On the spatial distribution and evolution of ultrafine aerosols in urban air

Abstract. Sources and evolution of ultrafine particles (<0.1 μ m diameter) were investigated both horizontally and vertically in the large urban agglomerate of Barcelona, Spain. Within the SAPUSS project (Solving Aerosol Problems by Using Synergistic Strategies), a large number of instruments was deployed simultaneously at different monitoring sites (road, two urban background, regional background, urban tower 150 μa.s.l., urban background tower site 80 m a.s.l.) during a 4 week period in September-October 2010. Particle number concentrations (N>5nm) are highly correlated with black carbon (BC) at all sites only under strong vehicular traffic influences. By contrast, under clean atmospheric conditions (low condensation sinks, CS) such correlation diverges towards much higher N/BC ratios at all sites, indicating additional sources of particles including secondary production of freshly nucleated particles. This is also evident in the urban background annual mean diurnal trend of N/BC, showing a midday peak in all seasons. Size-resolved aerosol distributions (N10-500) as well as particle number concentrations (N>5nm) allow us to identify two types of nucleation and growth events: a regional type event originating in the whole study region and impacting almost simultaneously the urban city of Barcelona and the surrounding background area; and an urban type which originates only within the city centre but whose growth continues while transported away from the city to the regional background. Furthermore, during these clean air days, higher N are found at tower level than at ground level only in the city centre whereas such a difference is not so pronounced at the remote urban background tower. In other words, this study suggests that the column of air above the city ground level possesses the best compromise between low CS and high vapour source, hence enhancing the concentrations of freshly nucleated particles. By contrast, within stagnant polluted atmospheric conditions, higher N and BC concentrations are always measured at ground level relative to tower level at all sites. Our study suggests that the city centre is a source of both non-volatile traffic primary (29–39%) and secondary freshly nucleated particles (up to 61–71%) at all sites. We suggest that organic compounds evaporating from freshly emitted traffic particles are a possible candidate for new particle formation within the city and urban plume.

Download Full-text

On the spatial distribution and evolution of ultrafine particles in Barcelona

Atmospheric Chemistry and Physics ◽

10.5194/acp-13-741-2013 ◽

2013 ◽

Vol 13 (2) ◽

pp. 741-759 ◽

Cited By ~ 57

Author(s):

M. Dall'Osto ◽

X. Querol ◽

A. Alastuey ◽

C. O'Dowd ◽

R. M. Harrison ◽

...

Keyword(s):

Ultrafine Particles ◽

Particle Number ◽

Ground Level ◽

City Centre ◽

Atmospheric Conditions ◽

Urban Background ◽

Urban City ◽

Regional Background ◽

The City ◽

Background Area

Abstract. Sources and evolution of ultrafine particles were investigated both horizontally and vertically in the large urban agglomerate of Barcelona, Spain. Within the SAPUSS project (Solving Aerosol Problems by Using Synergistic Strategies), a large number of instruments was deployed simultaneously at different monitoring sites (road, two urban background, regional background, urban tower 150 m a.s.l., urban background tower site 80 m a.s.l.) during a 4 week period in September–October 2010. Particle number concentrations (N>5 nm) are highly correlated with black carbon (BC) at all sites only under strong vehicular traffic influences. By contrast, under cleaner atmospheric conditions (low condensation sink, CS) such correlation diverges towards much higher N/BC ratios at all sites, indicating additional sources of particles including secondary production of freshly nucleated particles. Size-resolved aerosol distributions (N10–500) as well as particle number concentrations (N>5 nm) allow us to identify three types of nucleation and growth events: (1) a regional type event originating in the whole study region and impacting almost simultaneously the urban city of Barcelona and the surrounding urban background area; (2) a regional type event impacting only the regional background area but not the urban agglomerate; (3) an urban type event which originates only within the city centre but whose growth continues while transported away from the city to the regional background. Furthermore, during these clean air days, higher N are found at tower level than at ground level only in the city centre whereas such a difference is not so pronounced at the remote urban background tower. In other words, this study suggests that the column of air above the city ground level possesses the optimal combination between low CS and high vapour source, hence enhancing the concentrations of freshly nucleated particles. By contrast, within stagnant polluted atmospheric conditions, higher N and BC concentrations are always measured at ground level relative to tower level at all sites. Our study suggests that the city centre of Barcelona is a source of non-volatile traffic primary particles (29–39% of N>5 nm), but other sources, including secondary freshly nucleated particles contribute up to 61–71% of particle number (N>5 nm) at all sites. We suggest that organic compounds evaporating from freshly emitted traffic particles are a possible candidate for new particle formation within the city and urban plume.

Download Full-text

Quantification of an atmospheric nucleation and growth process as a single source of aerosol particles in a city

Atmospheric Chemistry and Physics ◽

10.5194/acp-17-15007-2017 ◽

2017 ◽

Vol 17 (24) ◽

pp. 15007-15017 ◽

Cited By ~ 4

Author(s):

Imre Salma ◽

Veronika Varga ◽

Zoltán Németh

Keyword(s):

Growth Process ◽

Particle Number ◽

Urban Climate ◽

Particle Diameter ◽

City Centre ◽

Time Interval ◽

Single Source ◽

Mean Values ◽

Early Afternoon ◽

The City

Abstract. Effects of a new aerosol particle formation (NPF) and particle diameter growth process as a single source of atmospheric particle number concentrations were evaluated and quantified on the basis of experimental data sets obtained from particle number size distribution measurements in the city centre and near-city background of Budapest for 5 years. Nucleation strength factors for a nucleation day (NSFNUC) and for a general day (NSFGEN) were derived separately for seasons and full years. The former characteristic represents the concentration increment of ultrafine (UF) particles specifically on nucleation days with respect to accumulation-mode (regional background) concentrations (particles with equivalent diameters of 100–1000 nm; N100−1000) due solely to the nucleation process. The latter factor expresses the contribution of nucleation to particle numbers on general days; thus, it represents a longer time interval such as season or year. The nucleation source had the largest effect on the concentrations around noon and early afternoon, as expected. During this time interval, it became the major source of particles in the near-city background. Nucleation increased the daily mean concentrations on nucleation days by mean factors of 2.3 and 1.58 in the near-city background and city centre, respectively. Its effect was largest in winter, which was explained by the substantially lower N100−1000 levels on nucleation days than those on non-nucleation days. On an annual timescale, 37 % of the UF particles were generated by nucleation in the near-city background, while NPF produced 13 % of UF particles in the city centre. The differences among the annual mean values, and among the corresponding seasonal mean values, were likely caused by the variability in controlling factors from year to year. The values obtained represent the lower limits of the contributions. The shares determined imply that NPF is a non-negligible or substantial source of particles in near-city background environments and even in city centres, where the vehicular road emissions usually prevail. Atmospheric residence time of nucleation-mode particles was assessed by a decay curve analysis, and a mean of 02:30 was obtained. The present study suggests that the health-related consequences of the atmospheric NPF and growth process in cities should also be considered in addition to its urban climate implications.

Download Full-text

Modelling the dispersion of particle numbers in five European cities

Geoscientific Model Development ◽

10.5194/gmd-9-451-2016 ◽

2016 ◽

Vol 9 (2) ◽

pp. 451-478 ◽

Cited By ~ 20

Author(s):

J. Kukkonen ◽

M. Karl ◽

M. P. Keuken ◽

H. A. C. Denier van der Gon ◽

B. R. Denby ◽

...

Keyword(s):

Particle Number ◽

Atmospheric Dispersion ◽

Emission Inventories ◽

Vehicular Traffic ◽

Background Concentrations ◽

Reasonable Accuracy ◽

Annual Average ◽

Urban Background ◽

European Cities ◽

Regional Background

Abstract. We present an overview of the modelling of particle number concentrations (PNCs) in five major European cities, namely Helsinki, Oslo, London, Rotterdam, and Athens, in 2008. Novel emission inventories of particle numbers have been compiled both on urban and European scales. We used atmospheric dispersion modelling for PNCs in the five target cities and on a European scale, and evaluated the predicted results against available measured concentrations. In all the target cities, the concentrations of particle numbers (PNs) were mostly influenced by the emissions originating from local vehicular traffic. The influence of shipping and harbours was also significant for Helsinki, Oslo, Rotterdam, and Athens, but not for London. The influence of the aviation emissions in Athens was also notable. The regional background concentrations were clearly lower than the contributions originating from urban sources in Helsinki, Oslo, and Athens. The regional background was also lower than urban contributions in traffic environments in London, but higher or approximately equal to urban contributions in Rotterdam. It was numerically evaluated that the influence of coagulation and dry deposition on the predicted PNCs was substantial for the urban background in Oslo. The predicted and measured annual average PNCs in four cities agreed within approximately ≤  26 % (measured as fractional biases), except for one traffic station in London. This study indicates that it is feasible to model PNCs in major cities within a reasonable accuracy, although major challenges remain in the evaluation of both the emissions and atmospheric transformation of PNCs.

Download Full-text

Eddy covariance measurements of the net turbulent methane flux in the city centre – results of 2 years campaign in Łódź, Poland

10.5194/acp-2015-979 ◽

2016 ◽

Author(s):

W. Pawlak ◽

K. Fortuniak

Keyword(s):

Carbon Dioxide ◽

Eddy Covariance ◽

Vertical Component ◽

Ground Level ◽

Methane Flux ◽

City Centre ◽

Carbon Dioxide Fluxes ◽

Methane Fluxes ◽

Eddy Covariance Measurements ◽

The City

Abstract. In the period between July 2013 and August 2015, continuous measurements of turbulent methane exchange between an urbanised area and the atmosphere were carried out in Łódź. Such long, continuous measurement series of turbulent methane exchange between the city and the atmosphere are still a rarity. The measurement station was located in the centre of the city, where fluxes of energy (sensible and latent heat) and fluxes of mass (carbon dioxide) have been continuously measured since 2000 and 2007, respectively. In the immediate vicinity of the measurement station there are potential sources of methane, such as streets with vehicle traffic or dense sewerage and natural gas networks. To determine the fluxes, the eddy covariance technique was used; the measurement station was equipped with instruments for recording fluctuations in the vertical component of the wind speed (an ultrasonic 3D anemometer, RM Young 81000, RM Young, USA) as well as the concentration of methane in the air (an open path Li 7700 CH4 Analyser, Li-cor, USA). The devices were mounted on a mast at a height of 37 metres above ground level and, on average, 20 metres over the roofs of the surrounding buildings. The results were therefore averaged for an area with a diameter of approximately 1 kilometre. Our aim was to investigate the temporal variability of the turbulent exchange of methane in the city-atmosphere system. The results show in the first place that positive methane fluxes (turbulent gas transport from the surface to the atmosphere) definitely dominate compared with negative fluxes. This indicates that the study area of the centre of Łódź is a net source of methane to the troposphere. The measurements also indicated the existence of a clear annual rhythm of the turbulent flux of methane in the centre of Łódź (on average, the values observed in winter amounted to ~40–60 nmol m−2 s−1 and were significantly larger than in summer). The daily variability of the flux of CH4 (FCH4) is faintly visible throughout the year. The studied area of the centre of Łódź is also characterised by a cycle of methane exchange – the values measured on working days were higher by 6.6 % (winter) to 5.6 % (summer) than those observed at weekends. The largest monthly exchange was characteristic of winter months (from 2.0 to 2.7 g m−2 month−1) and the lowest occurred in summer (from 0.8 to 1.0 g m−2 month−1). The mean daily patterns of FCH4 in consecutive months were used to determine the cumulative annual exchange. In 2014, the centre of Łódź emitted a net quantity of almost 18 g m−2. Furthermore, the study analyses the covariability of methane and carbon dioxide fluxes.

Download Full-text

Fossil fuel combustion, biomass burning and biogenic sources of fine carbonaceous aerosol in the Carpathian Basin

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-4295-2020 ◽

2020 ◽

Vol 20 (7) ◽

pp. 4295-4312 ◽

Cited By ~ 2

Author(s):

Imre Salma ◽

Anikó Vasanits-Zsigrai ◽

Attila Machon ◽

Tamás Varga ◽

István Major ◽

...

Keyword(s):

Biomass Burning ◽

Fossil Fuel ◽

Fuel Combustion ◽

Carpathian Basin ◽

Total Carbon ◽

Carbonaceous Aerosol ◽

City Centre ◽

Fossil Fuel Combustion ◽

Regional Background ◽

The City

Abstract. Fine-fraction aerosol samples were collected, and air pollutants and meteorological properties were measured in situ in the regional background environment of the Carpathian Basin, a suburban area and central part of its largest city, Budapest, in each season for a 1-year-long time interval. The samples were analysed for PM2.5 mass, organic carbon (OC), elemental carbon (EC), water-soluble OC (WSOC), radiocarbon, levoglucosan (LVG) and its stereoisomers, and some chemical elements. Carbonaceous aerosol species made up 36 % of the PM2.5 mass, with a modest seasonal variation and with a slightly increasing tendency from the regional background to the city centre (from 32 % to 39 %). A coupled radiocarbon-LVG marker method was applied to apportion the total carbon (TC = OC + EC) into contributions of EC and OC from fossil fuel (FF) combustion (ECFF and OCFF, respectively), EC and OC from biomass burning (BB) (ECBB and OCBB, respectively), and OC from biogenic sources (OCBIO). Fossil fuel combustion showed rather constant daily or monthly mean contributions (of 35 %) to the TC in the whole year in all atmospheric environments, while the daily contributions of BB and biogenic sources changed radically (from <2 % up to 70 %–85 %) at all locations and over the years. In October, the three major sources contributed equally to the TC in all environments. In January, it was the BB that was the major source, with a share of 70 % at all sites. The contributions from biogenic sources in January were the smallest. In April, FF combustion and biogenic sources were the largest two contributors at all locations with typical shares of 45 %–50 % each. In July, biogenic sources became the major source type with a monotonically increasing tendency (from 56 % to 72 %) from the city centre to the regional background. The share of BB was hardly quantifiable in July. The ECFF made up more than 90 % of EC in April and July, while in October and January, the contributions of ECBB were considerable. Biomass burning in winter and autumn offers the largest and most considerable potential for improving the air quality in cities as well as in rural areas of the Carpathian Basin.

Download Full-text

Mobile measurements of ship emissions in two harbour areas in Finland

Atmospheric Measurement Techniques ◽

10.5194/amt-7-149-2014 ◽

2014 ◽

Vol 7 (1) ◽

pp. 149-161 ◽

Cited By ~ 40

Author(s):

L. Pirjola ◽

A. Pajunoja ◽

J. Walden ◽

J.-P. Jalkanen ◽

T. Rönkkö ◽

...

Keyword(s):

Baltic Sea ◽

Particle Number ◽

City Centre ◽

Particulate Emissions ◽

Size Distributions ◽

So2 Emissions ◽

The Baltic Sea ◽

Eu Directive ◽

The Baltic ◽

The City

Abstract. Four measurement campaigns were performed in two different environments – inside the harbour areas in the city centre of Helsinki, and along the narrow shipping channel near the city of Turku, Finland – using a mobile laboratory van during winter and summer conditions in 2010–2011. The characteristics of gaseous (CO, CO2, SO2, NO, NO2, NOx) and particulate (number and volume size distributions as well as PM2.5) emissions for 11 ships regularly operating on the Baltic Sea were studied to determine the emission parameters. The highest particle concentrations were 1.5 × 106 and 1.6 × 105 cm−3 in Helsinki and Turku, respectively, and the particle number size distributions had two modes. The dominating mode peaked at 20–30 nm, and the accumulation mode at 80–100 nm. The majority of the particle mass was volatile, since after heating the sample to 265 °C, the particle volume of the studied ship decreased by around 70%. The emission factors for NOx varied in the range of 25–100 g (kg fuel)−1, for SO2 in the range of 2.5–17.0 g (kg fuel)−1, for particle number in the range of (0.32–2.26) × 1016 # (kg fuel)−1, and for PM2.5 between 1.0–4.9 g (kg fuel)−1. The ships equipped with SCR (selective catalytic reduction) had the lowest NOx emissions, whereas the ships with DWI (direct water injection) and HAMs (humid air motors) had the lowest SO2 emissions but the highest particulate emissions. For all ships, the averaged fuel sulphur contents (FSCs) were less than 1% (by mass) but none of them was below 0.1% which will be the new EU directive starting 1 January 2015 in the SOx emission control areas; this indicates that ships operating on the Baltic Sea will face large challenges.

Download Full-text

Quantification of atmospheric nucleation and growth process as a single source of aerosol particles in a city

10.5194/acp-2017-819 ◽

2017 ◽

Author(s):

Imre Salma ◽

Veronika Varga ◽

Zoltán Németh

Keyword(s):

Growth Process ◽

Particle Number ◽

Particle Diameter ◽

City Centre ◽

Background Concentration ◽

Time Interval ◽

Single Source ◽

Mean Values ◽

Early Afternoon ◽

The City

Abstract. Effects of new aerosol particle formation (NPF) and particle diameter growth process as a single source on atmospheric particle number concentrations were evaluated and quantified on the basis of experimental data sets obtained from particle number size distribution measurements in the city centre and near-city background of Budapest for 5 years. Nucleation strength factors separately for nucleation days (NSFnucl days) and for all days (NSFall days) were derived for seasons and full years. The former characteristics represents the concentration increment of ultrafine (UF) particle numbers with respect to background concentration due solely to nucleation specifically on nucleation days. The latter factor expresses the contribution of nucleation process to the background particle number concentrations in general, thus on a longer time interval such as season or year. The nucleation source had the largest effect on particle concentrations around noon and early afternoon as expected. During this time interval, it became the major source of particles in the near-city background. Nucleation increased the daily mean particle number concentrations on nucleation days by mean factors of 2.3 and 1.58 in the near-city background and city centre, respectively. Its effect was the largest in winter, which was explained with the substantially lower background concentration levels on nucleation days than that on non-nucleation days. On an annual time scale, 37 % of the UF particles were generated by nucleation in the near-city background, while NPF produced 13 % of UF particles in the city centre. The differences among the annual mean values, and among the corresponding seasonal mean values were likely caused by the variability in controlling factors from year to year. The values obtained represent lower limits of contributions. The shares determined imply that NPF is a non-negligible or substantial source of particles in near-city background environments and even in city centres, where the vehicular road emissions usually prevail. Atmospheric residence time of nucleation-mode particles was assessed by decay curve analysis of N6–25 concentrations in time, and a mean of 2:30 was obtained. The present study suggests that the health-related consequences of atmospheric NPF and growth process in cities should also be considered in addition to its urban climate implications.

Download Full-text

Important Factors for Tornadogenesis as Revealed by High-Resolution Ensemble Forecasts of the Tsukuba Supercell Tornado of 6 May 2012 in Japan

Monthly Weather Review ◽

10.1175/mwr-d-17-0254.1 ◽

2018 ◽

Vol 146 (4) ◽

pp. 1109-1132 ◽

Cited By ~ 9

Author(s):

Sho Yokota ◽

Hiroshi Niino ◽

Hiromu Seko ◽

Masaru Kunii ◽

Hiroshi Yamauchi

Keyword(s):

Ground Level ◽

Gradient Force ◽

Pressure Gradient Force ◽

Atmospheric Conditions ◽

Ensemble Forecasts ◽

Near Surface ◽

Large Water ◽

Perturbation Pressure ◽

The Relationship ◽

The City

To identify important factors for supercell tornadogenesis, 33-member ensemble forecasts of the supercell tornado that struck the city of Tsukuba, Japan, on 6 May 2012 were conducted using a mesoscale numerical model with a 50-m horizontal grid. Based on the ensemble forecasts, the sources of the rotation of simulated tornadoes and the relationship between tornadogenesis and mesoscale environmental processes near the tornado were analyzed. Circulation analyses of near-surface, tornadolike vortices simulated in several ensemble members showed that the rotation of the tornadoes could be frictionally generated near the surface. However, the mechanisms responsible for generating circulation were only weakly related to the strength of the tornadoes. To identify the mesoscale processes required for tornadogenesis, mesoscale atmospheric conditions and their correlations with the strength of tornadoes were examined. The results showed that two near-tornado mesoscale factors were important for tornadogenesis: strong low-level mesocyclones (LMCs) at about 1 km above ground level and humid air near the surface. Strong LMCs and large water vapor near the surface strengthened the nonlinear dynamic vertical perturbation pressure gradient force and buoyancy, respectively. These upward forces made contributions essential for tornadogenesis via tilting and stretching of vorticity near the surface.

Download Full-text

Breasts and the City: An Urban Ethnography of Infant Feeding in Public Spaces within Cardiff, United Kingdom

10.21203/rs.3.rs-111326/v1 ◽

2020 ◽

Author(s):

Aimee Grant

Keyword(s):

Infant Feeding ◽

Public Transport ◽

Large City ◽

Department Store ◽

Public Spaces ◽

Barriers And Facilitators ◽

City Centre ◽

Breastfeeding In Public ◽

Urban City ◽

The City

Abstract Background: Internationally, women report challenges breastfeeding in public spaces. This study aimed to investigate the social-spatial aspects of public spaces in one UK city, Cardiff, in order to suggest possible barriers and facilitators to breastfeeding in public spaces.Methods: The study observation location guide prioritised places that had been reported as hostile to breastfeeding or breastfeeding friendly in the existing literature. Data were collected at various times of day, in several areas of the city, and included transport (n=4), transport hubs (n=3), high streets (n=4), cafes (n=2), a large city centre shopping complex, comprising of three joined shopping centres and a large city centre department store (containing a third café). Low inference field notes were written on an encrypted smart phone and expanded soon after. Data were analysed thematically using deductive codes based on the observation schedule. Additional inductive codes relating to places were added.Results: Overall, public transport and the city centre were inhospitable environments for those who might need to breastfeed, and even more so for those who need to express breastmilk. The core barriers and facilitators running through the data appeared to be the availability of appropriate seating coupled with either high privacy or civil inattention. The one variation to this model arose from the department store café, where civil inattention was not performed and there was low privacy, but breastfeeding occurred anyway. Conclusions: This research highlights the physical and social barriers to breastfeeding within one urban city centre in the UK and its associated transport links. It is clear that there is an urgent need for change in urban city centres and public transport if countries are to meet their aims in relation to increasing breastfeeding rates. Interventions will need to be multifaceted, accounting for social norms relating to infant feeding as well as changes to the physical environment, policy and potentially legal change. Further research should be undertaken in other countries to examine the extent to which hostile environments exist, and if correcting these could facilitate breastfeeding and reduce gender based violence.

Download Full-text

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

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-27387-2013 ◽

2013 ◽

Vol 13 (10) ◽

pp. 27387-27422

Author(s):

M. Brines ◽

M. Dall&apos;Osto ◽

D. C. S. Beddows ◽

R. M. Harrison ◽

X. Querol

Keyword(s):

Spatial Distribution ◽

Urban Areas ◽

Aerosol Size Distribution ◽

Size Distributions ◽

Urban Background ◽

Background Site ◽

Regional Background ◽

Background Pollution ◽

Aerosol Sources ◽

The City

Abstract. The analysis of aerosol size distributions is a useful tool for understanding the sources and the processes influencing particle number concentrations (N) in urban areas. Hence, during the one month SAPUSS campaign (Solving Aerosol Problems by Using Synergistic Strategies, EU Marie Curie Action) in autumn 2010 in Barcelona (Spain), four SMPS (Scanning Mobility Particle Sizers) were simultaneously deployed at four monitoring sites: a road side (RSsite), an urban background site located in the city (UBsite), an urban background located in the nearby hills of the city (Torre Collserola, TCsite) and a regional background site located about fifty km from the Barcelona urban areas (RBsite). The spatial distribution of sites allows study of the aerosol temporal variability as well as the spatial distribution, progressively moving away from urban aerosol sources. In order to interpret the datasets collected, a k-means cluster analysis was performed on the combined SMPS datasets. This resulted in nine clusters describing all aerosol size distributions from the four sites. In summary there were three main categories (with three clusters in each category): "Traffic" (Traffic 1 "Tclus1" – 8%, Traffic 2 "Tclus2" – 13%, Traffic 3, "Tclus3" – 9%), "Background Pollution" (Urban Background 1 "UBclus1" – 21%, Regional Background 1, "RBclus1" – 15%, Regional Background 2, "RBclus2" – 18%) and "Special cases" (Nucleation "NUclus" – 5%, Regional Nitrate, "NITclus" – 6%, and Mix "MIXclus" – 5%). As expected, the frequency of traffic clusters (Tclus1–3) followed the order RSsite, UBsite, TCsite, and RBsite. These showed typical traffic modes mainly distributed at 20–40 nm. The urban background sites (UBsite and TCsite) reflected also as expected urban background number concentrations (average values, N = 2.4×104 cm−3 relative to 1.2×105 cm−3 seen at RSsite). The cluster describing the urban background pollution (UBclus1) could be used to monitor the sea breeze circulation towards the regional background study area. Overall, the RBsite was mainly characterised by two different regional background aerosol size distributions: whilst both exhibited low N (2.6×103 for RBclus1 and 2.3×103 cm−3 for RBclus2), RBclus1 had average PM10 concentrations higher than RBclus1 (30 vs. 23 μg m−3). As regards the minor aerosol size distribution clusters, the "Nucleation" cluster was observed during daytime whilst the "Regional Nitrate" was mainly seen at night. The ninth cluster ("Mix") was the least well defined and likely composed 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 with values progressively increasing from the regional site RBsite to the road site RSsite was found. This points to vehicular traffic as the main source of both N and NO2. By contrast, such an association does not exist for the case of the nucleation cluster, where the highest N is found with low NO2 and PM. Finally, the clustering technique allowed study of the impact of meteorological parameters on the traffic N emissions. This study confirms the shrinking of freshly emitted particles (by about 20% within 1 km in less than 10 min; Dall'Osto et al., 2011a) as particles are transported from the traffic hot spots towards urban background environments. Additionally, for a given well defined aerosol size distribution (Tclus2) associated to primary aerosol emissions from road traffic we found that N5–15 nm concentrations can vary up to a factor of eight. Within our measurement range (5–228 nm), we found that ultrafine particles within the range 5–15 nm are the most dynamic, being a complex ensemble of primary evaporating traffic particles, traffic tailpipe new particle formation and non-traffic new particle formation.

Download Full-text