Speciation of organic compounds in aerosols from urban background sites in the winter season

• Winter PM2.5 samples were collected at two urban background sites (Oporto and Coimbra).
• More than 200 organic compounds were quantified in PM2.5 from both cities.
• The daily benzo[a]pyrene equivalent concentrations often exceeded the WHO threshold.
• Higher levels of exhaust emission markers were observed in Oporto.
• In colder days, biomass burning can contribute up to 74% of the organic carbon mass.

Winter aerosol samples were daily collected during one-month long campaign in Oporto and Coimbra. The high-volume PM2.5 samples were solvent extracted and their organic content separated into several functional groups, which were then analysed by gas chromatography–mass spectrometry. The organic compounds identified and quantified revealed some differences between samples from the two urban areas. In general, the levels of total hydrocarbons in the urban background station of Oporto were higher than those of Coimbra. Concentration ratios between specific compounds and the presence of molecular markers derived from petroleum, such as hopanes, pristane and phytane, point out vehicles as the main source of pollutants. The contribution of biogenic compounds, mainly hydrocarbons associated with the waxy cuticle of vegetation, is also observable in both cities. The benzo[a]pyrene equivalent daily values were frequently higher than 1 ng m− 3 in Oporto suggesting an additional cancer risk for the population. The PM2.5 mass attributable to vehicle emissions is higher in the background atmosphere of Oporto than in Coimbra. On weekends, biomass burning emissions could represent up to 74% of the organic carbon content of the urban aerosols.