The characteristics of aerosol particles formed during combustion of moist forest residue were studied as a function of load in a moving grate boiler and at almost full load in a similar larger boiler. The coarse () particle number and mass concentration increased by more than one order of magnitude and the fine particle mean diameter, total volume and mass decreased when the load was increased from 50% to 75%. The combined effects led to an increase in PM10 with increasing load. The number and mass concentration and the size distribution were similar in the two boilers when operated at high load. The dominant elements (Z>12) were K, S and Cl in the fine mode and Ca, K and S in the coarse mode. The dominant ions in the fine mode were K+, SO42− and CO32−. The fine mode particles had hygroscopic growth factors of around 1.65 at RH=90%, with a deliquescence point at a relative humidity between 30% and 60%. It was assessed that K2CO3 is responsible for the low deliquescence point. Fine mode particles of a given dry diameter had similar chemical composition