Abstract. This study investigated the sources of black carbon (BC) at two
contrasting urban environments in Helsinki, Finland: residential area and
street canyon. The measurement campaign in the residential area was
conducted in winter–spring 2019, whereas in the street canyon the
measurements were carried out in autumn 2015. The sources of BC were
explored by using positive matrix factorization (PMF) for the organic and
refractory black carbon (rBC) mass spectra collected with a soot particle
aerosol mass spectrometer (SP-AMS). Based on the PMF analysis, two sites had different local BC sources; the largest fraction of BC originated from
biomass burning at the residential site (38 %) and from the vehicular
emissions in the street canyon (57 %). Also, the mass size distribution
of BC diverged at the sites as BC from traffic was found at the particle
size of ∼100–150 nm whereas BC from biomass combustion was
detected at ∼300 nm. At both sites, a large fraction of BC
was associated with urban background or long-range-transported BC indicated
by the high oxidation state of organics related to those PMF factors. The results from the PMF analysis were compared with the source
apportionment from the Aethalometer model calculated with two pairs of
absorption Ångström values. It was found that several PMF factors
can 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.