An instrument based on Plasma Excited Atomic Resonance Line Spectroscopy (PEARLS) was applied to measurements of vapor and aerosol phase Na and K in atmospheric combustion conditions. The PEARLS method is based on dissociation of alkali compounds by a direct current plasma torch and subsequent determination of alkali concentration by atomic absorption spectroscopy. The measurements were carried out in a 20 kW bubbling fluidized bed reactor during co-combustion of biomass and coal. The capability of the instrument to operate under atmospheric high-ash conditions was demonstrated. The analysis was made directly from hot (880 degreesC) particulate laden sample gas. The fuels were pine bark mixed with South African coal and Finnish forest residue mixed with Polish lignite. Thirty percent addition of coal or lignite to biomass reduced measured alkali concentrations by 44-64%. The reduction is explained by conversion of alkali chlorides and hydroxides to sulfates and silicates. (C) 2004 Elsevier Ltd. All rights reserved.