Product gas obtained through biomass gasification can be upgraded to hydrogen-rich synthesis gas. The synthesis gas can be further converted to liquid or gaseous fuels. However, the raw product gas contains both gas- and particle-phase impurities that can negatively affect both catalysts and hot-gas filters used for upgrading and cleaning. The present study aimed to characterize, both physically and chemically, aerosol particles formed during the steam- and oxygen-blown biomass gasification of wood pellets in an atmospheric 20 kW bubbling fluidized bed (BFB) gasifier. The product gas from the gasifier was sampled upstream from the cyclone at 500 degrees C. The particle number size distribution determined using a scanning mobility particle sizer (SNIPS) was bimodal, with modes at 20-30 and 400 nm, mobility equivalent diameters (d(B)). The total mean number concentration of particles with dB = 15-670 nm was approximately 7 x 105 particles/cm(3); however, the concentration of particles with dB < 80 nm fluctuated. The particle mass size distribution determined using a low-pressure impactor (LPI) was bimodal, and the total mass concentration of particles with aerodynamic diameters (d(ae)) < 5 mu m was 310 mg/m(3). Microscopy analysis of particulate matter on the lower LPI stages, expected to sample particles with d(ae) < 0.4 mu m, revealed structures approximately 10 mu m in diameter. In addition, the mass concentration of particles with d(ae) < 0.5 mu m determined using a LPI was higher than that estimated using a SNIPS, possibly because of the bounce-off or re-entrainment of coarser particles from higher LPI stages. Elementary analysis of the particulate matter indicated that it was dominated by carbon. The collected particulate matter was stable when heated in nitrogen to 500 degrees C, indicating that the carbon was not present as volatile tars but more likely as char or soot. The particulate matter collected on all LPI stages contained a small percentage of ash (noncarbonaceous inorganic material), with calcium as the dominant element.