The distribution of daytime and nighttime sources of PM10 collected from January to March 2021 at an urban background site in the city of Braganca, Portugal, was performed using positive matrix factorisation (PMF). Additional data of PM2.5, NOx and meteorological variables were collected to support the interpretations. A solution with 5 factor profiles was found: traffic (33%), dust (24%), biomass burning (21%), secondary inorganic aerosol (SIA) (12%) and sea salt (10%). Mean daytime and nighttime PM10 concentrations were 43.1 mu g m(-3) and 46.1 mu g m(-3), respectively. Nighttime concentrations were dominated by residential biomass combustion. Vehicle traffic and dust factors showed significantly greater contributions during the day (+12% and +4%, respectively), suggesting that exhaust and non-exhaust emissions and long-range transport are important contributors to daytime PM10 levels. In contrast, there were no significant differences between day and night for SIA and sea salt. Exceedances of the daily limit to PM10 (50 mu g m(-3)) and PM2.5 (15 mu g m(-3)) were observed in 22 (33%) and 27 (40%) days of the campaign, respectively, mostly associated with biomass burning for residential heating, but also with Saharan dust outbreaks. The application of the Aliivibrio fischeri bioluminescence inhibition assay for ecotoxicity assessment allowed classifying 70% of the samples as toxic, especially those from the nocturnal period, indicating that biomass burning is one of the main sources responsible for PM10 toxicity. Both the contributions from biomass burning estimated by the PMF and multiple tracers of this source showed statistically significant correlations with the toxicity units.
