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. This study investigated the sources of black carbon (BC) at twocontrasting urban environments in Helsinki, Finland: residential area andstreet canyon. The measurement campaign in the residential area wasconducted in winter–spring 2019, whereas in the street canyon themeasurements were carried out in autumn 2015. The sources of BC wereexplored by using positive matrix factorization (PMF) for the organic andrefractory black carbon (rBC) mass spectra collected with a soot particleaerosol mass spectrometer (SP-AMS). Based on the PMF analysis, two sites had different local BC sources; the largest fraction of BC originated frombiomass burning at the residential site (38 %) and from the vehicularemissions in the street canyon (57 %). Also, the mass size distributionof BC diverged at the sites as BC from traffic was found at the particlesize of ∼100–150 nm whereas BC from biomass combustion wasdetected at ∼300 nm. At both sites, a large fraction of BCwas associated with urban background or long-range-transported BC indicatedby the high oxidation state of organics related to those PMF factors. The results from the PMF analysis were compared with the sourceapportionment from the Aethalometer model calculated with two pairs ofabsorption Ångström values. It was found that several PMF factorscan be attributed to wood combustion and fossil fuel fraction of BC provided by the Aethalometer model. In general, the Aethalometer model showed less variation between the sources within a day than PMF, indicating that it was less responsive to the fast changes in the BC sources at the site, or it could not distinguish between as many sources as PMF due to the similar optical properties of the BC sources. The results of this study increase understanding of the limitations and validity of the BC source apportionment methods in different environments. Moreover, this study advances the current knowledge of BC sources and especially the contribution of residential combustion in urban areas.

Sources of black carbon at residential and traffic environments obtained by two source apportionment methods