure wood combustion (without coal ash addition) consisted primarily of irregularly shaped aggregates rich in K, Cl and S (probably KCl and K2SO4). The addition of coal fly ash mainly affected the submicron aerosols in two ways: the relative amount of spherical particles originated from molten minerals was increased; and the composition of the aggregates was changed from K-Cl-S rich to Ca-PSi rich. In conclusion, the results confirm that coal fly ash is effective in capturing gaseous K, presumably by incorporating gaseous K into solid potassium-aluminumsilicates, thereby reducing significantly the amount of submicron aerosols formed from homogeneous nucleation, coagulation and condensation of KCl and K2SO4. A potential for optimizing the coal fly ash concentration exists.