Particles in the atmosphere are often divided into two categories, depending on whether they are directly emitted into the atmosphere or formed there by condensation. Primary organic aerosol (POA) particles are generally understood to be those that are released directly from various sources. Secondary organic aerosol (SOA) is formed in the atmosphere by condensation of low vapour pressure products from the oxidation of organic gases. The quantification of different types of aerosols such as SOA and POA (or more classes if possible) is important as source identification is the first step in all mitigation activities. Furthermore, SOA and POA may be associated with different sizes, chemical composition and physical properties and thus may have different effects on climate or health.

Submicron ambient aerosol (PM1) was characterized in summer 2005 and in winter 2006 at the NABEL site Zurich-Kaserne (urban background) during three-week measurement campaigns. Highly time-resolved samples of non-refractory aerosol components were analyzed with an Aerodyne aerosol mass spectrometer (AMS).

For the summer campaign, positive matrix factorization (PMF) was used for the first time for AMS spectra to identify the main components of the total organic aerosol and their sources (Lanz et al., 2007a). The PMF retrieved factors were compared to measured reference mass spectra and were correlated with tracer species of the aerosol and gas phase measurements from collocated instruments of the NABEL site. Our analysis suggests that at the measurement site only a small (<10%) fraction of organic PM1 originates from freshly emitted fossil fuel combustion. Other primary sources are of similar or even higher importance are charbroiling (10–15%) and wood burning (~10%). A high fraction (60–69%) of the measured organic aerosol mass is OOA (oxygenated organic aerosol) which is interpreted mostly as SOA. This oxygenated organic aerosol can be separated into a highly aged fraction, OOA I, (40–50%) with low volatility and a mass spectrum similar to fulvic acid (model substance for highly aged aerosol), and a more volatile and probably less processed fraction, OOA II (on average 20%) (Fig. 1a).

A novel hybrid receptor model that incorporates a priori known source composition was needed to resolve aerosol sources in wintertime (Lanz et al., 2007b). The major component was again OOA accounting on average for 52–57% of the particulate organic mass (Fig. 1b). OOA estimates were strongly correlated with measured particulate ammonium. Particles from wood combustion (35–40%) and 3–13% traffic-related hydrocarbon-like organic aerosol (HOA) fractions accounted for the other half of measured organic matter (OM). Emission ratios of modelled HOA to measured nitrogen oxides (NOx) and OM from wood burning to levoglucosan (organic wood burning tracer) from filter analyses were found to be consistent with literature values. Radiocarbon (14C) measurements of organic carbon (OC) indicated that ~31% and ~69% of OOA originated from fossil (via the oxidation of VOCs from traffic and oil heating) and non-fossil sources (VOCs from wood burning and vegetation), respectively.

_{V. A. Lanz, M. R. Alfarra, U. Baltensperger, B. Buchmann, C. Hueglin, and A. S. H. Prévôt: Source apportionment of submicron organic aerosols at an urban site by factor analytical modelling of aerosol mass spectra, Atmos. Chem. Phys., 7, 1503-1522, 2007a. www.atmos-chem-phys.net/7/1503/2007/}

_{V. A. Lanz, M. R. Alfarra, U. Baltensperger, B. Buchmann, C. Hueglin, S. Szidat, M. N. Wehrli, L. Wacker, S. Weimer, and A. S. H. Prévôt: Source apportionment of submicron organic aerosols during wintertime inversions by advanced factor analysis of aerosol mass spectra, Environ. Sci. Technol., in press, 2007b.}

Further information
Please direct questions and/or comments about this project:

Valentin Lanz
Tel. ++41 44 823 40 46

Christoph Hüglin
Tel. ++41 44 823 46 54