A one-year record of carbonaceous components and major ions in aerosols from an urban kerbside location in Oporto, Portugal

Abstract

PM2.5 aerosol samples were collected from January 2013 to January 2014 on the kerbside of a major arterial route in the city of Oporto, Portugal, and later analyzed for carbonaceous fractions and water soluble ions. The average concentrations of organic carbon (OC), elemental carbon (EC) and water soluble organic carbon (WSOC) in the aerosol were 6.2μg/m3, 5.0μg/m3 and 3.8μg/m3, respectively, and fit within the range of values that have been observed close to major roads in Europe, Asia and North America. On average, carbonaceous matter accounted for 56% of the gravimetrically measured PM2.5 mass. The three carbon fractions exhibited a similar seasonal variation, with high concentrations in late autumn and in winter, and low concentrations in spring. SO42− was the dominant water soluble ion, followed by NO3−, NH4+, Cl−, Na+, K+, oxalate, Ca2+, Mg2+, formate, methanesulfonate and acetate. Some of these ions exhibited a clear seasonal trend during the study period. The average OC/EC ratio for the entire set of samples was 1.28±0.61, which was consistent with a significant influence of vehicle exhaust emissions on aerosol composition. On the other hand, the average WSOC/OC ratio was 0.67±0.23, reflecting the influence of other emitting sources. WSOC was highly correlated with nssK+, a tracer of biomass combustion, and was not correlated with nssSO42−, a species associated with secondary processes, suggesting that the main source of WSOC was biomass burning. Most of the SO42− was anthropogenic in origin and was closely associated with NH4+, pointing to the formation of secondary aerosols. Na+, Cl− and methanesulfonate were clearly associated with marine sources while NO3− was related with combustion of both fossil and non-fossil fuels. Mixed sources explained the occurrence of the other water soluble ions.