Ambient Ultrafine Particle Levels at Residential ... - ACS Publications

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Ambient Ultrafine Particle Levels at Residential and Reference Sites in Urban and Rural Switzerland Reto Meier,†,‡ Marloes Eeftens,†,‡ Inmaculada Aguilera,†,‡ Harish C. Phuleria,†,‡,§ Alex Ineichen,†,‡ Mark Davey,†,‡ Martina S. Ragettli,†,‡,∥ Martin Fierz,⊥ Christian Schindler,†,‡ Nicole Probst-Hensch,†,‡ Ming-Yi Tsai,†,‡ and Nino Künzli*,†,‡ †

Swiss Tropical and Public Health Institute, Basel 4002, Switzerland University of Basel, Basel 4003, Switzerland § Centre for Environmental Science and Engineering, Indian Institute of Technology, Mumbai 110 016, India ∥ Department of Environmental and Occupational Health, School of Public Health, University of Montreal Montreal QC H3T 1J4, Canada ⊥ University of Applied Science Northwestern Switzerland, Windisch 5210, Switzerland ‡

S Supporting Information *

ABSTRACT: Although there is evidence that ultrafine particles (UFP) do affect human health there are currently no legal ambient standards. The main reasons are the absence of spatially resolved exposure data to investigate long-term health effects and the challenge of defining representative reference sites for monitoring given the high dependence of UFP on proximity to sources. The objectives of this study were to evaluate the spatial distribution of UFP in four areas of the Swiss Study on Air Pollution and Lung and Heart Diseases in Adults (SAPALDIA) and to investigate the representativeness of routine air monitoring stations for residential sites in these areas. Repeated UFP measurements during three seasons have been conducted at a total of 80 residential sites and four area specific reference sites over a median duration of 7 days. Arithmetic mean residential PNC scattered around the median of 10,800 particles/cm3 (interquartile range [IQR] = 7800 particles/cm3). Spatial within area contrasts (90th/10th percentile ratios) were around two; increased contrasts were observed during weekday rush-hours. Temporal UFP patterns were comparable at reference and residential sites in all areas. Our data show that central monitoring sites can represent residential conditions when locations are well chosen with respect to the local sources−namely traffic. For epidemiological research, locally resolved spatial models are needed to estimate individuals’ long-term exposures to UFP of outdoor origin at home, during commute and at work.

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INTRODUCTION Fine particulate matter (PM) poses a threat to human health.1 Recent attention has been given to the ultrafine particle fraction with aerodynamic particle diameters below 100 nm which usually dominate total ambient particle number counts.2 The small size of these ultrafine particles (UFP) facilitates translocation into the bloodstream and secondary organs3 and their large surface area increases the particle toxicity.4 There is evidence that UFP are associated with acute cardiovascular morbidity.5 While legal ambient standards regulating PM2.5 or PM10 exist in most countries, there are no regulations for UFP. Limited epidemiologic data on health impacts of long-term exposure to UFP and the existing regulations for PM are the main reasons for the absence of legislation for UFP. Other reasons are the disagreement on the particle metric to be regulated (particle number concentration [PNC], particle surface area, lung deposited surface area [LDSA], particle specific compounds such as elemental carbon or metals) and insufficient standardization in methods for UFP © XXXX American Chemical Society

measurement. Setting air quality standards for pollutants with high spatial variability comes with a further challenge, namely to define the density of the monitoring network and the selection of representative measurement sites. We now conducted for the first time a dense UFP monitoring campaign within four Swiss communities. The objective of these analyses were to evaluate the spatial and temporal patterns of UFP within communities and to investigate the representativeness of UFP concentrations at routine monitoring stations, operated by Swiss authorities, for the general population. Outdoor levels of particles in the size range from ∼15 to 300 nm at 80 homes in three urban areas (Basel, Geneva, Lugano) and one rural area (Wald ZH, hereafter referred to as Wald) of the Swiss Study on Air Pollution and Lung and Heart Diseases in Adults (SAPALDIA) Received: October 27, 2014 Revised: January 26, 2015 Accepted: February 3, 2015

A

DOI: 10.1021/es505246m Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology have been compared to reference measurements colocated with central monitoring stations and air pollution data from routine monitoring at those sites.

Table 1. Routine Monitoring Sites Selected for Co-Located UFP Reference Measurements

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MATERIALS AND METHODS Study Design. Repeated residential outdoor UFP measurements were conducted with miniature diffusion size classifiers6 (miniDiSC) at 80 sites during up to three seasons at each site in three urban areas (Basel, Geneva, Lugano) and one rural area (Wald) in Switzerland. In addition, reference measurements with miniDiSCs were conducted at reference sites colocated with area specific routine air monitoring stations of Swiss authorities. A total of 72 residential sites were homes of SAPALDIA subjects and eight sites were homes in proximity to selected SAPALDIA subjects which stopped participation. Maps showing all measurement locations in the four areas are provided in Supporting Information (SI) Figure S1. Criteria for site selection for residential measurements were (1) geographic coverage of the SAPALDIA cohort within each study area; (2) ensuring the representation of determinants of air pollution, such as population density, nearby traffic intensity, altitude and proximity to industry, and (3) agreement of subjects to participate. Residential UFP measurements were conducted for one- or two-week periods during three seasons in 2011 (Basel and Geneva) and 2012 (Lugano and Wald). Seasonal measurements campaigns within one area were timed within 4 weeks during which reference measurements were running continuously; exact timing of the measurements can be retrieved from SI Figures S2 and S3. All study participants gave written informed consent. Measurement Methods. Ultrafine particles at residential and reference sites were measured with a total of 14 miniDiSCs, providing the particle metrics PNC and LDSA. Side-by-side comparison under laboratory conditions showed good agreement of all 14 devices with a deviation from the lowest to the highest measured PNC within 25% (SI Figure S4). The miniDiSC has been shown to provide accurate results in the particle size range from 16 to 300 nm.7 Raw data collected at 1 s logging intervals have been processed with the provided software and averaged over 1 min. Minute-averages were excluded if a critical error was reported by the measurement device. Longer-term averages (entire measurements, daily, hourly) were excluded if more than 20% of minute-averages were missing or invalid, or if the sampling duration was shorter than 5.6 days (80% of 7 days). Reference UFP measurements were colocated with routine air monitoring stations operated by cantonal authorities and the National Air Pollution Monitoring Network (NABEL), described in Table 1. PNC data provided by authorities were measured with condensation particle counter (CPC) model #3022 (Basel St. Johannplatz) and model no. 3775 (NABEL stations) from TSI Incorporated (Shoreview, MN). Declared particle size ranges of these two CPCs are 7 nm to 1 μm for model no. 3022 and 4 nm to 1 μm for model no. 3775 (technical specifications from TSI). No comparable UFP data were available from routine monitoring stations in Geneva and Wald (not measured by local authorities). All reported averages of PNC and LDSA (entire measurements, daily, hourly) are based on arithmetic means. PNC reported in the text have been rounded to hundreds. All statistical analyses were conducted using STATA release 12 (StataCorp LP, College Station, TX). Temporal Adjustment for the Evaluation of Spatial Contrasts. Temporally adjusted PNC were used for the

area

station

type of location

Basel Basel Geneva Lugano Wald

St. Johannplatz Binningenb Ste-Clotilde Università Höhenklinik

urban suburban urban urban rural

distance to major roada

station operated by

18 m 224 m 11 m 50 m 1990 m

Canton of Basel NABELc Canton of Geneva NABELc Ostluftd

a Road with ≥5000 vehicles per 24 h. bOnly routine monitoring data used; no comparative UFP reference measurement at this site; the site is located in a green zone on the border of suburban Basel. cNational Air Pollution Monitoring Network. dCooperation of cantonal authorities in eastern Switzerland.

Table 2. Spatial Contrasts of Mean PNC within Areas Stratified by Weekday Rush-Hour and Non-Rush-Hour Periods Including Weekendsa

Basel

Geneva

Lugano

Wald

spring summer winter spring summer winter spring summer winter spring summer winter

sites

all time windows pooled

non rushhours

weekday rush-hour 6 am -9 am

weekday rush-hour 4 pm -7 pm

16 12 19 11 16 18 16 15 14 17 18 15

1.3 1.5 1.4 1.8 1.6 2.4 1.8 1.5 1.8 2.5 2.0 2.8

1.3 1.4 1.4 1.7 1.7 2.5 1.7 1.6 1.7 2.3 1.9 2.6

1.8 1.6 1.5 2.5 2.7 3.0 3.5 2.8 3.2 3.3 4.7 3.5

1.7 1.6 1.6 2.0 1.8 3.7 2.0 1.8 1.9 2.7 2.2 3.7

a

Contrasts are 90th/10th percentile ratios of averages adjusted for temporal variation within the seasonal measurement campaign.

evaluation of spatial contrasts of nonsimultaneously measured UFP levels within seasons shown in Table 2. Adjustments were based on the temporal variation of the area specific reference UFP measurement which was running continuously during the entire 4 weeks of each seasonal measurement campaign. Adjusted seasonal averages were calculated by multiplying the uncorrected concentration at site i (i = 1. . .80) in period j by the ratio between the concentration at the reference site during the four week long seasonal measurement campaign (“seasonal refsite concentration”) and the concentration measured at that same reference site during period j: adjusted seasona laveragei , j =

seasonal refsite concentration refsite concentrationj × uncorrected concentrationi , j

Diurnal PNC Patterns. Diurnal PNC patterns shown in Figure 3 are based on hourly averages which were combined according to the time of the day when they were measured. They combine nonsimultaneously measured PNC from all three seasons and all homes in an area. Road and Traffic Data. Road data with resolution 1:25 000 and traffic data were received from Swiss Topo (Federal Office of Topography) at a national level for the year 2008, with B


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Environmental Science & Technology modeled traffic intensity data for the same year. Major roads were defined as roads with ≥5000 vehicles per 24 h. This criterion has been adopted from the ESCAPE study.8

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RESULTS Data set. Repeated residential outdoor UFP measurements were conducted at 80 sites during up to 3 seasons at each site, resulting in a total of 187 valid measurements with a median duration of 167 h (range from 142 to 377 h). The total data set consists of 1558 daily and 41 732 hourly averages (partial days with less than 19 hourly averages were not considered for daily averages). Two 6 h outlier episodes at two sites and the corresponding daily averages were excluded from the comparative analysis with reference sites. The cause of these outlier peak events with PNC >1.0 × 106 particles/cm3 remains unclear, but may have been the consequence of a fire/barbeque in the garden. Reference UFP measurements, colocated with area specific routine monitoring stations, were available for Basel St. Johannplatz (50 days; reference measurement in summer not valid), Geneva Ste-Clotilde (80 days), Lugano Università (80 days) and Wald Höhenklinik (76 days). Air pollution data from routine monitoring stations were available for the entire years of 2011 and 2012, except for PNC at Basel St. Johannplatz for which we only had PNC data from March 2011 to December 2012. UFP at Residential Sites. Arithmetic mean PNC at residential sites scattered around the median of 10 800 particles/cm3 (interquartile range [IQR] = 7800 particles/ cm3), corresponding to a LDSA of 28.1 μm2/cm3 (IQR = 18.8 μm2/cm3). PNC were higher in winter than during spring and summer, most pronounced for Geneva and Lugano. Measured PNC stratified for area and season are shown as boxplots in Figure 1 and selected percentiles of PNC and LDSA are also

Figure 2. Mean PNC (miniDiSC measurements) as a function of their distance to the nearest major road with nonlinear regression line for all areas pooled. Total of 187 measurements at residential sites in Basel (23 sites), Geneva (20), Lugano (18), Wald (19), and 11 measurements at reference sites.

concentrations within areas (90th/10th percentile ratios) ranged from 1.3 to 2.8 (Table 2). Diurnal PNC Patterns. Diurnal PNC patterns had distinct morning and evening rush-hour peaks in all urban areas (Figure 3). PNC decreased to levels only slightly above night-time levels between the two peaks. Rush hour peaks were also visible in Wald but much less pronounced. Spatial contrasts within areas increased during weekday rush-hours to ratios from 1.5 to 4.7 (Table 2). PNC peaks and spatial contrasts were usually higher in the morning than in the evening. PNC at Reference Sites. Mean PNC measured at reference sites in Lugano and Wald were within the upper and lower quartiles of residential measurements. Measurements at reference sites in Basel and Geneva were usually higher than the upper quartile of the residential PNC (between the 80th and 90th percentile, Figure 1). The temporal pattern of PNC at reference sites was comparable to residential sites and showed corresponding high and low pollution episodes (exemplary time-series is shown in Figure 4, time-series for all areas and seasons are shown in SI Figure S3). Reference miniDiSC measurements colocated with routine monitoring stations in Basel St. Johannplatz and Lugano Università were in good agreement with data of CPCs from the routine measurement stations (Pearson correlation r > 0.97 for daily averages and r > 0.86 for hourly averages (SI Figure S5a and S5c)). However, average PNC from routine monitoring stations were lower in Basel (difference of 4000 particles/cm3) and higher in Lugano (difference of 1900 particles/cm3) compared to our colocated devices. We also found a high correlation between the reference measurement taken centrally at Basel St. Johannplatz and data from the more remote hillside routine monitoring station Basel Binningen located at the border of the city (r = 0.81 for daily averages and r = 0.61 for hourly averages (SI Figure S5b; distance between the two stations: 2.7 km). The average level of PNC at Basel Binningen was slightly lower than one of our reference measurement at St. Johannplatz (difference of 1200 particles/cm3). Correlation of UFP with Air Pollutants at Reference Sites. Ultrafine PNC and LDSA were highly correlated with a Person correlation of r = 0.91 for daily averages. The relationship between these two ultrafine particle metrics is

Figure 1. Average PNC from miniDiSC measurements at residential sites (box with quartiles and median; whiskers including most extreme values within 1.5 × IQR). Average miniDiSC PNC at area specific reference sites are indicated with a diamond (Basel St. Johannplatz, Geneva Ste-Clotilde, Lugano Università, Wald Höhenklinik). Total of 187 measurements during three seasons (spring, summer and winter) at residential sites in Basel (23 sites), Geneva (20), Lugano (18), Wald (19) and 11 measurements at reference sites.

provided in SI Tables S1 and S2). PNC decreased rapidly with increasing distance to major roads (Figure 2). Distances of residential sites to major roads were between 3 and 1200 m in urban areas and between 130 and 2600 m in Wald (SI Table S3). Distances of reference sites to major roads were between 11 and 1,990 m (Table 1). Spatial contrasts of average PNC C


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Environmental Science & Technology

Figure 3. Diurnal PNC patterns (hourly averages): 10th to 90th percentile range of levels measured at residential sites (dark gray area with dashed black line for median) overlaid with 10th to 90th percentile range of levels measured at reference site (light gray area with dashed gray line for median). Note the different scaling of Y-axes for individual areas.

the urban areas (r > 0.8) but not in Wald (Table 3). Very high correlations between PNC and routinely monitored CO and SO2 were found at the reference site in Lugano (no available data for those gases from other stations). Correlations between PNC and PM10 were low in Wald and Basel, and moderate to high in Geneva and Lugano (Table 3). Correlations between hourly averages were comparable to daily correlations but consistently lower.

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DISCUSSION Levels of UFP have been measured at residential sites in three urban areas and one rural area in Switzerland. Comparable residential PNC were found in urban areas, PNC were generally low at rural sites. Rapidly decreasing levels of UFP with increasing distance to major roads supports the notion that road traffic is a very relevant source of UFP in our measurement setting. Observed spatial contrasts of average PNC within areas were relatively low, despite the rapid decrease in proximity to traffic, which has been well described previously.9−11 However, increased spatial contrasts during rush-hours indicate the importance of temporal variation when evaluating exposure to UFP. Spatial and temporal contrasts within the four areas might have been considerably higher if nucleation mode particles had been taken into account. A recent study in Dresden, Germany, reported spatial long-term contrasts for PNC in the size range from 20 to 800 nm between

Figure 4. PNC time-series from the measurement period during Spring 2012 in Lugano. Daily average PNC of residential outdoor measurements (dark gray area for full range and black lines for the median), reference measurements colocated with routine monitoring stations (blue lines) and routine monitoring stations operated by authorities (red and orange lines). The number of parallel residential outdoor measurements is indicated by the light gray area at the bottom.

shown in SI Figure S6. Daily average PNC measured at reference sites showed high Pearson correlations with daily mean NO2 concentrations at the routine monitoring stations in

Table 3. Pearson Correlations of Daily and Hourly miniDiSC PNC Data Measured at Reference Sites and Air Pollution Data from Co-Located Routine Monitoring Stations (In Parentheses the Number of Compared Averages) PNC at Basel St. Johannplatz daily PNCstation PM10 station NO2 station NOstation NOx station COstation SO2 station

0.97 0.19 0.81 0.73 0.79

(23) (50) (48) (48) (48)

hourly 0.87 0.30 0.78 0.67 0.77

(592) (1253) (1234) (1234) (1234)

PNC at Geneva Ste-Clotilde daily

hourly

0.62 (81) 0.85 (81) 0.87 (81)

0.74 (1962) 0.80 (1962)

0.88 (79)

0.83 (1989)

D

PNC at Lugano Università daily 0.99 0.76 0.88 0.73 0.86 0.93 0.95

(80) (80) (80) (80) (80) (80) (80)

hourly 0.90 0.50 0.74 0.72 0.63 0.74 0.81

(1993) (1981) (1903) (1903) (1903) (1986) (1903)

PNC at Wald Höhenklinik daily 0.07 0.45 0.49 0.49

(73) (76) (76) (76)

hourly 0.13 0.35 0.46 0.45

(1864) (1896) (1896) (1896)


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cumulative daily exposure dose.17 Time spent indoors further complicates exposure estimation because indoor concentrations of UFP of outdoor origin are usually lower than outdoors and the indoor/outdoor ratio may vary.18 In addition, it is important to consider specific indoor sources if the total exposure to UFP from all sources is of interest: major contributions from cooking and candle burning to the total daily dose have recently been reported.19−21 We found high Pearson correlations between daily and hourly averages of PNC and NO2 at reference sites in the three urban areas. High correlations between UFP and nitrogen oxides in traffic influenced environments have been reported previously22,23 and accurate predictions of UFP levels based on nitrogen oxides have recently been reported for street sites in Antwerp, Belgium.24 Given the large number of studies publishing associations between local NO2 and various health outcomes,25 it will be relevant to know to what extent NO2 might have served as a marker of exposure to UFP and whether our findings can be generalized beyond our four areas. While no predictions based on other pollutants can be made for rural sites, as correlations between UFP and other pollutants are low, the link between NO2 and UFP may be more consistent within cities. It is, nevertheless, important to investigate the health effects of UFP with personal UFP exposure estimates and to analyze associations also in two-pollutant models, including NO2, to understand the specific contributions of UFP. This is a prerequisite for estimating the public health benefits of regulations and standards targeting UFP specifically. We have also seen very high correlations between PNC, CO and SO2 in Lugano for which we do not have a clear explanation. There are no major industrial sources nearby which would explain high correlations of these three pollutants. Local emissions from road traffic and boats on the nearby lake may explain the correlation of UFP and CO. The correlation with SO2 may also relate to such local sources from water-traffic. Else, SO2, is rather influenced by long-range transport as the region is strongly influenced by emissions from northern Italy.26 As those may also contribute to the formation of secondary UFP correlations with SO2 may also be partly explained by the characteristics of the background levels of pollution in the air shed of the larger region. Our data support the selection of a few monitoring locations per area to control compliance of ambient UFP standards. These sites have to be well chosen in respect to local UFP sources, namely traffic, and should be exposed to average and elevated traffic levels in the respective area. The total number of sites required for UFP monitoring depends on the size and geographical heterogeneity of an area, prevailing wind directions and local topography (vertical gradients). However, data from central sites are not sufficient for epidemiological research due to relatively large contrasts within small distances to roads. For this purpose more spatially resolved measurements and models are needed to estimate individuals’ longterm exposures to UFP of outdoor origin at home, during commute and at work.

urban and suburban street and background sites which are very similar to the within area contrasts we report here. Yet, in the same study much higher contrasts, up to a ratio of nine, were found for particles in nucleation mode (5 to 20 nm).12 Small changes in the measured particle size range can lead to large changes in absolute PNC which is a consequence of the large number of nucleation mode particles in traffic environments. Differently measured particle size ranges may also be the reason for the relatively constant shifts between our miniDiSC measurements and data from colocated CPCs of routine monitoring stations in Basel and Lugano. Similar shifts between different measurement devices have been reported previously.7 The evaluation of the temporal changes at the various sites revealed rather interesting findings. Although residential sites covered a large range of absolute levels of UFP and of distances to major roads, high and low pollution episodes coincided across the various homes within each of these four areas (Figure 4 and SI Figure S3). The temporal patterns also coincided with those observed at the area specific reference sites. This is likely explained by the similar weather patterns within these areas, and the fact that temporal variation of traffic density correlates across the areas. Nevertheless, given the substantial spatial contrasts of PNC within areas, the definition of “representative sites” remains a challenge. As shown in our data, reference sites in Lugano and Wald had very similar absolute UFP levels compared to the median residential site. This confirms that well-chosen monitoring sites, in respect to local traffic and other potential UFP sources, can reflect average UFP levels in an area. A recent study in Antwerp, Belgium, reporting representative PNC in the 25−300 nm size range at a central monitoring station compared to three sites within one kilometer came to the same conclusion.13 In contrast, the reference sites in Basel and Geneva represented conditions in the upper tail of the distribution of values seen at residential sites. These differences reflect the increased traffic exposure of these two sites. Traffic loads in the proximity of these two sites were about four times (Basel) and seven times (Lugano) higher than at the median residential site (SI Table S4). However, one has to keep in mind that the selection of residential sites, based on long-term participants of SAPALDIA, is not fully representative of the population at large. As usual in cohort studies, participants are a selected subgroup. Loss to follow-up was higher in the least affluent14 which may lead to underrepresentation of the more polluted areas. The objective of clean air policies is to protect all people from adverse health effects, thus, monitoring is needed also at the most polluted sites as those reflect the living conditions of the most exposed part of the population. The substantial spatial and temporal variation of UFP may be a relevant source of exposure misclassification15 and needs to be taken into account in epidemiological studies investigating the long-term health effects of traffic-related UFP. In such studies, long-term exposure to UFP needs to be estimated for each study participant, thus, exposure assessments cannot solely be based on data from any reference site. More precise exposure estimates can be achieved with land use regression (LUR) models considering local traffic variables and building structures.16 A further challenge in the estimation of personal exposure to traffic related UFP relates to the dependence of exposure on daily time-activity patterns which is possibly more substantial than in the case of more homogeneously distributed pollutants such as PM2.5. As shown for Basel, activities such as commuting can contribute to a significant amount of the

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ASSOCIATED CONTENT

S Supporting Information *

Table S1, Average PNC at residential sites. Table S2, Average LDSA at residential sites. Table S3, Distances between homes and major roads. Table S4, Traffic load in proximity to measurement sites. Figure S1, Maps with measurement sites. Figure S2, Timing of UFP measurements. Figure S3, PNC time E


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(6) Fierz, M.; Houle, C.; Steigmeier, P.; Burtscher, H. Design, calibration, and field performance of a miniature diffusion size classifier. Aerosol Sci. Technol. 2011, 45 (1), 1−10. (7) Meier, R.; Clark, K.; Riediker, M. Comparative testing of a miniature diffusion size classifier to assess airborne ultrafine particles under field conditions. Aerosol Sci. Technol. 2013, 47 (1), 22−28. (8) Beelen, R.; Hoek, G.; Vienneau, D.; Eeftens, M.; Dimakopoulou, K.; Pedeli, X.; Tsai, M.-Y.; Künzli, N.; Schikowski, T.; Marcon, A.; Eriksen, K. T.; Raaschou-Nielsen, O.; Stephanou, E.; Patelarou, E.; Lanki, T.; Yli-Tuomi, T.; Declercq, C.; Falq, G.; Stempfelet, M.; Birk, M.; Cyrys, J.; von Klot, S.; Nádor, G.; Varró, M. J.; Dėdelė, A.; Gražulevičienė, R.; Mölter, A.; Lindley, S.; Madsen, C.; Cesaroni, G.; Ranzi, A.; Badaloni, C.; Hoffmann, B.; Nonnemacher, M.; Krämer, U.; Kuhlbusch, T.; Cirach, M.; de Nazelle, A.; Nieuwenhuijsen, M.; Bellander, T.; Korek, M.; Olsson, D.; Strömgren, M.; Dons, E.; Jerrett, M.; Fischer, P.; Wang, M.; Brunekreef, B.; de Hoogh, K. Development of NO2 and NOx land use regression models for estimating air pollution exposure in 36 study areas in EuropeThe ESCAPE project. Atmos. Environ. 2013, 72 (0), 10−23. (9) Zhu, Y.; Hinds, W. C.; Kim, S.; Sioutas, C. Concentration and size distribution of ultrafine particles near a major highway. J. Air Waste Manage Assoc 2002, 52 (9), 1032−42. (10) Hagler, G. S. W.; Baldauf, R. W.; Thoma, E. D.; Long, T. R.; Snow, R. F.; Kinsey, J. S.; Oudejans, L.; Gullett, B. K. Ultrafine particles near a major roadway in Raleigh, North Carolina: Downwind attenuation and correlation with traffic-related pollutants. Atmos. Environ. 2009, 43 (6), 1229−1234. (11) Zhou, Y.; Levy, J. I., Factors influencing the spatial extent of mobile source air pollution impacts: A meta-analysis. Bmc Public Health 2007, 7. (12) Birmili, W.; Tomsche, L.; Sonntag, A.; Opelt, C.; Weinhold, K.; Nordmann, S.; Schmidt, W. Variability of aerosol particles in the urban atmosphere of Dresden (Germany): Effects of spatial scale and particle size. Meteorol. Z. 2013, 22 (2), 195−211. (13) Mishra, V. K.; Kumar, P.; Van Poppel, M.; Bleux, N.; Frijns, E.; Reggente, M.; Berghmans, P.; Panis, L. I.; Samson, R. Wintertime spatio-temporal variation of ultrafine particles in a Belgian city. Sci. Total Environ. 2012, 431, 307−313. (14) Ackermann-Liebrich, U.; Kuna-Dibbert, B.; Probst-Hensch, N. M.; Schindler, C.; Felber Dietrich, D.; Stutz, E. Z.; Bayer-Oglesby, L.; Baum, F.; Brandli, O.; Brutsche, M.; Downs, S. H.; Keidel, D.; Gerbase, M. W.; Imboden, M.; Keller, R.; Knopfli, B.; Künzli, N.; Nicod, L.; Pons, M.; Staedele, P.; Tschopp, J. M.; Zellweger, J. P.; Leuenberger, P.; Team, S. Follow-up of the Swiss Cohort Study on Air Pollution and Lung Diseases in Adults (SAPALDIA 2) 1991−2003: Methods and characterization of participants. Soz Praventivmed 2005, 50 (4), 245−63. (15) Fuller, C. H.; Brugge, D.; Williams, P. L.; Mittleman, M. A.; Durant, J. L.; Spengler, J. D. Estimation of ultrafine particle concentrations at near-highway residences using data from local and central monitors. Atmos. Environ. 2012, 57 (0), 257−265. (16) Ragettli, M. S.; Ducret-Stich, R. E.; Foraster, M.; Morelli, X.; Aguilera, I.; Basagaña, X.; Corradi, E.; Ineichen, A.; Tsai, M.-Y.; ProbstHensch, N.; Rivera, M.; Slama, R.; Künzli, N.; Phuleria, H. C. Spatiotemporal variation of urban ultrafine particle number concentrations. Atmos. Environ. 2014, 96 (0), 275−283. (17) Ragettli, M. S.; Corradi, E.; Braun-Fahrländer, C.; Schindler, C.; de Nazelle, A.; Jerrett, M.; Ducret-Stich, R. E.; Künzli, N.; Phuleria, H. C. Commuter exposure to ultrafine particles in different urban locations, transportation modes and routes. Atmos. Environ. 2013, 77 (0), 376−384. (18) Meier, R.; Eeftens, M.; Phuleria, H. C.; Ineichen, A.; Corradi, E.; Davey, M.; Fierz, M.; Ducret-Stich, R. E.; Aguilera, I.; Schindler, C.; Rochat, T.; Probst-Hensch, N.; Tsai, M.-Y.; Künzli, N., Differences in indoor versus outdoor concentrations of ultrafine particles, PM2.5, PMabsorbance and NO2 in Swiss homes. J. Exposure Sci. Environ. Epidemiol. 2015; doi: 10.1038/jes.2015.3.

series of all measurements. Figure S4, Side-by-side comparison of miniDiSCs. Figure S5, Comparison of miniDiSCs and CPCs at reference sites. Figure S6, Comparison of PNC and LDSA. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +41 61 284 83 99; fax: +41 61 284 81 01; e-mail: nino. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the field workers carrying out the air pollution measurements, the whole SAPALDIA team (listed in the Supporting Information), and in particular the late Prof. LeeJane Sally Liu (1965-2011) who initiated this exposure assessment project. We also thank the cantonal and national authorities for providing routine monitoring data and the study participants for their help during the data collection period. This study was funded by Grant No. 324730_135673 from the Swiss National Science Foundation (SNSF) and supported by the Federal Office for the Environment (FOEN). SNF is the main funder of SAPALDIA since its start in 1990 with grants no 33CSCO-134276/1, 33CSCO-108796, 3247BO-104283, 3247BO-104288, 3247BO-104284, 3247-065896, 3100059302, 3200-052720, 3200-042532, 4026-028099, PMPDP3_129021/1, PMPDP3_141671/1.

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ABBREVIATIONS IQR interquartile range LDSA lung deposited surface area NABEL National Air Pollution Monitoring Network NO2 nitrogen dioxide PM2.5 mass concentration of particles less than 2.5 μm in size PM10 mass concentration of particles less than 10 μm in size PNC particle number concentration SAPALDIA Swiss Cohort Study on Air Pollution and Lung and Heart Diseases in Adults UFP ultrafine particles

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