Quantification of the contribution of biomass burning aerosols to PM levels on an annual basis was possible by means of the Multilinear Engine (ME). Biomass burning emissions accounted for 3%of PM10 and PM2.5 (annual mean), while this percentage increased up to 5%of PM1. During the winter period, regional-scale biomass burning emissions (agricultural waste burning) were estimated to contribute with 7 卤 4%of PM2.5 aerosols during night-time (period when emissions were clearly detected). Long-range transported biomass burning aerosols (possibly from forest fires and/or agricultural waste burning) accounted for 5 卤 2%of PM2.5 during specific episodes. Annually, biomass burning emissions accounted for 19%-21%of OC levels in PM10, PM2.5 and PM1. The contribution of this source to K+ ranged between 48%for PM10 and 97%for PM1 (annual mean). Results for K+ from biomass burning evidenced that this tracer is mostly emitted in the fine fraction, and thus coarse K+ could not be taken as an appropriate tracer of biomass burning.