© 2021 Elsevier LtdThis work investigated the particulate matter formation during co-combustion of thermally pre-treated biomass with coal. Olive residue was chosen as agricultural waste biomass, and Tunçbilek lignite as the coal. The biomass was thermally pretreated to assess the influence of the pretreatment temperature on particulate matter formation during co-combustion. Specifically, the olive residue was torrefied (at 275 °C) and pyrolyzed (at 500 °C) using a tubular oven. The biomass-coal blends in a 50:50 wt% ratio were co-fired in a drop tube furnace operated at 1200 °C, heating rate of ∼ 104 °C/s, residence time of ∼ 3 s, and dry air atmospheric conditions. The particulate matter was collected at the bottom of the reactor using a three-stage stack impactor which allowed to quantify the relevant levels of PM2.5, PM2.5-10, and PM10. The results showed that co-combustion resulted in clear reduction of PM2.5 emission to values close to those of the biomass fuel. Specifically, combustion of blends of Tunçbilek lignite with olive residue, torrefied olive residue, and olive residue char resulted in 646, 408, and 559 mg/MJ input, respectively. Moreover, the mechanisms responsible for the formation of PM2.5 during biomass and coal combustion were found applicable to biomass-coal blends. The shift from fine to coarser particles with co-firing is likely to allow the capture of PM from biomass-coal co-firing by conventional coal-PM traps in existing coal power plants.