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The analysis of aerosol size distributions is a useful tool forunderstanding the sources and the processes influencing particle numberconcentrations (N) in urban areas. Hence, during the one-month SAPUSScampaign (Solving Aerosol Problems by Using Synergistic Strategies, EU MarieCurie Action) in autumn 2010 in Barcelona (Spain), four SMPSs (ScanningMobility Particle Sizer) were simultaneously deployed at four monitoringsites: a road side (RSsite), an urban background site located in thecity (UBsite), an urban background site located in the nearby hills of thecity (Torre Collserola, TCsite) and a regional background site locatedabout 50 km from the Barcelona urban areas (RBsite). The spatialdistribution of sites allows study of the aerosol temporal variability aswell as the spatial distribution, progressively moving away from urbanaerosol sources. In order to interpret the data sets collected, a k-meanscluster analysis was performed on the combined SMPS data sets. This resultedin nine clusters describing all aerosol size distributions from the foursites. In summary there were three main categories (with three clusters ineach category): "Traffic" (Traffic 1, "Tclus_1" – 8%;Traffic 2, "Tclus_2" – 13%; and Traffic 3,"Tclus_3" – 9%) "Background Pollution" (UrbanBackground 1, "UBclus_1" – 21%; Regional Background 1,"RBclus_1" – 15%; and Regional Background 2,"RBclus_2" – 18%) and "Special Cases" (Nucleation,"NUclus" – 5%; Regional Nitrate, "NITclus" – 6%; andMix, "MIXclus" – 5%). As expected, the frequency of traffic clusters(Tclus_1–3) followed the order RSsite, UBsite,TCsite, and RBsite. These showed typical traffic modes mainlydistributed at 20–40 nm. The urban background sites (UBsite andTCsite) reflected also as expected urban background numberconcentrations (average values, N = 1.0 × 104 cm−3and N = 5.5 × 103 cm−3, respectively, relative to1.3 × 104 cm−3 seen at RSsite). The clusterdescribing the urban background pollution (UBclus_1)could be used to monitor the sea breeze circulation towards the regionalbackground study area. Overall, the RBsite was mainly characterised bytwo different regional background aerosol size distributions: whilst bothexhibited low N (2.7 × 103 for RBclus_1and 2.2 × 103 cm−3 for RBclus_2),RBclus_1 had average PM10 concentrations higherthan RBclus_2 (27 vs. 23 μg m−3). As regardsthe minor aerosol size distribution clusters, the "Nucleation" cluster wasobserved during daytime, whilst the "Regional Nitrate" was mainly seen atnight. The ninth cluster ("Mix") was the least well defined and likelycomposed of a number of aerosol sources. When correlating averaged values of N, NO2 and PM (particulate mass)for each k-means cluster, a linear correlation between N and NO2 withvalues progressively increasing from the regional site RBsite to theroad site RSsite was found. This points to vehicular traffic as themain source of both N and NO2. By contrast, such an association doesnot exist for the case of the nucleation cluster, where the highest N isfound with low NO2 and PM. Finally, the clustering technique allowed study of the impact ofmeteorological parameters on the traffic N emissions. This study confirmsthe shrinking of freshly emitted particles (by about 20% within 1 km inless than 10 min; Dall'Osto et al., 2011a) as particlesare transported from the traffic hot spots towards urban backgroundenvironments. Additionally, for a given well-defined aerosol sizedistribution (Tclus_2) associated with primary aerosolemissions from road traffic we found that N5–15 nm concentrations canvary up to a factor of eight. Within our measurement range of SMPSs (N15–228 nm) and CondensationParticle Counters (CPCs, N>5 nm), we found that ultrafine particleswithin the range 5–15 nm in urban areas are the most dynamic, being acomplex ensemble of primary evaporating traffic particles, traffic tailpipenew particle formation and non-traffic new particle formation

Simplifying aerosol size distributions modes simultaneously detected at four monitoring sites during SAPUSS