Spatial variation of chemical composition and sources of submicron aerosol in Zurich during wintertime using mobile aerosol mass spectrometer data
Abstract. Mobile measurements of PM1 (particulate matter with an aerodynamic diameter <1 μm) chemical composition using a quadrupole aerosol mass spectrometer and a multi-angle absorption photometer were performed using the PSI mobile laboratory during winter 2007/2008 and December 2008 in the metropolitan area of Zurich, Switzerland. Positive matrix factorization (PMF) applied to the organic fraction of PM1 yielded 3 factors: Hydrocarbon-like organic aerosol (HOA) related to traffic emissions; organic aerosol from wood burning for domestic heating purposes (WBOA); and oxygenated organic aerosol (OOA), assigned to secondary organic aerosol formed by oxidation of volatile precursors. The chemical composition of PM1 was assessed for an urban background site and various sites throughout the city. The background site is dominated by secondary inorganic and organic species (57 %), BC, HOA, and WBOA account for 15 %, 6 %, and 12 %, respectively. As for the other sites, HOA is important along major roads (varying between 7 and 14 % of PM1 for different sites within the city, average all sites 8 %), domestic wood burning makes up between 8–15 % of PM1 for different sites within the city (average all sites 10.5 %). OOA makes up the largest fraction of organic aerosol (44 % on average). A new method allows for the separation and quantification of the local fraction of PM1 emitted or rapidly formed in the city, and the fraction of PM1 originating from the urban background. The method is based on simultaneous on-road mobile and stationary background measurements and the correction of small-scale meteorological effects using the ratio of on-road sulfate to stationary sulfate. Especially during thermal inversions over the Swiss plateau, urban background concentrations contribute substantially to particulate number concentrations (between 40 and 80 % depending on meteorological conditions and emissions, 60 % on average) as well as to the mass concentrations of PM1 components measured on road in downtown Zurich (between 30 and 90 %, on average 60 % for black carbon and HOA, and between 90 and 100 % for WBOA, OOA, and the measured inorganic components). The results emphasize, on a scientific level, the advantage of mobile measurements for distinguishing local from regional air pollution research, and on a political level, the importance of regional collaboration for mitigating air pollution issues.