Fine and ultrafine particles in the Zürich (Switzerland) area

On occasion of the project YOGAM (year of gas phase and aerosol measurements), the spatial and temporal variation of selected aerosol and gas phase parameters was assessed for the Z ¨ urich (Switzerland) area with a new mobile pollutant measurement laboratory. This assessment based on on-road measurements along a specified route 5 on selected days during different seasons in 2001/2002, covering urban, suburban and rural regions. Special focus was put on the investigation and characterization of particles in the fine (< 2.5 µm) and ultrafine (< 100 nm) size ranges. Analysis of Variance (ANOVA) showed that the variance of all considered fine and ultrafine aerosol parameters (i.e. particle background and total number concentration for particles larger than 10 3 nm, number concentrations in the size ranges 7–30 nm and 80–140 nm, as well as the active surface area concentration) was considerably larger for day-today than for spatial variation. However, Principal Component Analysis (PCA) found a similar regional pollution pattern within every single measuring day. Lowest particle background levels were found in rural areas at higher elevation (15 000 cm −3), while corresponding mean 15 background values for urban and freeway-influenced areas were typically 35 000 cm −3 and > 80 000 cm −3 , respectively. Meteorology, i.e. prevailing weather conditions not only governed the day-today concentration variations in the selected area, but also influenced the formation of both primary (directly traffic-related) and secondary (biogenic or indirectly traffic-related) ultrafine particles. Overall, low temperatures regularly en-20 hanced primary ultrafine particle formation in urban areas. There was however indication for relatively low number concentrations of secondary ultrafine particles during only a few warm and sunny spring days. Mobile measurements as described in this study have been shown to be suitable for long-term pollutant assessments, to obtain good information on spatial variability and reasonable information on the temporal variabil-25 ity. For aspects concerning spatial resolution, an experimental design using a mobile measurement may even be more appropriate than a network of stationary measuring sites.