In Portugal, during summer 2003, unusually large forested areas (>300,000 ha) were destroyed by fire, emitting pollutants to the atmosphere. During this period, aerosol samples were collected in the Aveiro region, and analysed for total mass and a set of inorganic and organic compounds, including tracers of biomass burning. Comparisons of aerosol size distributions, levels of particulate mass and chemical aerosol composition between heavily smoke-impacted periods and the rest of the summer permit to evaluate the contribution of forest fires to the regional aerosol load. The absolute and relative variability of the particulate inorganic and organic constituents were used to evaluate the importance of wildfires as emission sources responsible for the presence of compounds such as molecular tracers in the summer atmosphere. From organic carbon to levoglucosan or to potassium ratios it was estimated that 40–55% of primary organic carbon could be attributed to wood smoke. The large fraction of secondary organic carbon suggested that forest fires may strongly contribute to gas-to-particle processes. It was found a better correlation of organic carbon with potassium than with levoglucosan, indicating that, during more complete combustive processes, potassium is possibly a more reasonable biomass burning tracer.