The goal of this research is to determine trends and sources of airborne particulates in the centre of Lisbon, by using speciated
particulate-matter data and back-trajectory analyses. Results showed that, in 2007, the annual PM2.5 concentration exceeded the World Health Organization recommended levels. PM2.5 diurnal variability and the ratio between weekdays’ and weekends’ concentrations indicated that traffic contributed highly
to decreasing air quality. Air back-trajectory analysis showed that maritime air mass transport had a significant role on
air quality in Lisbon, promoting the decrease of anthropogenic aerosol concentrations.