Concentration of Ultrafine Particles near Roadways in an Urban Area in Chicago , Illinois

Monitoring ambient concentrations of ultrafine particles (UFPs) is important due to their negative impact on human health. This paper provides measurements of UFP concentrations near roadways because emissions from light duty vehicles (LDVs) and heavy duty vehicles (HDVs) have been shown to be a major source of UFPs. The concentration of UFPs was measured near two different roadways in Chicago IL for 2 to 4 hours on 52 days between 2014 and 2016. One of the sites was restricted to LDVs (Lake Shore Drive, LSD) and had a near roadway concentration from vehicles (background subtracted) that averaged nearly 8,000 particles cm. The other site had a mix of LDVs and HDVs (Dan Ryan Expressway, DRE) and the near roadway concentration from the vehicle fleet (background subtracted) averaged nearly 11,000 particles cm. The contribution of UFPs from HDVs was almost 80% of the total emissions on the DRE demonstrating that HDVs emit many more UFPs than LDVs. Background concentrations of UFP were measured upwind of the near roadway sampling sites and were subtracted from the near roadway measurements in order to determine the vehicle contribution to the total UFP concentration. The background concentrations varied with wind direction and therefore were divided into ambient background categories based on wind direction. The four different background categories are defined as remote, lake, industrial and urban. Each category has a distinct different average ambient background concentration (particles cm) as follows: remote, 2,700; lake, 6,000; industrial 12,000 and urban 11,000. The large background concentrations in urban areas indicate that total near roadway measurements are generally near 20,000 particles cm with 50 to 60% from vehicles and reach to 60,000 particles cm depending on the background and traffic emission. The results indicate high UFP readings near roadway and one possible solution is mitigation of traffic congestion.


INTRODUCTION
Ultrafine particles (UFPs) are defined as particles with diameter less than 0.1 µm and recent studies have shown an association between UFPs and adverse health effects (Pope III et al., 2002;Oberdörster et al., 2004;Sioutas et al., 2005;Pope III and Dockery, 2006;Samet et al., 2009;Brook et al., 2010;Chen et al., 2017).The epidemiological studies indicate that particulate pollution may be responsible and attributed to asthma and cardiovascular effects, hospitalizations and mortality rates.Primary sources of UFP in urban region include power generation, motor vehicles, domestic heating and industrial facilities.Besides direct emissions, secondary sources formed by gas-to-particle conversion can also result in the production of UFPs (e.g., condensation of sulfuric acid on pre-existing particle) (Seinfeld and Pandis, 2016).Due to the diverse emission sources and complex transport mechanism, it is difficult to identify the origin of UFPs but source apportionment (Cass et al., 2000;Pakkanen et al., 2001;Hu et al., 2014;Liu et al., 2014) and modeling studies (Marmur et al., 2006;Wang et al., 2009;Posner and Pandis, 2015) indicate that vehicle emissions are the major source of UFPs in urban areas.
To mitigate human exposure, vehicle emission of UFPs from individual vehicles need to be restricted.An UFPs number emission standard has been established in Europe since 2014 (Euro VI) (ICCT, 2016), however, it is still an unregulared pollutant in National Ambient Air Quality Standards (NAAQS) of the United States (NAAQS, 2015).To develop a standard for UFPs, numerous data collection and evaluation are needed.On and near roadway field measurements of UFP concentrations have been made by a number of investigators (Morawska et al., 1999;Zhu et al., 2002a, b;Kittelson et al., 2004;Westerdahl et al., 2005;Zhang et al., 2005;Zhai et al., 2015;Xiang et al., 2018).They demonstrate that UFPs are significantly elevated on and near roadways and decrease as a function of distance from the road and are indistinguishable from background concentration at downwind distances greater than 300 m.They also determined that heavy duty vehicles (HDVs) produce higher emissions of UFPs than light duty vehicles (LDVs) and that vehicle mode of operation (e.g., high frequency of acceleration and deceleration) is also a critical factor in producing high emissions.These studies verified laboratory studies of particle emission factors (EFs, pollutant emitted per unit distance) as a function of vehicle mode of operation (Morawska et al., 2005).Also, design of roadway structures is suggested as a practical solution to reduce human exposure from roadway (Hagler et al., 2009;Hagler et al., 2012;Lin et al., 2016).Results suggest that solid barriers and roadway vegetation can reduce UFP concentrations in close vicinity of the roadway.However, the critical parameter of this affect is not explicit and needs more investigation.
In this study, the concentration of UFPs was measured near two different roadways in Chicago IL for 2 to 4 hours on 52 days between 2014 and 2016.One of the sites was restricted to LDVs (Lake Shore Drive.LSD).The other site had a mix of LDVs and HDVs (Dan Ryan Expressway, DRE).During our study, the ambient temperature was 25 (± 3)°C in summer, 21 (± 4)°C in fall; the relative humidity was 66 (± 9)% in summer, 64 (± 7)% in fall.Background concentrations of UFP were measured upwind of the sampling sites and were subtracted from the near roadway measurements in order to determine the vehicle contribution to the total UFP concentrations.The background concentrations varied with wind direction and therefore were divided into ambient background categories based on wind direction.The four different background categories are defined as remote, lake, industrial and urban.Each category has a distinct different average ambient background concentration (particles cm -3 ).Based on the emission factors and measured traffic flow rate for LDVs and HDVs, the measured concentrations of UFPs are separated into the contribution from LDVs and HDVs.

METHODS
A three-year sampling program was conducted between 2014 and 2016 (summer and fall) near two roadways in Chicago, IL (see Fig. 1) using handheld, fast response instruments.LSD is a two-directional roadway close to Lake Michigan with four lanes each direction, restricted to LDVs.DRE is a two-directional roadway a few kilometers south of the city center with seven lanes each direction and allows both LDVs and HDVs.Information from the Illinois Department of Transportation (IDOT, 2016), provides the annual average daily traffic flow rate is 160,000 veh day -1 for LSD and for DRE is 300,000 veh day -1 with 8.4% of HDVs.Measurements of the UFP concentration were made upwind and downwind from the roadways (Fig. 2), The upwind measurements were considered background and divided into ambient background categories based on wind direction (see Fig. 1 and Table 1).Fig. 1 indicates that remote background occurred for winds from N to NE where the nearest emission sources were more than 450 km on the other side of Lake Michigan.For winds coming from ENE to E, the ambient background categories were characterized as lake because of the shorter trajectory (80 km) over the lake.The winds from the ESE to SSE sector were categorized as industrial background due to atmospheric emissions from Fig. 1.Map of sampling sites (Lake Shore Drive and Dan Ryan Expressway) is shown with ambient background categories (Remote, Lake, Industry, Urban).Distances to the nearest sources are identified.industrial sources (two large steel mills, two power plants, a major oil refinery and numerous other smaller industrial sources) that emit large amount of UFPs and sulfur dioxides and nitrogen oxides resulting in gas-to-particle conversion.All of the westerly wind directions at the sampling sites provide air flow from the Chicago urban area and therefore are classified as urban background.
All samples were averaged for 5-min intervals and were collected on 52 sampling day (19 days for LSD, 33 days for DRE) in the afternoon and evening depending on traffic and meteorological conditions.Ambient background samples were collected at the beginning and the end of each sampling day at background sampling sites, assuming the mass in the ambient air is constant for 2 to 4 hours.The average of the background measurements were used to compute traffic contribution.The locations of background, near road sampling sites and the surrounding areas are presented in Fig. 2. For LSD, background was measured 100 m upwind, depending on the wind direction.Near roadway sampling location A was located at the west side (shown in Fig. 2(a)) and east side (not shown) of the roadway for easterly wind and westerly wind, respectively.When sampling near DRE, the background concentrations were measured upwind 200 m on the east side and 100 m on the west side of the roadway.Near roadway sampling location B (Fig. 2(b)) was at the edge of the subway platform and in the middle of the roadway.The DRE site analysis only used one-side traffic and pollutant measurements.
Total UFPs number concentrations were measured by a condensation particle counter (CPC) (TSI 8525).Daily zero check of the CPC was performed.CPC measurements were periodically compared to two other CPCs (TSI 3007, size range: 10-1000 nm) and a SMPS (TSI 3910, size range: 10-420 nm) in laboratory.Wind speed and direction, temperature and relative humidity were measured and logged with a hand-held weather station (Kestrel 4500).Traffic flow and speed were obtained from video recordings (Sony HDR-CX330) taken simultaneously with other measurements.Each 5-min is a separate sampling period.The length of the sampling (usually 2 to 4 hours) on the meteorology and traffic condition.Sampling was conducted only when the wind direction was within 45° of normal to the roadway.This allows averaging data for different days (Zhu et al., 2002a, b).
Subtraction of the background UFP concentrations from near roadway UFP measurements provides the ambient concentration of UFPs from the roadway traffic to be determined (see Eq. ( 1)).Categorizing the data by different ambient background sectors allows the evaluation of human exposure to UFPs in urban area.
In Eq. ( 1) the index t refers to a particular time of the sampling period.Where C UFP,t is UFP near roadway measurements (particle cm -3 ).C UFP,t,bg is UFPs measured at background location (particle cm -3 ).C UFP,t,traffic is UFP concentrations due to the traffic (particles cm -3 ).
When the measurements of traffic flow rate for LDVs and (2) EF UFP,LDV and EF UFP,HDV are the UFP emission factors for LDVs and HDVs expressed as particles emitted per kilometer (particles vehicle -1 km -1 ), respectively.N LDV,t and N HDV,t are the total traffic flow rate for each type vehicle composition (3,600 × vehicle s -1 ).K m,t is eddy diffusivity (m 2 s -1 ) which has been related to vehicle-induced turbulence by a number of investigators (Pavageau and Schatzmann, 1999;Sahlodin et al., 2007;Gordon et al., 2012) and is considered to be a measure of the strength of the turbulent flow in analogy with molecular viscosity.The value of emission factors and eddy diffusivity have been evaluated in a previous paper (Xiang et al., 2018) and are available for use in this study.

Concentration of UFPs in Ambient Air by Different Ambient Background Categories
Table 2 records a comparison of UFP concentrations measured near different roadways.The concentration measured from roadways is influenced by both vehicle emission and dispersion of pollutants.The differences between the results can be explained by the traffic composition, road geometry, variability in meteorology, lack of unity of measuring instruments and different fuel content between studies can also results in significant differences in the measured concentrations (Morawska et al., 2005;Kumar et al., 2010).Fig. 3(a) shows the average UFP concentrations measured for the four different ambient background categories measured at the LSD and DRE background sampling sites (Table 1).Each category has a distinct average ambient background concentration (particles cm -3 ) reported using average background values of LSD and DRE: remote, 2,700; lake, 6,000; industrial 12,000 and urban 11,000.The lake background sector has larger concentrations than the remote background sector.This is probably due to the longer trajectory over water for the remote site (450 km) than for the lake sector (80 km).In a study by Ketzel et al. (2004), a near water site was identified with similar UFP concentrations.The UFP background concentrations for the industrial sector are much higher than the lake sector backgrounds and also shows significantly more variation as indicated by the higher standard deviations.The larger number of UFP emission sources in this sector and the much shorter trajectory over water (25 km) can account for this difference.It is also important that the DRE site has much larger concentrations from the industrial sector than the LSD site.The DRE is further from the lake and closer to the emission sources than the LSD site.Young and Keeler (2007) also measured high UFP concentrations at a distance of 5 km from industrial sources as shown in Fig. 3(a).Measurements of urban background concentrations were similar at both sampling locations and also similar to a study by Morawska et al. (2008) who reported UFP concentrations in urban areas of 1 to 2 × 10 4 particle cm -3 .Fig. 3(b) provides the UFP concentrations with background subtracted for the two roadway monitoring sites, using Eq. ( 1).The results indicated that roadway measurements are near 10,000 particles cm -3 which is comparable to the urban background values that are near 12,000 particles cm -3 .When the roadway concentrations are added to the background concentrations the average concentrations near the roadways were between 20,000 and 25,000 particles cm -3 .The UFP concentrations near LSD were much lower than near DRE even though the total vehicle flow rate on LSD is nearly two times greater than on the DRE.This difference is due to the fact that the UFP emission factors for HDVs is 100 times larger than the UFP emission factors for LDVs (Wang et al., 2010).

UFP Emissions from Urban Roadways
Fig. 4 provides information on the large variation in measured UFP concentrations due to variations in background and traffic conditions.The figure shows the difference in concentration between and within each sampling day.Changes in background concentration had a large effect on the concentration of UFPs measured near the roadways.This is indicated in the figure by the large difference in the contribution of traffic to the total concentration on different days.When the background concentrations are from the remote sector the roadway contributes between 80 and 95% of the measured values.This is reduced to as little as 5% for the industrial sector.The figure also indicates that there is a large difference in concentration during each sampling day (between 10,000 and 20,000 particles cm -3 ).This difference is due to short term differences in traffic flow rate and percentage of trucks in the fleet mix during the 2 to 4 hours sampling period each day.The overall range of concentrations near the two roadways varied from 5000 to 60,000 particles cm -3 due to differences in background concentration and traffic emissions.For both roadways, when background concentrations came from remote sector associated with low vehicle emission, the roadway measurements can be as low as 5,000 particles cm -3 .When high vehicle emissions occurred with industrial background concentrations, the near roadway measurements ranged from 35,000 to 60,000 particles cm -3 , for LSD and DRE, respectively.

Contribution of LDVs and HDVs to Roadway Concentrations
Fig. 5 provides the variations in UFP concentrations for LDV and HDV (3-12%) as a function of vehicle mode of operation using Eq. ( 2) and emission factors in Table 3.The contribution of UFP concentrations from HDVs were Fig. 5. UFP concentrations from HDV and LDV calculated using Eq. ( 2) and emission factors in Table 3 for 3-12 HDV% as a function of vehicle mode of operation (congestion and free flow) on DRE.
between 80% and 90% due to much larger emission factor for HDVs (Imhof et al., 2005;Morawska et al., 2005;Birmili et al., 2009;Keogh et al., 2010).HDVs not only generate more emissions than LDVs they also produce significantly more turbulent mixing (Gordon et al., 2012) and this causes the minimum UFP concentration to occur when the fleet flow rate is highest.Fig. 5 also indicates that traffic congestion conditions generates significant amounts of UFPs from HDVs.For HDVs, UFP emission factors are more than two times higher in congestion than in free flow, indicating more emissions during congestion.On the contrary, emission factors of LDV show higher value in free flow than in congestion (Zhai et al., 2015;Xiang et al., 2018).And these differences between LDV and HDV emission factors not only result in distinct UFP emission contribution but also the different trend of UFP production.

CONCLUSIONS
This study demonstrates that there is a significant number of UFPs in urban air.The average concentration in urban air was near 12,000 particles cm -3 and increased to 20,000 to 25,000 particles cm -3 near roadways.The overall concentration of UFPs was highly variable (5,000-60,000 particles cm -3 ) due to differences in background concentrations.The highest concentrations near roadways occurred when the background concentrations were high due to emissions of UFPs from industrial sources.HDVs have a large impact on near roadway concentrations due to the large amounts of UFPs that they emit.

DISCLAIMER
Reference to any companies or specific commercial products does not constitute endorsement.

Fig. 3 .
Fig. 3. Concentrations of UFP (a) subdivide by ambient background categories measured at background sites and (b) near roadway measurements with ambient background subtracted.Average values are shown with one standard deviation.Literature results for remote and urban categories fromMorawska et al. (2008); Lake category fromKetzel et al. (2004); Industry category fromYoung et al. (2007).

Fig. 4 .
Fig. 4. Variation in measured UFPs near LSD (left) and DRE (right) with different ambient background sectors for each sampling day.The measured near roadway concentrations including background ranged from 5,000 to 60,000 particles cm -3 .The near roadway concentrations vary by 30 to 40% on most days due to variation in traffic flow rate.The near roadway concentrations represent 80 to 95% of the total concentration for remote background conditions and only 5 to 40 % for industrial background conditions.The Lowest and highest background concentrations for each category are shown.

Table 1 .
Wind direction related to ambient background categories.HDVs are available, data are subdivided into different traffic intervals (1000 veh h -1 ) based on the total flow rates for outliers and spikes to analyze.And the UFP concentrations due to the emissions from different vehicle types determined by Eq. (2):

Table 2 .
Compare particle number concentrations in this study to other studies.

Table 3 .
Average particle number emission factors (EFs) for different vehicle type under different traffic conditions based on previous studies.