Atmospheric Research 90 (2008) 159–169

Contents lists available at ScienceDirect

Atmospheric Research

j ourna l homepage: www.e lsev ie r.com/ locate /atmos

Using Föhn conditions to characterize urban and regional sources of particles

D. Mira-Salama, R. Van Dingenen, C. Gruening, J.-P. Putaud, F. Cavalli, P. Cavalli, N. Erdmann 1,A. Dell'Acqua, S. Dos Santos, J. Hjorth, F. Raes, N.R. Jensen⁎European Commission — Joint Research Centre, Institute for Environment and Sustainability, Climate Change Unit, TP 290, I-21027 Ispra (VA), Italy

a r t i c l e i n f o

⁎ Corresponding author. European Commission —

Institute for Environment and Sustainability, ClimateI-21027 Ispra (VA), Italy. Tel.: +39 0332789225; fax:

E-mail address: niels.jensen@jrc.it (N.R. Jensen).1 Present address: European Commission— Joint Res

for Transuranium Elements, P.O. Box 2340, D-76125 K

0169-8095/$ – see front matter © 2008 Elsevier B.V.doi:10.1016/j.atmosres.2008.02.007

a b s t r a c t

Article history:Received 4 September 2007Received in revised form 18 January 2008Accepted 4 February 2008

Characterizations of urban and regional sources of particulate matter (PM) were performedin the Milan area (North of Italy) during Föhn and stagnant (non-Föhn) conditions. Themeasurements were performed at two different places: in an urban area North of Milan(Bresso) and in a regional area at the EMEP-GAW station in Ispra (about 65 km NW fromMilan)during the winter periods of the years 2002–2007. Particle size distributions and chemical bulkanalysis of aerosols are combined with single particle mass spectrometry to obtain informationabout the chemical content of the particles and their mixing state. Föhn conditions arecharacterized by extremely clean background air fromwhich background aerosol is scavenged,and consequently local sources (here defined as sources between the sampling sites and themountain range top about 100–150 km away depending on the wind direction) determine theaerosol properties.It was observed that during Föhn events the accumulation mode in the size range50 nmbdb300 nm practically disappears and that the size fraction below 50 nm dominatesthe total number distribution. The significant change in the number size distribution and thelarge decrease in PM10mass during Föhn events are accompanied by a significant change in thechemical composition of the particles. Results from bulk chemical analysis showed highamounts of carbonaceous compounds and very low concentrations of ammonium nitrate (asindicator for secondary chemistry) during Föhn episodes, in contrast to stagnant conditions,when secondary components are dominating the aerosol composition. Single particlemeasurements confirm the high contribution of carbonaceous compounds in locally emittedparticles.It was concluded that particles that originated in the urban area come mainly from combustionprocesses, especially direct traffic emissions, domestic heating and industrial activities,whereas the regionally emitted particles are different with much less traffic contribution.We estimate that under prevailing (non-Föhn) winter conditions, about 50–65% of the aerosolmass load in the city of Milan are caused by local emissions, and about 35–50% come fromregional background. This finding suggests that in order to improve air quality in a big city likeMilan, it is important to combine local traffic restriction interventions with other long-termregional scale air-quality-measures.

© 2008 Elsevier B.V. All rights reserved.

Keywords:FöhnUrban pollutionAerosols size distributionsBulk analysisSingle particlesAerosol characterization

Joint Research Centre,Change Unit, TP 290,+39 0332785837.

earch Centre Institutearlsruhe, Germany.

All rights reserved.

1. Introduction

Atmospheric particles play an important role in chemicalas well as physical processes related to air pollution, humanhealth and climate change (Novakov and Penner, 1993; Tegenet al., 1997; Lippmann et al., 2000).

High levels of particulate matter (PM10, the sum of themass of particles with a diameter below 10 μm) have been a

160 D. Mira-Salama et al. / Atmospheric Research 90 (2008) 159–169

serious problem in big cities for decades. The levels of PM10 ata certain location can be considered as a sum of the regionalbackground and locally emitted particles. Traffic is known tobe a very significant local PM10 contributor in the urbanenvironments, be it rather in particle number than in particlemass (Wåhlin et al., 2001; Baltensperger et al., 2002; VanDingenen et al., 2004).

The Po Valley in Northern Italy is known for its highPM levels, especially during winter, when stagnant meteor-ological conditions prevail. During the cold season, PM10levels frequently exceed 50 μg m−3 (the EU 24-hours limitvalue of 50 μg m−3, is not to be exceeded for more than35 days year–1), invoking traffic ban measures in the urbancentres. In this study we investigate the contribution of theregional background and the local urban emissions to thetotal PM load at an urban location (Bresso) in Milan, Italyduring winter and also at a regional EMEP-GAW station inIspra (EMEP-GAW: Co-operative programme for monitoringand evaluation of the long-range transmissions of airpollutants in Europe — Global Atmospheric Watch).

We take advantage of particularmeteorological conditions,the so-called “Föhn” episodes, typically occurring for somedays per month in winter time (see e.g. Weber and Prévôt,2002). In Italy, Föhn winds develop when two phenomenaoccur concomitantly: (1) Moist air masses are pushed upslopeNorth of the Alps, water vapour condenses, latent heat ofcondensationwarms the air mass above the temperature thatwould result from adiabatic expansion only, and liquid waterprecipitates; (2) The synoptic pressure system pulls down-slope theAlps this liquidwater–free airmasswhichundergoesadiabatic compression and thus warming. Föhn episodes aretherefore characterized by a windy, sunny, dry and warmweather, as well as clean air, since water soluble gases andactivated particles have been scavenged in clouds at themountain range top and removed by precipitation. Theclean air mass brought by the Föhn wind reduces regionalPM10 levels from approximately 50 μg m−3 down to the orderof 1–5 μg m−3 during a typical winter day. In such conditions,only local (urban) emissions contribute to the urban aerosolload since the regional background aerosol has been removedby the Föhn. The comparison of physical and chemical proper-ties of the urban aerosol during Föhn and stagnant conditionsallows us to evaluate the contribution of local and regionalPM on the urban background aerosol in the Milan area.

Previous studies have also used “unusual wind conditions”to evaluate or characterize aerosols in a specific area: e.g. aninvestigation by Guazzotti et al. (2001) used a transportableATOFMS (Aerosol Time-Of-Flight Mass Spectrometer, TSI) toevaluate the aerosol chemical composition with and withouta Santa Ana wind episode in California, US, during the year1998. In this way they could study themixed sea-salt particlesthat were present in conditions without the Santa Ana wind(air masses with marine characteristics) and the dust andcarbon-containing particles during a Santa Ana wind episode(air masses with inland characteristics). They observed that asubstantial change in aerosol composition occurred during ashort time frame (approximately 30 min.) and they were ableto study the rapid evolution of particles from one type(marine conditions) to the other type (inland condition).

In another study by Furger et al. (2005), an aircraft wasused to investigate particle number by using optical particle

counters and backscattering by using a backscatter lidarduring Föhn episodes in a region north of the Alps in theRhine Valley in Germany. They concluded that thermalstratification of the valley atmosphere plays a crucial role forthe observed aerosol concentration and that advection andsubsidence also alter the aerosol characteristics significantly.

Some preliminary results of the present study have beendiscussed by Erdmann et al. (2005) although the paper wasmainly dealing with instrumental development and charac-terization of the single particle and sizing system (SPASS).SPASS measurements indicated that in the Milan area,particles present a high degree of internal mixing. Resultsfrom bulk-chemistry analysis as well as from SPASS bothshowed that during a Föhn episode organic matter (carbon) islargely dominating the chemical composition.

Rodriguez et al. (2005) studied the relationship betweenaerosol number size distribution and air quality in terms ofparticulate matter mass concentration at a rural site, Ispra, inNorthern Italy. This study shows that the submicron aerosolsize distribution is strongly influenced by two main compet-ing processes: nucleation of new particles and condensationof gas-phase components onto pre-existing particles (result-ing in particle growth). In addition, it was concluded thatnucleation occurred under clean air conditions prompted byNorth–Föhn meteorology.

In addition a number of articles have tried to address thesource apportionment of PM in the Milan area: e.g. Lonatiet al. (2005) found a strong PM contribution from secondarysources and a small contribution from the emissions ofstationary sources inwinter. They also found that the primarycontribution of traffic, in terms of carbonaceous matter (ECand primary organic aerosols), is estimated to be 6% and 11%of the total PM2.5 mass in the cold and warm seasonsrespectively. In addition, they found that secondary organicaerosols are about 85% of total particulate organic matter and30% of the total PM2.5 mass. From all of their findings theyconclude that traffic restriction interventions are not effectiveto provide relevant short-term air quality improvements andthat it is important to control the emission of the precursorsof PM2.5 secondary components. Giugliano et al. (2005) foundthat organic and inorganic secondary production is contribut-ing for about 75% of PM2.5 mass in winter and 40% in summerand therefore concluded that effective long-term actions, stillcontrolling the emissions of primary pollutants too, arerequired for air quality standards attainment. In addition theyconcluded that the potentiality of short-term interventions, astraffic restrictions, appears quite limited. Marcazzan et al.(2003), using receptor modelling, found that the mostimportant sources were “soil dust”, “traffic”, “industry” and“secondary compounds” for PM10 and “soil dust”, “anthro-pogenic” and “secondary compounds” for PM2.5, constitutingthe major fraction of the PM mass (91% and 75%, respectively).

The present article tries to address the question: In anurban area, howmuch aerosol in terms of mass and number isdue to local urban emissions and how much is due to theregional background aerosol? We present an investigation ofaerosol data obtained during Föhn episodes, as well as stag-nant conditions in the Milan area, North of Italy. The mea-surements were performed as case studies in the time frame2002–2007. From the results wewere able to characterize andevaluate the urban local emissions of particles.

161D. Mira-Salama et al. / Atmospheric Research 90 (2008) 159–169

2. Experimental

2.1. Location and meteorology of the sites

Milan lies in the upper Po basin and is surrounded by thecentral European Alps to the north and west and by theApennines to the south. In order to characterize typicallyurban-produced aerosols, intensive winter campaigns werecarried out in January and February in 2002 and 2003 inBresso (45°32′N, 9°12′E; elevation 130 m a.s.l.), approxi-mately 6 km north of the Milan city centre, and 1 km south ofthe motorway Milan–Venice. Despite the distance from thecity centre, Bresso is a typical urban mix of industry andresidential zones. Also during northerly winds, it is exposed tothe large and densely populated Milan metropolitan areawhich extends north of the site.

Atmospheric regional background conditions are continu-ously monitored at the Ispra EMEP-GAW (Co-operativeprogramme for monitoring and evaluation of the long-rangetransmissions of air pollutants in Europe — Global Atmo-spheric Watch) station (45°49′N, 8°38′E, elevation 209m a.s.l.),located at the border of Lake Maggiore, near the southernfoothills of the Alps, 30 km north and 47 kmwest of Bresso. TheIspra EMEP-GAW station has been running since 1985, andsome of the measurements in the time frame 2002–2007 arepresented here. In this way, we have the data collected onintensive winter campaigns from a typical urban site, andcontinuous data froma typical regional site. The twomeasuringsites are shown in Fig. 1.

The meteorology of the sites shows the following commonunique characteristics: During anticyclonic conditions, whichoccur frequently in winter, stagnating air masses prevail withlow wind speed over the whole Po valley. In winter/springtime, the atmospheric boundary layer can be as low as a few100 m or even lower (Moussiopoulos et al., 2000) leadingto strong accumulation of atmospheric pollutants over thearea. These stagnant conditions are occasionally interrupted

Fig. 1. Population density in Italy and in the study area. Darkest area indicates highemetodologie statistiche e le tecnologie informatiche http://demo.istat.it.

by Föhn events, where relatively warm and dry down-slopewind from the Alps flows over the area, with the air tem-perature rising to 15°–20 °C in January when it reaches thevalley (see Fig. 2). As no significant aerosol mass flows outfrom mountain top clouds during Föhn events, it can beconsidered that the contribution of long-range transport toPM mass is negligible, and therefore that the PM observedduring these Föhn episodes results from sources andprocesses occurring locally, i.e. between the mountain rangetop (the top is about 100–150 km upwind, depending on thewind direction) and the sampling sites. Under North–Föhnconditions, the urbanized area expected to contribute to‘local’ emissions, ranges less than 50 km upwind the urbanmeasurement site. For what concerns the regional site, nourban agglomerations occur upwind the site during Föhnconditions, and the rural built-up area extends about 40 kmupwind the site.

2.2. Instrumentation

2.2.1. Number and size distributions of the aerosolsBoth during the intensive winter campaign in Bresso and

in the Ispra site the aerosol number size distributions (sizerange from 6 nm to 600 nm) were measured with a custom-built medium size Vienna-type differential mobility analyzer(DMA), in combination with a TSI model 3010 condensationparticle counter (CPC). The system is operated in a closed-looparrangement with dry sheath air (relative humidity b20%)and hence delivers dry size distributions of the aerosols. Byapplying enhanced cooling to the CPC saturator, particlesdown to 6 nm can be detected.

An optical particle counter (OPC, Grimm dust monitormodel 1.108) was used simultaneously to measure the sizedistribution of the aerosol to cover a wider range, from300 nm to 10 μm. The Grimm detects the amount of scatteredlight from aerosol particles at an angle of 90° and classifiesthem into 15 size classes. In general, the overlapping size

st density (N500 inhabitants/km2). Source: ISTAT — Direzione centrale per le

Fig. 2. Examples of events observed at Bresso (urban BG site) and Ispra (regional BG site). Wind speed, relative humidity and temperature during Föhn events(symbols) present anomalies compared to average (thick line with standard deviations) from 17:00 to 01:00 (1st day) in Bresso and from 04:00 to 17:00 (2nd day)in Ispra.

162 D. Mira-Salama et al. / Atmospheric Research 90 (2008) 159–169

range between the Grimm and DMA (300 – 600 nm) showedgood agreement.

During the Bresso campaign in 2003, a ‘volatility’-DMA(V-DMA) was used in parallel to the normal DMA to measurethe refractory part of the aerosol when heated to N270 °C.The system has an identical configuration of the DMA atambient temperature, apart from a tube furnace throughwhich the sample line (quartz tube inside the furnace) is ledbefore entering the DMA system. The oven temperature of300 °C leads to a temperatureN270 °C inside the sample stream,causing the evaporation of H2SO4, ammonium sulphate, bi-sulfate, ammonium nitrate, and other semi-volatile organiccompounds.

The remaining aerosol after the heating treatment iscommonly referred to as refractory aerosol, and may containnon-volatile organic carbon, elemental carbon, non-volatilesalts and mineral dust. Laboratory tests with an internalmixture of NaCl aerosol and ammonium sulphate have shownthat the tube furnace arrangement is able to evaporate andremove volatile compounds without re-nucleation or re-condensation. During overnight measurements the furnacewas switched off for safety reasons.

2.2.2. Equivalent black carbon (EBC)The black carbon concentration was measured with a

commercial GIV Aethalometer™ (incandescent lamp) with10 minute time interval and a flow rate of 5 L min−1. No pre-

conditioning was applied to the sample air. The Aethalometerreports the “Equivalent Black Carbon” concentration (EBC) inμg m−3 based on an assumed attenuation cross section of theblack carbon aerosol filter deposit of 19 m2 g−1. In this studywe use EBC as a tracer for locally emitted primary aerosol.Further, due to the high time resolution of the measurements,the dataset allows us to identify accurately the start and endof clean Föhn episodes.

2.2.3. Bulk aerosol chemical analysisAerosol particles were collected for chemical analysis at a

flow rate of 20 Lmin−1 onto 47mmquartz filters and cellulosefibre filters.

Organic and elemental carbon (OC and EC) were measuredfrom an 8 mm diameter section of the quartz filters using amulti-step thermal method with CO2 detection, according tothe method described by Putaud et al. (2002). Carbon evolvedat up to about 340 °C in He/O2 and up to 650 °C in pure Hewas classified as OC. EC was defined as the carbonaceousfraction remaining after this treatment and evolved at 650 °Cin a He/O2 carrier gas.

The remaining pieces of quartz filters were extracted inultra pure water (18.2 MΩ cm) and the extracts analysed byion chromatography (IC) formajor species (Na+, NH4

+, K+, Mg2+,Ca2+, Cl−, NO3

− and SO42−). Mineral dust was determined by

weighting with a microbalance the residue obtained afterashing in quartz crucible at 600 °C the paper filters previously

Fig. 3. Median number size distributions during Föhn and stagnant conditions at the urban (Bresso) and regional (Ispra) background site. (a) Data from Jan.–Feb.2002 UBG and Jan.–Feb. 2004 RBG. (b) Data from Jan.–Feb. 2002. (c) Data from Jan.–Feb. 2002 and 2004.

Fig. 4. Volume and number data during Föhn and stagnant conditions atBresso (urban BG site, 2002, 2003) and Ispra (regional BG site, 2004).

163D. Mira-Salama et al. / Atmospheric Research 90 (2008) 159–169

extracted in ultra pure water. The IC was calibrated by usingNIST commercially available synthetic rainwater standards.

2.2.4. Single particle measurementsThe operational set-up of the Single Particle Analysis and

Sizing System (SPASS) has been described in details elsewhere(Erdmann et al., 2005). In brief, aerosols are introduced intothe SPASS using an aerodynamic lens system. Particleswith anaerodynamic diameter of more than approximately 300 nmare then sized with a two beam laser velocimeter. This in-formation is further used to trigger a frequency quadrupledNd:YAG laser (wavelength λ=266 nm, pulse length T=8 ns,pulse energy E=20mJ)which desorbs and ionizes the particle.Positive and negative ions are simultaneously detected by twotime-of-flight (TOF) mass spectrometers.

Thus the size and chemical composition of single aerosolparticles can be characterized simultaneously and on-line.The SPASS has been installed inside a truck to create a mobileunit.

Inherent to the laser ionization process in this type ofaerosol mass spectrometers, the SPASS delivers only qualita-tive information about the particles composition with a biastowards compounds that are easy to ionize (elements andmolecular fragments). Mass number calibration was per-formed by spraying a variety of aerosols of known chemicalcomposition into the system.

Because the SPASS determines size and chemical proper-ties of individual particles, it allows evaluating their mixingstate (internal or external mixing), which is an importantparameter e.g. to evaluate the “age” of the aerosol mixture(coming from nearby sources or from long-range transport).In addition, by collecting characteristic spectra for a variety ofaerosol sources, the technique can also be applied for sourceapportionment studies.

To interpret the vast amount of single particle mass spec-tra regularly obtained during field campaigns the k-meansclustering algorithm is used. The peak intensities for everyparticle are first normalized separately for the positive andnegative mass spectrum to form a vector of unit length. Thenthe k-means clustering algorithm sorts all particles into classes

according to the similarity of theirmass spectra (Erdmann et al.,2005; Hinz et al., 2006). As a result, the numbers of mainparticle classes present in the aerosol population under in-vestigation as well as the mass spectra representing theseclasses are obtained.

3. Results and discussion

Wewill present results from the intensive winter campaignat the urban background site (UBG) in Bresso, and at theregional background site (RBG) in Ispra during Föhn andstagnant conditions. During Föhn, the size distributions areaveraged for the duration of each event, which typically lastsbetween 6 and 12 h. Night-time refers to 0200–0500 anddaytime to 0800–1800 local time. Fig. 3 shows for both sites themedian number size distributions measured during wintertime under different conditions. It illustrates how local and

Table 1Average physical aerosol properties at the Bresso Urban Background stationduring summer smog (Baltensperger et al., 2002), winter Föhn and wintersmog

Summerstagnant 1998

WinterFöhn 2003

Winterstagnant 2003

BC μg/m3 4.4 0.34 6.0Namb 104/cm3 3.0 2.1 3.3Namb,b20 nm 104/cm3 0.6 2.0 1.9Vamb μm3/cm3 12 2.6 37Vrefr/Vamb [ ] 0.32 0.48 0.33Nrefr/Namb [ ] 0.24 0.20 0.69[Nrefr/Namb]b20 nm [ ] 0.16 0.18 0.80

164 D. Mira-Salama et al. / Atmospheric Research 90 (2008) 159–169

regional contributions affect different size ranges of the aerosolsize distribution. Fig. 3a shows night time distributions understagnant conditions at the UBG and RBG site; Fig. 3b and cshows the day time size distribution for stagnant and Föhnconditions at the UBG and RBG sites respectively.

During the night, when local emissions are minimal, theaccumulation mode (100 nmbdb1 μm) of the urban BGaerosol distribution under normal, stagnant conditions, isvery similar to the one at the regional BG site (Fig. 3a),indicating that the urban accumulationmode is dominated bythe regional background. During daytime, local emissionscontribute significantly to the Aitken mode (particle diameterb100 nm). At both sites, an increase in the daytime number ofAitken mode particles is observed, especially clear at theurban site (Fig. 3b and c, full line). Earlier studies have alreadyhighlighted the role of primary traffic emissions as well asphotochemical processes and gas-to-particle conversion inshaping the ultra-fine particle size range, explaining observeddifferences between urban–regional background, and day–night (Baltensperger et al., 2002; Bukowiecki et al., 2002; VanDingenen et al., 2004).

Under the particular Föhn conditions, the accumula-tion mode is mostly absent at both sites, (dashed lines inFig. 3b and c), and the locally produced particles are standingout clearly. Hence, Föhn conditions offer a unique occasion toinvestigate more in detail the properties of the freshlyemitted urban aerosol. As can be seen in Fig. 4, there is anobvious and significant difference in volume and numberconcentration between Föhn/non-Föhn conditions, both inBresso (urban BG site) and in Ispra (regional BG site). DuringFöhn, the aerosol volume is reduced by a factor of 13 (Bresso)to 30 (Ispra), the number (only) by a factor of 1.7 (Bresso) to 6

Fig. 5. Aerosol volume versus equivalent black carbon (EBC) during Föhn events atlocally emitted volume in terms of EBC.

(Ispra). Looking at ultra-fine particles only (diameterb20 nm),we find a similar relative decrease in Bresso as for the totalnumber, whereas in Ispra the ultra-fine number even in-creases with a factor of 1.5 (not shown in the figure). Hence,on a timescale of hours, local emissions affect rather theaerosol number and much less the volume (or mass).

Table 1 summarizes aerosol properties for winter stagnantand Föhn conditions, together with the results from an earliercampaign at the same location during summer stagnantconditions (Baltensperger et al., 2002). Although the refrac-tory aerosol volume fraction is equal in summer and winter,the refractory number fraction is significantly higher inwinter,in particular the ultra-fine size range (b20 nm). The refractorynumber fraction during Föhn conditions on the other handresembles the summer smog conditions (around 20%). Bothduring summer smog and winter Föhn, nucleation of con-densable compounds is favoured, albeit for different reasons:during summer the photochemical production of H2SO4 andother condensable species may be fast enough to build up acritical concentration for nucleation to happen. During winterFöhn, photochemical production is much lower, but the lackof background aerosol surface area, in combination withlower temperatures, strongly favours gas-to-particle conver-sion of emitted or produced condensable compounds. Duringwinter smog, condensable species would rather go directly tothe accumulation mode particles, which explains the lowfraction of externally mixed volatile particles in the ultra-fineparticle number during such episodes.

Interestingly, the refractory volume fraction is roughlyequal during winter and summer smog. However, in contrastto the summer conditions where BC could reasonably accountfor the total refractorymass (Baltensperger et al., 2002), this isnot the case during winter smog. Although the refractoryvolume is highly correlated with BC (R2=0.91), the slope of BCvs. Vrefr is only 0.45, i.e. too low to represent the density of BC(during summer a slope of 1.1 was found). This indicates thatduring winter other compounds, different from, but asso-ciated with BC, contribute to the refractory aerosol mass.

The lower refractory volume fraction during summer andwinter smog – compared to Föhn – indicates a larger con-tribution of semi-volatile (secondary) aerosol components,formed on larger temporal and spatial scales. In the followingwe will estimate the local and regional contribution to theurban aerosol volume during winter smog, assuming thatFöhn conditions are indicative for the local contribution.More in particular, we assume that a) both during Föhn and

the urban background site. The fitted line represents a parameterisation of

Fig. 6. Scatter plot of all hourly aerosol volume data versus BC at the urban background site. The grey line is the extrapolation of the V-EBC relation during Föhn, andrepresents the locally emitted fraction of the aerosol volume.

165D. Mira-Salama et al. / Atmospheric Research 90 (2008) 159–169

stagnant conditions, black carbon can be considered to be atracer for locally emitted aerosol mass/volume, (i.e. the re-gional contribution to the urban black carbon is negligible)and b) during Föhn, the regional contribution to the aerosolvolume is negligible. Although the total aerosol volumeduring Föhn contains a secondary (volatile) fraction (52%,see Table 1; 1−0.48=0.52), the very high correlation betweentotal volume and refractory volume during Föhn (R2=0.94)indicates that this secondary formation is associated with theBC emissions and happens on the same spatial scale, hencealso this fraction is interpreted as being locally produced. Weparameterise the locally emitted aerosol volume by Vlocal=a⁎EBC+b, where Vlocal is the aerosol volume and EBC is theequivalent black carbon, measured during Föhn conditions atthe UBG site. The results of this fitting for each of the wintercampaigns are shown in Fig. 5. The regression line thus re-presents the aerosol volume associated with local blackcarbon emissions.

Fig. 6 shows a scatter-plot of all hourly V versus EBC (bothFöhn and stagnant conditions) during both winter campaigns

Fig. 7. Average diurnal trend of the estimated locally emitted sub-μm aerosol volumestandard deviation).

at the urban background station. The line is an extrapolationof the EBC-V relationship described above, representing thelocally produced UBG aerosol volume. Note that the Föhnconditions are represented by a small number of data pointsnear the origin; it is remarkable howwell the extrapolation ofthe Föhn EBC-V relationship describes the lower boundary ofV, even at elevated aerosol levels. The deviation of data pointsfrom the ‘local emission’ line may be interpreted as the con-tribution of the regional background (aged, secondary com-ponents) to the aerosol volume.

The large variability in the data is related to the variabilityin emission strength (e.g. traffic density) and meteorologicalconditions. This is illustrated in Fig. 7, showing the averagediurnal trend of the local sub-μm volume fraction duringstagnant conditions. Despite the large standard deviation,both campaigns show a very similar diurnal profile, with thehighest local contribution during the rush hours (8:00 and20:00), and minimal local contribution at 3:00 and 15:00.From our simple parameterisation we estimate that localemissions account for about 65% of the urban sub-μm

at the urban background site during both winter campaigns (error bars: one

Fig. 8. Bulk (PM10) chemical composition observed at the urban BG site in Milan (left) and the regional BG site in Ispra (right) during winter time stagnantconditions (PM10N50 μgm−3) and typical Föhn events (2003–2004). POM: Particulate organic matter. EC: Elemental carbon. Salt: Na++Cl−+K++Mg2++Ca2+. Unacc:Unaccounted mass.

166 D. Mira-Salama et al. / Atmospheric Research 90 (2008) 159–169

(diameter b1 μm) aerosol volume (and mass) during normalmeteorological winter conditions.

The dramatic change in the number size distribution andthe significant decrease in PM10 mass during Föhn events areaccompanied by a significant change in the chemical com-position of the particles (Fig. 8). During the Föhn events, thecontribution to PM10 of secondary inorganic compounds likeammonium nitrate is strongly reduced at both the urban andthe regional background sites (Table 2). In contrast, thecontribution of EC more than doubles during Föhn episodescompared to stagnant conditions at both sites.

Making the same assumption as for the estimation of thelocal aerosol volume contribution, namely that 100% of the ECobserved during stagnant conditions comes from localsources; the chemical composition of PM during the Föhnepisodes is used as a fingerprint of the local PM sourcecomposition to assess their contribution to the PM observedin stagnant conditions. Table 2 shows that the maximumcontribution of these local sources to the secondary inorganicspecies (NO3

−, SO42−, NH4

+) concentrations observed duringstagnant conditions is small. In contrast, the maximumcontributions of local sources to the POM (particulate organicmatter) concentrations observed during stagnant conditionsare ca. 50% and 75% at the urban and regional BG sites, res-pectively. The larger contribution of local sources to POM atthe regional backgroundmight be related tomorewidespreaduse of wood for domestic heating in the rural areas. The

Table 2PM10 chemical composition at the urban site and the regional site during stagnant clocal sources to PM10 component during stagnant conditions (all in %), assuming 10

Urban BG site (Bresso), Jan. 2003

Stagnant Foehn Local contribut

Dust 11 29 N/ASalt 8 13 N/ANH4 7 0 1nssSO4 8 7 42NO3 20 0 0POM 39 38 51EC 9 18 100Unaccounted −2 −5 N/A

N/A=not applicable.

contributions of mineral dust and sea-salt like componentsare also larger during Föhn than during stagnant conditions.However, the contributions of local sources to the concentra-tions of these components during stagnant conditions werenot estimated, because the source strength of these twocomponents is most probably larger during Föhn episodes,due to enhanced resuspension by strong winds. Based on themaximum contributions listed in Table 2 and the meanchemical composition presented in Fig. 8, it can be estimatedthat local sources are responsible for ca. 50% of the PM10massconcentrations observed during stagnant conditions at bothsites, which is consistent with the estimates based on numbersize distribution measurements.

Fig. 8 shows that during Föhn episodes, at the regionalbackground site, the secondary inorganic compounds likeammonium nitrate are strongly reduced, and sometimes evenvirtually absent. In addition, it is observed that during Föhnepisodes the EC becomes a major component compared tonon-Föhn conditions, very significantly evident at the urbanBG site where the percentage of EC increases from 9% to 18%during Föhn events and the regional BG site from 4% to 10%.

The observation of a larger contribution of EC during Föhnevents is confirmed by the single particle instrument. Theparticle mass spectra obtained with the SPASS during theintensive winter campaign at Bresso and during measure-ments at the Ispra regional BG site were analyzed using the k-means clustering algorithm.

onditions and Foehn episodes, and estimates of the maximum contribution of0% for EC

Regional BG site (Ispra), winter 2004

ion Stagnant Foehn Local contribution

3 10 N/A4 6 N/A9 1 66 5 35

23 0 042 72 764 10 1009 −4 N/A

167D. Mira-Salama et al. / Atmospheric Research 90 (2008) 159–169

Fig. 9 shows the average mass spectra of the main particleclasses that were obtained during the campaign in Bresso.Mass spectrum a) represents secondary aerosol particles as

Fig. 9. Average mass spectra of particle classes. a) ‘Secondary’. b) ‘Internal mixture ofof the positive mass spectrum of plot a).

the spectrum is dominated by nitrate and sulfate with verysmall peaks of positive ions (note the enlarged y-axis com-pared to b) and c)). Particles consisting of an internal mixture

combustion/traffic with secondary’. c) ‘Combustion’. Note the zoomed y-scale

Table 3Single particle measurements at the urban site in Bresso (winter 2003) and atthe regional site in Ispra (winter 2007)

Name of class Bresso (urbanbackground)

Ispra (regionalbackground)

Föhn,%

Stagnant,%

Föhn,%

Stagnant,%

Combustion, traffic 70 a 16 82 a 50Internal mixture of combustion/

traffic with secondary30 25 12 28

Secondary – 59 – 20Dust/soil – – 5 –

Unaccounted – – 1 2

Explanations: “Combustion, traffic”: Carbon clusters. “Internal mixture ofcombustion with secondary chemistry”: Carbon clusters together withnitrate and sulphate peaks. “Secondary”: Predominantly nitrate andsulphate peaks. “Dust/Soil”: Mg, Al, Fe, etc. peaks.

a During Föhn a significantly higher traffic contribution vs. combustionwas observed in Bresso (more carbon clusters with Ca/CaO peaks observedduring Föhn) compared to Ispra (see also text).

168 D. Mira-Salama et al. / Atmospheric Research 90 (2008) 159–169

of combustion/traffic with secondary components, i.e. nitrateand sulfate, are shown in plot b). The relatively high Ca peaktogether with carbon clusters indicates that diesel exhaust isan important fraction of this particle type (see Vogt et al.,2003). Plot c) depicts the mass spectrum of rather freshparticles from combustion other than traffic as indicated bythe absence of intense nitrate or Ca peaks in the spectrum.

Table 3 summarizes the major particle classes observedduring Föhn and non-Föhn conditions at the urban BG site inBresso and at the regional BG site in Ispra. The major particleclasses during Föhn conditions are carbonaceous classes atboth sites. At the urban site, all spectra contain carbon clusters(of which 30%mixedwith sulphate/nitrate), and no externallymixed sulphate/nitrate particles are found. At the regionalsite, 82% of the spectra are predominantly carbon-containing,and 12% mixed with sulphate and/or nitrate, during Föhnconditions.

Furthermore, at the urban BG site, Ca/CaO peaks togetherwith carbon peaks were simultaneously present in the massspectra of many particles that were characterized during Föhnepisodes. As Ca/CaO together with carbon on the same particleis a good indicator for diesel exhaust (as observed in severalstudies, see e.g. Vogt et al., 2003), this finding suggests thatmany particles originate from traffic. At the regional BG site onthe other hand, carbon clusters during Föhn are observedmainly as pure carbon spectra without Ca/CaO peaks, or asso-ciated with potassium, which indicates main local sources ofparticles other than traffic. These are especially stoves, domesticheating, wood burning and industrial combustion sources,which are all known to emit black carbon/carbonaceous par-ticles (e.g. see discussions in Mira-Salama et al., in press).

During non-Föhn conditions, a carbonaceous class (alsowith Ca/CaO peaks) appears as amajor class at the regional site,representing 50% of the spectra. The rest is 20% of (externallymixed) inorganic compounds (predominantly sulphate and/ornitrate, secondary chemistry) and finally 28% of a mixture ofthe two classes. At the urban site, 59% of the spectra aredominated by nitrate and/or sulphate (secondary chemistry),and 16% are predominantly carbonaceous (with small peaks ofCa/CaO) and finally 25% mixture of the two classes.

Summarizing, the big difference in carbonaceous particlespresent during Föhn conditions compared to stagnant con-

ditions at both locations has to be stressed: The urban siteshows 100% of the particles containing carbon clusters duringFöhn, against 41% during stagnant conditions. At regional theregional site difference is 82% vs. 50%. On the other hand,predominantly nitrate/sulfate containing particles (as an indi-cation of secondary chemistry) are much higher during non-Föhn conditions compared to Föhn conditions: Urban site:59% vs. 0% and at regional site: 20% vs. 0%. It is thus clearlyobserved that there are particle classes that are observedduring non-Föhn (“secondary compounds”) that are comple-tely absent during Föhn.

4. Summary and conclusions

Föhn events offer a unique opportunity to evaluate andcharacterize the local emissions of particles.We have used theoccurrence of Föhn events and stagnant conditions during theyears 2002–2007 to evaluate the local emissions at an urbanBG site in Bresso (Milan) and at a regional BG site in Ispra inNorthern Italy.

Physical and chemical characterization of the urban back-ground aerosol indicate consistently that local urban emissionsaccount for 50–65% of the PM mass in the Milan urbanbackground during winter time, whereas the regional back-ground accounts for 35–50% of themass. Local emissions occuras fine (Aitken mode) particles, consisting mainly of carbonac-eousmaterial (EC+POM).Volatilitymeasurements indicate thatfreshly emitted elemental carbon is not internally mixed withthe volatile (organic carbon and sulfate) particles. Particlesoriginated in the urban area come mainly from combustionprocesses, especially direct traffic emissions, domestic heating(includingwood burning) and industrial activities, whereas theregionally emitted particles are different with much less trafficcontribution. Nitrate, after POM the major secondary compo-nent during stagnant conditions, can be entirely apportioned tothe regional background aerosol.

Our findings suggests that, in order to improve air qualityin a big city like Milan, it is important to combine local trafficrestriction interventions with other long-term regional scaleair-quality-measures. This statement is in agreement with thestudies by Lonati et al. (2005) and Giugliono et al. (2005), whoconcluded that it is important also to look at the regional airpollution and contribution as well, when evaluating the effectof restrictions/air-pollution-measures in a big city.

Acknowledgement

We would like to thank the EU-III project EUROCHAMP(contract no. 505968) for the financial support to write thispaper.

References

Baltensperger, U., Streit, N., Weingartner, E., Nyeki, S., Prevot, A.S.H., VanDingenen, R., Virkkula, A., Putaud, J.-P., Even, A., ten Brink, H., Blatter, A.,Neftel, A., Gaggeler, H.W., 2002. Urban and rural aerosol characteriza-tion of summer smog events during the PIPAPO field campaign in Milan,Italy. Journal of Geophysical Research 107 (D22), 8193. doi:10.1029/2001JD001292.

Bukowiecki, N., Dommen, J., Prévôt, A.S.H., Richter, R., Weingartner, E.,Baltensperger, U., 2002. A mobile pollutant measurement laboratory —

measuring gas phase and aerosol ambient concentrations with highspatial and temporal resolution. Atmospheric Environment 36, 5569–5579.

169D. Mira-Salama et al. / Atmospheric Research 90 (2008) 159–169

Erdmann, N., Dell'Acqua, A., Cavalli, P., Gruening, C., Omenetto, N., Putaud,J.-P., Raes, F., Van Dingenen, R.A.S.H., 2005. Instrument characterisationand first application of the single particle analysis and sizing system(SPASS) for atmospheric aerosols. Aerosol Science and Technology 39,377–393.

Furger, M., Werner, R., Mitev, V., Frioud, M., Neininger, B., Prevot, A.S.H., 2005.Aerosol measurements in the Rhine valley during Foehn — another formperspective. Proceeding from the conference ICAMAP in Zadar, Croatia,from 23rd to 27th of May 2005, pp. 272–275.

Giugliano, M., Lonati, G., Butelli, P., Romele, L., Tardivo, R., Grosso, M., 2005.Fine particulate (PM2.5–PM1) at urban sites with different traffic ex-posure. Atmospheric Environment 39, 2421–2431.

Guazzotti, S.A., Whiteaker, J.R., Suess, D., Coffee, K.R., Prather, K.A., 2001. Real-time measurements of the chemical composition of size-resolvedparticles during a Santa Ana wind episode, California, USA. AtmosphericEnvironment 35, 3229–3240.

Hinz, K.-P., Erdmann, N., Grüning, C., Spengler, B., 2006. Comparative parallelcharacterization of particle populations with two mass spectrometricsystems LAMPAS 2 and SPASS. International Journal of Mass Spectro-metry 258, 151–166.

Lippmann, M., Ito, K., Nadas, A., Burnett, R.T., 2000. Association of ParticulateMatter Components with Daily Mortality and Morbidity in Urban Pollu-tion, Res., Rep., 95. Health Effects Institute, Cambridge, MA.

Lonati, G., Giugliano, M., Butelli, P., Romele, L., Tardivo, R., 2005. Majorchemical components of PM2.5 in Milan (Italy). Atmospheric Environ-ment 39, 1925–1934.

Marcazzan, G.M., Ceriani, M., Valli, G., Vecci, R., 2003. Source apportionmentof PM10 and PM2.5 in Milan (Italy) using receptor modelling. Science ofthe Total Environment 317, 137–147.

Mira-Salama, D., Grüning, C., Jensen, N.R., Cavalli, P., Putaud, J.-P., Larsen,B.R., Raes, F., Coe, H., in press. Source attribution of urban smog epi-

sodes caused by coal combustion. Atmospheric Research. doi:10.1016/j.atmosres.2007.11.025.

Moussiopoulos, N., Papalexiou, S., Lammel, G., Arvanitis, T., 2000. Simulationof nitrous acid formation taking into account heterogeneous pathways:application to the Milan metropolitan area. Environmental Modelling &Software 15, 629–637.

Novakov, T., Penner, J.R.E., 1993. Large contribution of organic aerosols tocloud-condensation-nuclei concentrations. Nature 365, 823–826.

Rodriguez, S., Van Dingenen, R., Putaud, J.-P., Martins-Dos Santos, S., Roselli,D., 2005. Nucleation and growth of new particles in the rural atmosphereof North Italy — relationship to air quality monitoring. AtmosphericEnvironment 39, 6734–6746.

Tegen, I., Hollrig, P., Chin, M., Fung, I., Jacob, D., Penner, J., 1997. Contribution ofdifferent aerosol species to the global aerosol extinction optical thick-ness: estimates from model results. J. Geophys. Res. 102, 23895–23915.

Putaud, J.-P., Van Dingenen, R., Raes, F., 2002. Submicron aerosol massbalance at urban and semirural sites in the Milan area (Italy). Journal ofGeophysical Research 107, 8198. doi:10.1029/2000JD000111.

Van Dingenen, R., et al., 2004. A European aerosol phenomenology — 1: phy-sical characterization of particulate matter at kerbside, urban, rural andbackground sites in Europe. Atmospheric Environment 38, 2561–2577.

Vogt, R., Kirchner, U., Scheer, V., Hinz, K.P., Trimborn, A., Spengler, B., 2003.Identification of diesel exhaust particles at an Autobahn, urban and rurallocation using single-particle mass spectrometry. Journal of AerosolScience 34, 319–333.

Wåhlin, P., Palmgren, F., Van Dingenen, R., 2001. Experimental studies ofultrafine particles in streets and the relationship to traffic. AtmosphericEnvironment 35 (1), 63–69.

Weber, R.O., Prévôt, A.S.H., 2002. Climatology of ozone transport from the freetroposphere into the boundary layer south of the Alps during North Foehn.Journal of Geophysical Research 107, 4030. doi:10.1029/2001JD000987.