Skip to content U.S. Department of Transportation/Federal Highway AdministrationU.S. Department of Transportation/Federal Highway Administration

Office of Planning, Environment, & Realty (HEP)
PlanningEnvironmentReal Estate

HEP Events Guidance Publications Awards Contacts

FHWA and EPA National Near-Road Study Las Vegas

7 Results and Discussion

The study design, methods, and general data trends are the focus of this report. An assessment of one of the data quality indicator (DQI) goals as stated in the quality assurance project plan (QAPP) is shown in Table 15 for certain major parameters. As may be seen in Table 15, the DQI goal for data completeness was met or exceeded.

7.1 Traffic Activity

Traffic data (Figure 3) indicated a tri-modal traffic distribution as opposed to a bi-modal distribution. This is believed to be the result of several factors: 1) Las Vegas is not typical commuter city; 2) Las Vegas is a recreation destination for many travelers; 3) shift changes in Las Vegas are later or earlier in the day depending on the employer; 4) study site is along an interstate that carries both inter- and intra- state traffic; and 5) I-15 is a North American Free Trade (NAFTA) corridor.

Monthly traffic volumes shown in Figure 20 also indicate a tri-modal traffic distribution as opposed to a bi-modal distribution. Figure 21 shows the average daily traffic volume by the day of the week for each month. Even though the traffic data for December is only for part of the month (site operation began mid-December, 2008), traffic volume for December is much higher than for the remainder of the project. This is believed to be the result of Las Vegas being a recreation destination during the December holiday period.

Box-whisker plots for weekday and weekend traffic volume (Figure 22) and seasonal traffic volume (Figure 23) are shown on the following pages.

Average Hourly Traffic Volume by Month at I-15 Site - Description: The patterns are very similar, with most of the traffic volume occuring during mid day. May sees the highest traffic volume, which November sees the lowest.

Figure 20. Average Hourly Traffic Volume by Month at I-15 Site.

Average Daily Traffic Volume by Day-of-Week at I-15 Site - Description: Traffic is comparative over the course of the week, with no noticeable differences. July sees the most volume while October sees the least.

Figure 21. Average Daily Traffic Volume by Day-of-Week at I-15 Site.

Box-Whisker Plot -- average hourly traffic volume by weekday and weekend. The majority of the ovlume is between 140000 and 160000.

Figure 22. Box-Whisker Plot -- average hourly traffic volume by weekday and weekend[1].

Box-Whisker Plot -- average hourly traffic volume by weekday and weekend. The majority of the ovlume is between 140000 and 160000.,Box-Whisker Plot -- average hourly traffic volume by season. Fall sees the majority fall between 3500 and 7000. Spring sees the majority fall between 6000 and 10000. Summer sees the majority fall between 5500 and 9500. Winter sees the majority fall between 5000 and 8000.

Figure 23. Box-Whisker Plot -- average hourly traffic volume by season.

7.2 Meteorology

Las Vegas meteorological conditions can be problematic as the city is located in a valley surrounded by mountains that channel the wind. This channeling of wind presented technical challenges in site selection3 and operation and achieving proper wind flow from the source to the detector (i.e., gas analyzers). The topographic and meteorological conditions are shown in Figure 13.

Meteorological data from nearby McCarran International Airport shows (Figure 14) that the wind blows predominately from the southwest quadrant until approximately 3PM. Beginning at about 3PM in the afternoon, the winds shift and become more variable. In the late afternoon early evening (6-9PM), the winds are predominately from the east and northeast. Then, after 9PM the winds shift again into a more variable pattern. After midnight the winds blow predominately from the southwest.

Figure 4 shows wind rose data collected by the R.M. Young sonic anemometers at the closest station to I-15, approximately 20 m to the east of the highway. During the study, winds were predominately from the southwest quadrant, indicating that the monitoring transect typically contained one upwind station 100 m west of I-15 and three downwind stations east of the highway. Figure 4 does show an occasionally strong easterly wind during the time period from midnight until approximately 3 p.m. During this time, the station to the west of I-15 experienced impacts from the highway.

7.3 Continuous Analyzers

7.3.1 CO and NOX

Figure 24 and Figure 25 show the mean hourly CO concentrations by site from all wind directions and winds from road, respectively. The mean CO concentration for the 20 meter road is approximately 26 percent higher than the 100-meter upwind site (Figure 24) for all wind directions. The mean CO concentration for the 20-meter road is approximately 60 percent higher than the 100-meter upwind site (Figure 25) for downwind conditions (winds from road).

Figure 26 shows the mean CO concentration by hour for all stations when winds are from the road. Figure 27 shows the mean CO concentration by hour for all four stations when the winds are from the road vs. hourly average traffic.

Box-Whisker Plot Mean CO Concentration by Site (all wind directions). The majority of the values all fall with 0.2 and 0.3.

Figure 24. Box-Whisker Plot Mean CO Concentration by Site (all wind directions).

Box-Whisker Plot Mean CO Concentration by Site (winds from road). The majority of the values fall within 0.2 and 0.5.

Figure 25. Box-Whisker Plot Mean CO Concentration by Site (winds from road).

Mean CO Concentration by Hour: all stations (winds from road) - Description: The graph compares 100 meters upwind, 20 meters roadside, 100 meters downwind and 200 meters downwind. They all follow the same graph path, with the roadside values being the highest and the upwind values being the lowest.

Figure 26. Mean CO Concentration by Hour: all stations (winds from road).

Mean CO Concentration and Traffic Volume by Hour: all stations (winds from road) - Description: The graph compares 100 meters upwind, 20 meters roadside, 100 meters downwind and 200 meters downwind as well as traffic volume. The wind directions follow the same path, with the roadside values being the highest and upwind being the lowest.

Figure 27. Mean CO Concentration and Traffic Volume by Hour: all stations (winds from road).

When CO concentrations by hour are overlaid with traffic volume (Figure 27), an increase in CO concentration trends with the increase in traffic volume during morning commute hours (5-7 a.m.). However as the morning progresses and traffic volume continues to increase, CO concentrations appear to decrease. CO concentrations appear to peak around 7:00 am, decrease through the morning and early afternoon hours and begin to increase again beginning around 3:00 p.m. This is apparently due to meteorological influences. As the sun rises and solar energy is pumped into the atmosphere, the air is heated and mixing is occurring in the atmosphere both horizontally and vertically. Winds during these morning hours tend to be from the southwest quadrant and typically wind speed increases as the day progresses (Figure 4) until the mid to late afternoon hours. During the mid to late afternoon (3-6 p.m.) wind speed decreases and wind direction is more variable (Figure 4). Due to these meteorological conditions, pollutant concentrations tend to be higher during the morning commute (5-7 a.m.) and appear to decrease even though traffic volume increases throughout the day, peaking about 4 p.m. (Figure 27).

As evidence that meteorological factors play a role in observed pollutant concentrations, Figure 28 is shown. On April 21 and April 22, wind speed is very low and wind direction is variable. Pollutant concentrations (NO, CO and BC) are elevated for these days for Site 1 (20 meter roadside) and Site 4 (100-meter upwind). Pollutant concentrations decrease as the week progresses, This appears to be directly related to a significant increase in wind speed and resultant increased atmospheric mixing.

Mean NO, CO, BC Concentration, Traffic Volume and Wind Speed and Wind Direction for a Week in April, 2009 - Description: On April 21 and April 22, wind speed is very low and wind direction is variable. Pollutant concentrations (NO, CO and BC) are elevated for these days for Site 1 (20 meter roadside) and Site 4 (100-meter upwind). Pollutant concentrations decrease as the week progresses,. This appears to be directly related to a significant increase in wind speed and resultant increased atmospheric mixing.

Figure 28. Mean NO, CO, BC Concentration, Traffic Volume and Wind Speed and Wind Direction for a Week in April, 2009.[2]

Additional evidence that meteorological factors play a role in observed pollutant concentrations, Figure 29 is shown. During periods of low wind speed pollutant concentrations (NO, CO and BC) are elevated for Site 1 (20-meter roadside) and Site 4 (100-meter upwind). During periods of high wind speed and strong south-southwesterly winds, pollutant concentrations decrease. This appears to be directly related to a significant increase in wind speed and resultant increased atmospheric mixing.

Mean NO, CO, BC Concentration, Traffic Volume and Wind Speed and Wind Direction for a Week in July 2009 - Description: During periods of low wind speed pollutant concentrations (NO, CO and BC) are elevated for Site 1 (20-meter roadside) and Site 4 (100-meter upwind). During periods of high wind speed and strong south-southwesterly winds, pollutant concentrations decrease.

Figure 29. Mean NO, CO, BC Concentration, Traffic Volume and Wind Speed and Wind Direction for a Week in July 2009.[3]

Figure 30 and Figure 31 show the mean hourly NO concentrations by site from all wind directions and winds from road, respectively. The mean NO concentration for the 20-meter road is approximately 134 percent higher than the 100 meter upwind site (Figure 30) for all wind directions. The mean NO concentration for the 20-meter road is approximately 336 percent higher than the 100-meter upwind site (Figure 31) for downwind conditions (winds from road). Figure 32 shows the mean NO concentration by hour for all stations when winds are from the road.

Box-Whisker Plot for NO by Station (all wind directions). The majority of the values fall between 0 and 5 with peaks above 300.

Figure 30. Box-Whisker Plot for NO by Station (all wind directions).

Box-Whisker Plot for NO by Station (winds from road). The majority of the values fall between 0 and 20, with peaks around 300.

Figure 31. Box-Whisker Plot for NO by Station (winds from road).

Mean NO Concentration by Hour: all stations (winds from road) - Description: The graph compares 100 meters upwind, 20 meters roadside and 100 meters downwind. The paths mimick each other, with the roadside values being the highest and upwind being the lowest.

Figure 32. Mean NO Concentration by Hour: all stations (winds from road).

Figure 33 and Figure 34 show the mean hourly NO2 concentrations by site from all wind directions and winds from road, respectively. The mean NO2 concentration for the 20-meter road is approximately 16 percent higher than the 100-meter upwind site (Figure 33) for all wind directions. The mean NO2 concentration for the 20-meter road is approximately 46 percent higher than the 100-meter upwind site (Figure 34) for downwind conditions (winds from road). Figure 35 shows the mean NO2 concentration by hour for all stations when winds are from the road.

Box-Whisker Plot for NO2 by Station (all wind directions). The majority of the values fall between 0 and 5, with outliers near 250.

Figure 33. Box-Whisker Plot for NO2 by Station (all wind directions).

Box-Whisker Plot for NO2 by Station (winds from road). The majority fall between 0 and 10, with outliers near 100.

Figure 34. Box-Whisker Plot for NO2 by Station (winds from road).

Mean NO2 Concentration by Hour: all stations (winds from road) - Description: The graph compares 100 meters upwind, 20 meters roadsisde and 100 meters downwind. The paths all mimick each other, with the roadside values being the highest and upwind being the lowest.

Figure 35. Mean NO2 Concentration by Hour: all stations (winds from road).

Figure 36 and Figure 37 show the mean hourly NOX concentrations by site from all wind directions and winds from road, respectively. The mean NOX concentration for the 20-meter road is approximately 54 percent higher than the 100 meter upwind site (Figure 36) for all wind directions. The mean NOX concentration for the 20 meter road is approximately 121 percent higher than the 100-meter upwind site (Figure 37) for downwind conditions (winds from road). Figure 38 shows the mean NOX concentration by hour for all stations when winds are from the road.

Box-Whisker Plot for NOX by Station (all wind directions). The majority of the values fall between 0 and 10, with peaks nearing 400.

Figure 36. Box-Whisker Plot for NOX by Station (all wind directions).

Box-Whisker Plot for NOX by Station (all wind directions). The majority of the values fall between 0 and 10, with peaks nearing 400.,Box-Whisker Plot for NOX by Station (winds from road). The majority of the values fall between 0 and 15, with peaks around 350.

Figure 37. Box-Whisker Plot for NOX by Station (winds from road).

Mean NOX Concentration by Hour: all stations (winds from road) - Description: The graph compares 100 meters upwind, 20 meters roadside, 100 meters downwind and 200 meters downwint. The roadside values are the highest while the upwind values are the lowest.

Figure 38. Mean NOX Concentration by Hour: all stations (winds from road).

Long term averages for NO, NO2, NOx, and CO for all wind directions are shown in Table 16 and Table 17.

Example gradient plots for nitric oxide (NO) for two sample days are shown in Figure 39. These plots show that when the wind direction is from the roadway towards the downwind monitors with minimal wind speed, high NO concentrations are observed by the monitors (Figure 39) Moreover, when the wind direction is not from the roadway with higher wind speeds, lower NO concentrations are observed by the monitors (Figure 40). The concentration gradient is still observable in Figure 40, however just not as pronounced as in Figure 39.

Gradient plots for NO for two sample days at I-15 site - Description: Wind direction from roadway towards monitors and low wind speed (concentration in ppb vs time). The graph shows a much higher NO concentration when there is low wind speed.

Figure 39 Gradient plots for NO for two sample days at I-15 site—wind direction from roadway towards monitors and low wind speed (concentration in ppb vs time).

Gradient plots for NO for two sample days at I-15 site - Description: Wind direction not from roadway and higher wind speeds (concentration in ppb vs time).The graph shows a much lower NO concentration when there is high wind speed.

Figure 40 Gradient plots for NO for two sample days at I-15 site—wind direction not from roadway and higher wind speeds (concentration in ppb vs time).

Table 16. Long-term averages at near-road monitoring stations for NO, NO2, NOx, and CO - all wind directions.

Location

Time span

CO (ppm)

NO2 (ppb)

NOX (ppb)

N

Avg

95% CI

N

Avg

95% CI

N

Avg

95% CI

Station 1: 20 m east

12/15/2008 to 12/15/2009

8535

0.34

0.34 – 0.35

8535

24.10

23.84 – 24.36

8535

46.89

46.04 – 47.72

Station 2: 100 m east

12/15/2008 to 12/15/2009

8556

0.30

0.29 – 0.30

8593

21.26

20.99 – 21.53

8593

36.07

35.31 – 36.83

Station 3: 300 m east

12/15/2008 to 12/15/2009

8544

0.27

0.26 – 0.27

8518

18.69

18.42 – 18.95

8518

30.64

29.94 – 31.35

Station 4: 100 m west

12/15/2008 to 12/15/2009

8482

0.27

0.26 – 0.27

8459

20.73

20.44 – 21.03

8459

30.47

29.85 – 31.08

Table 17. Long-term averages a near-road monitoring stations for NO, NO2, NOx, and CO - winds from the West.

Location

Time span

CO (ppm)

NO2 (ppb)

NOX (ppb)

N

Avg

95% CI

N

Avg

95% CI

N

Avg

95% CI

Station 1: 20 m east

12/15/2008 to 12/15/2009

3472

0.40

0.39 – 0.40

3473

27.31

26.93 – 27.69

3473

56.08

54.82 – 57.33

Station 2: 100 m east

12/15/2008 to 12/15/2009

3523

0.32

0.32 – 0.33

3539

23.81

23.41 – 24.21

3539

41.16

40.00 – 42.32

Station 3: 300 m east

12/15/2008 to 12/15/2009

3526

0.30

0.29 – 0.31

3513

20.95

20.53 – 21.37

3513

34.86

33.72 – 35.99

Station 4: 100 m west

12/15/2008 to 12/15/2009

3461

0.25

0.25 – 0.26

3448

18.73

18.25 – 19.21

3448

25.32

24.46 – 26.18

7.3.2 Black Carbon

Summaries of the annual BC averages and confidence intervals at each site are presented in Table 18 and shown in Figure 41. Given that two of the stations did not have consistent data collection for the first month of sampling, an 11-month time frame (from 1/15/2009 to 12/15/2009) was selected to compare the average concentrations among the four stations. Data completeness was at 97 percent or higher for the four stations during this time frame. The data show that, on an annual average basis with winds from all directions, the BC annual average at 10 m from the highway is significantly higher than at further distances from the road. In addition, BC average values at 100 m in the predominant downwind direction (east of the highway) are significantly higher than at 100 m in the opposite direction, as well as higher than at 300 m on the downwind side of the road.

Table 18. BC averages for all data (1/15/2009-12/15/2009)

Site name

Distance from Road

Na

(hours)

Mean

(µg/m-3)

95% CI

(µg/m-3)

Station 4

100 Meter Upwind

8032

0.94

0.92-0.96

Station 1

20 Meter Roadside

7779

1.46

1.44-1.49

Station 2

100 Meter Downwind

7807

1.09

1.07-1.11

Station 3

300 Meter Downwind

7866

0.78

0.76-0.80

aA complete hour of sampling was set at a minimum of 10 five minute data points

BC hourly values were also isolated for time periods with winds from the west, designated as 270 ± 60 degrees. Selecting only the BC hourly data with 70% or greater of the time period having winds from the West, the mean values at each station provided in Table 19 and shown in Figure 41. On the downwind side of the road, BC values at Station 1 are significantly higher than all other stations. Designating Station 4 as representative of the urban background, under downwind conditions Station 1, Station 2, and Station 3 exceed the urban background by a factor of 2.2, 1.6, and 1.2, respectively. These results suggest that the impact of traffic emissions on near-road air quality likely extends beyond 300 m from the road.

Table 19. BC averages, wind from the West (1/15/2009-12/15/2009)

Site name

Distance from Road

Na

(hours)

Mean

(µg/m-3)

95% CI

(µg/m-3)

Station 4

100 Meter Upwind

1954

0.80

0.77-0.83

Station 1

20 Meter Roadside

1826

1.74

1.69-1.79

Station 2

100 Meter Downwind

1863

1.28

1.24-1.32

Station 3

300 Meter Downwind

1908

0.92

0.89-0.96

aA complete hour of downwind sampling was set at a minimum of 70% of the hour with winds from the West (210-330 degrees).

Average black carbon concentrations as a function of distance from the road for all data and during time periods with wind from the west. - Description: On the downwind side of the road, BC values at Station 1 are significantly higher than all other stations. Designating Station 4 as representative of the urban background, under downwind conditions Station 1, Station 2, and Station 3 exceed the urban background by a factor of 2.2, 1.6, and 1.2, respectively. These results suggest that the impact of traffic emissions on near-road air quality likely extends beyond 300 m from the road

Figure 41. Average black carbon concentrations as a function of distance from the road for all data and during time periods with wind from the West (210-330 degrees).

Figure 42 and Figure 43 show the mean hourly BC concentrations by site from all wind directions and winds from road, respectively. The mean BC concentration for the 20-meter road is approximately 55 percent higher than the 100-meter upwind site (Figure 42) for all wind directions. The mean BC concentration for the 20-meter road is approximately 118 percent higher than the 100-meter upwind site (Figure 43) for downwind conditions (winds from road). Figure 44 shows the mean BC concentration by hour for all stations when winds are from the road.

Box-Whisker Plot for BC by Station (all wind directions), The majority of the values fall between 0 and 2000, with peak values close to 15000.

Figure 42. Box-Whisker Plot for BC by Station (all wind directions).

Box-Whisker Plot for Hourly BC by Station (winds from road). The majority of the values fall within 0 and 2000 with peaks around 15000.

Figure 43. Box-Whisker Plot for Hourly BC by Station (winds from road).

Mean BC Concentration by Hour: all stations (winds from road) - Description: The graph compares 100 meters upwind, 20 meters roadside, 100 meters downwind and 200 meters downwind. The paths are similar to each other, with roadside having the highest values and upwind having the lowest values.

Figure 44. Mean BC Concentration by Hour: all stations (winds from road).

7.4 Continuous Particulate Data (TEOM)

Figure 45 shows box-whisker plots for PM10, PM2.5 and PM Coarse. Summaries of PM10, PM2.5 and PM Coarse averages and confidence intervals are shown in Table 20 and Table 21. These data were measured by a TEOM 1405 FDMS. Most analyzers deployed for this study performed well with the exception of the TEOMs. This instrument had both design and manufacturing issues that only became apparent after the instruments had been deployed. The remedy for this situation was that the manufacturer performed an “in the field upgrade” by technical staff from ThermoScientific in late November 2009 and early December 2009. While these upgrades improved instrument performance and stability, data collected prior to this time period is problematic. Results are presented in the main body of this report although it is very difficult to draw conclusions from the current analyses.

Table 20. PM10, PM2.5 and PM Coarse averages for all wind directions (12/15/2008-01/20/2010)

Site name

Distance from Road

N (hours)

Mean (µg/m3)

95% CI (µg/m3)

PM10

Station 4 a

100 Meter Upwind

326

20.26

19.12-21.40

Station 1

20 Meter Roadside

8228

22.62

22.16-23.07

Station 2

100 Meter Downwind

6517

18.10

17.60-18.59

Station 3 b

300 Meter Downwind

3158

20.75

20.12-21.38

PM2.5

Station 4 a

100 Meter Upwind

326

8.31

7.83-8.79

Station 1

20 Meter Roadside

8267

8.74

8.60-8.87

Station 2

100 Meter Downwind

6602

7.71

7.45-7.97

Station 3 b

300 Meter Downwind

3158

7.84

7.58-8.10

PM Coarse

Station 4a

100 Meter Upwind

326

11.95

11.10-12.80

Station 1

20 Meter Roadside

8317

13.94

13.56-14.33

Station 2

100 Meter Downwind

6368

11.15

10.78-11.52

Station 3 b

300 Meter Downwind

3158

13.10

12.59-13.61

a Data from Station 4 is from 12/01/2009-01/01/08/2010.

b Data from Station 3 is from 07/15/2009-01/01/20/2010

Table 21. PM10, PM2.5 and PM Coarse averages for winds from road (12/15/2008-01/20/2010)

Site Name

Distance from Road

N (hours)

Mean (µg/m3)

95% CI (µg/m3)

PM10

Station 4

100 Meter Upwind

190

20.36

18.81-21.90

Station 1

20 Meter Roadside

3528

25.08

24.35-25.81

Station 2

100 Meter Downwind

2704

19.80

19.09-20.52

Station 3

300 Meter Downwind

1466

21.55

20.70-22.39

PM2.5

Station 4

100 Meter Upwind

190

8.45

7.83-9.07

Station 1

20 Meter Roadside

3551

9.05

8.84-9.26

Station 2

100 Meter Downwind

2731

8.15

7.91-8.39

Station 3

300 Meter Downwind

1466

8.30

7.92-8.68

PM Coarse

Station 4

100 Meter Upwind

190

11.91

10.77-13.05

Station 1

20 Meter Roadside

3585

16.06

15.43-16.68

Station 2

100 Meter Downwind

2666

12.19

11.61-12.78

Station 3

300 Meter Downwind

1466

13.40

12.73-14.07

a Data from Station 4 is from 12/01/2009-01/01/08/2010.

b Data from Station 3 is from 07/15/2009-01/01/20/2010.

Figure 45 Box-Whisker Plots for PM10, PM2.5 and PM Coarse for all stations; all wind directions and winds from road.[4]

7.5 Integrated Sample Data

7.5.1 Integrated MSAT Data (TO-15 — VOC)

Table 22 shows the number of observations, mean and 95 percent confidence intervals for the VOC data (TO-15 method). As shown in Table 22, Station 2 exhibits higher values for benzene. This may be due to influences from other nearby sources such as Las Vegas Blvd., McCarran International Airport, nearby truck parking lot as shown in Figures 48 and 49.

Table 22. VOC -- averages for all wind directions (12/15/2008-12/15/2009)

Site name

Distance from Road

N (Obs.)

Mean (ppb)

95% CI (ppb)

1,3-Butadiene

Station 4

100 Meter Upwind

251

0.05

0.04-0.05

Station 1

20 Meter Roadside

276

0.06

0.05-0.07

Station 2

100 Meter Downwind

246

0.06

0.05-0.07

Station 3

300 Meter Downwind

246

0.03

0.03-0.04

Benzene

Station 4

100 Meter Upwind

251

0.20

0.18-0.22

Station 1

20 Meter Roadside

276

0.22

0.20-0.24

Station 2

100 Meter Downwind

246

0.32

0.29-0.35

Station 3

300 Meter Downwind

246

0.16

0.15-0.18

NOTE: Data are for valid samples only.

7.5.2 Data Caveats– Integrated Samples -- VOC

It should be noted that acrolein values for the TO-15 method (canister) are problematic. Prior to June 18, 2009 the GC/MS system was not optimized for acrolein analysis. In addition, there is low confidence with all acrolein values due to potential contamination of Summa passivated canisters associated with the “growth” of acrolein inside cleaned canisters. Acrolein concentrations inside cleaned canisters containing zero humidified air have been shown to increase over time due to unknown reasons. For these reasons, acrolein data is not reported for the TO-15 method (Table 22). All sample results are presented with no blank or recovery correction. This was deemed unnecessary as the field blank values were either zero or below the method detection limit. Blank and control values may be found in the SAS/JMP data sets.

Box-Whisker Plot for 1,3-Butadiene all stations, all sample times, all wind directions - Description: The majority of the values are in between 0.0 and 0.1 with an outlier near 0.6.

Figure 46 Box-Whisker Plot for 1,3-Butadiene all stations, all sample times, all wind directions.

Box-Whisker Plot for 1,3-Butadiene all stations, all sample times, downwind conditions. - Description: The majority of the values are in betweemnm 0.0 and 0.1, with outliers near 0.4.

Figure 47 Box-Whisker Plot for 1,3-Butadiene all stations, all sample times, downwind conditions.

Box-Whisker Plot for Benzene all stations, all sample times, all wind directions - Description: The majority of the values are in between 0.1 and 0.4 with an outlier near 1.8.

Figure 48 Box-Whisker Plot for Benzene all stations, all sample times, all wind directions.

Box-Whisker Plot for Benzene all stations, all sample times, downwind conditions - Description: The majority of the values lie between 0.1 and 0.5 with an outlier near 1.8.

Figure 49 Box-Whisker Plot for Benzene all stations, all sample times, downwind conditions.

7.5.3 Integrated MSAT Data (TO-11a — carbonyl)

Table 23 shows the number of observations, mean and 95 percent confidence intervals for the carbonyl data (TO-11a method). As shown in Table 23, the study did not provide any usable data for Station 3. The instrument at Station 3 had instrument problems throughout the life of the study. Acetaldehyde measurements at Station 1 were approximately 10% higher than at Station 4. Acetaldehyde measured at Station 1 versus Station 2 was virtually the same. Formaldehyde measurements at Station 1 were approximately 18 percent higher and 9 percent higher than at Station 2 and 4, respectively. Acrolein measurements at Station 1 were approximately 14 percent higher than at Station 2. Acrolein measured at Station 1 versus Station 4 was virtually the same. Box-whisker plots (Figures 50 – 55) show all three pollutants for all wind conditions and downwind conditions. All sample results are presented with no blank or recovery correction. Blank and control values may be found in the SAS/JMP data sets.

Table 23. Carbonyl -- averages for all wind directions (12/20/2008-12/16/2009)

Site name

Distance from Road

N (Obs.)

Mean (ppb)

95% CI (ppb)

Acetaldehyde

Station 4

100 Meter Upwind

279

1.02

0.94-1.11

Station 1

20 Meter Roadside

308

1.12

1.02-1.22

Station 2

100 Meter Downwind

225

1.11

1.00-1.21

Station 3

300 Meter Downwind

---

---

---

Formaldehyde

Station 4

100 Meter Upwind

279

2.91

2.73-3.10

Station 1

20 Meter Roadside

308

3.18

2.97-3.39

Station 2

100 Meter Downwind

225

2.62

2.45-2.79

Station 3

300 Meter Downwind

---

---

---

Acrolein

Station 4

100 Meter Upwind

279

0.27

0.26-0.29

Station 1

20 Meter Roadside

308

0.27

0.25-0.28

Station 2

100 Meter Downwind

225

0.27

0.25-0.29

Station 3

300 Meter Downwind

---

---

---

NOTE: Data are for valid samples only.

7.5.4 Data Caveats– Integrated Samples -- Carbonyl

As shown in Table 23, the study did not provide any usable data for Station 3. The instrument at Station 3 had problems throughout the life of the study. Thus, all carbonyl data collected at Station 3 is considered invalid. Background corrections were not performed on the formaldehyde data. This was deemed unnecessary as the field blank values were either zero or below the method detection limit. Background corrections were performed on the acetaldehyde and acrolein data. The EPA Compendium TO-11A DNPH carbonyl method was implemented in Las Vegas for the collection and analysis of air samples for acetaldehyde, formaldehyde and acrolein. A field blank is a DNPH cartridge that is treated in the same manner as a sample cartridge except no sample air is drawn through the field blank. These field blanks are sent back to the laboratory, analyzed and values were reported for acetaldehyde, formaldehyde, and acrolein.

DNPH cartridges are prepared in large batches by a manufacturer (e.g., Sigma-Aldrich). Properly stored DNPH cartridges may be used over several weeks or months. For a year-long study, we purchased DNPH cartridges multiple times to maintain the “freshness” of the cartridges. Thus, our data shows that we used cartridges from multiple batches. Ideally, field blanks should be drawn from the same batch as the sample cartridges. For the Las Vegas study, there were cases when no field blanks were drawn from batches used for sampling. Thus, we had two cases: 1) field blanks drawn from the same batch as field samples; and 2) no field blanks drawn from the same batch as field samples. For case 1, we used the median value by batch by pollutant of the reported field blank values as the background correction. For case 2, we used an overall median value by pollutant for the entire set of reported field blank values for the background correction. If corrected values were calculated as negatives or below the method detection limit, then the corrected values were replaced with the method detection limit value; otherwise the corrected value is the actual calculated value. The SAS/JMP data set reports the uncorrected and corrected data along with the method detection limit values and relevant data flags to indicate how the data were treated.

Box-Whisker Plot for Acetaldehyde all stations, all sample times, all wind directions - Description: The majority of the values are in between 0 and 2, with an outlier near 5.

Figure 50 Box-Whisker Plot for Acetaldehyde all stations, all sample times, all wind directions.

Box-Whisker Plot for Acetaldehyde all stations, all sample times, downwind conditions - Description: The majority of the values are in between 0 and 2, with an outlier near 5.

Figure 51 Box-Whisker Plot for Acetaldehyde all stations, all sample times, downwind conditions.

Box-Whisker Plot for Formaldehyde all stations, all sample times, all wind directions - Description: The majority of the values are in between 2 and 4, with an outlier near 17.

Figure 52 Box-Whisker Plot for Formaldehyde all stations, all sample times, all wind directions.

Box-Whisker Plot for Formaldehyde all stations, all sample times, downwind conditions - Description: The majority of the values are in between 2 and 4, with an outlier near 10.

Figure 53 Box-Whisker Plot for Formaldehyde all stations, all sample times, downwind conditions.

Box-Whisker Plot for Acrolein all stations, all sample times, all wind directions - Description: The majority of the values are in between 0.2 and 0.5, with an outlier near 1.2.

Figure 54 Box-Whisker Plot for Acrolein all stations, all sample times, all wind directions.

Box-Whisker Plot for Acrolein all stations, all sample times, downwind conditions - Description: The majority of the values are in between 0.2 and 0.5 with no outliers.

Figure 55 Box-Whisker Plot for Acrolein all stations, all sample times, downwind conditions.

7.5.5 Particulate Data (FRM Filters)

A summary of PM2.5 averages and confidence intervals are shown in Table 24. Figure 56 shows box-whisker plots PM2.5 integrated filter samples. As shown in Table 24 and Figure 56, the 20 m site (Station 1) has an observed higher mean PM2.5 concentration than the other sites. Station 1 is approximately 19%, 11%, and 9% higher than Stations 2, 3, and 4 respectively. Figure 57 shows PM2.5 data by date and site.

Table 24. PM2.5 Filters -- averages for all wind directions (12/15/2008-12/15/2009)

Site name

Distance from Road

N (Obs.)

Mean (µg m3)

95% CI (µg m3)

Station 4

100 Meter Upwind

30

9.20

7.63-10.79

Station 1

20 Meter Roadside

29

10.03

8.56-11.50

Station 2

100 Meter Downwind

30

8.42

6.99-9.85

Station 3

300 Meter Downwind

29

9.05

7.51-10.58

NOTE: Data are for valid samples only.

Box-Whisker Plot for PM2.5 for all stations, all sample times, all wind directions - Description: The majority of the values fall between 6 and 11, with outliers at 3 and 20.

Figure 56 Box-Whisker Plot for PM2.5 for all stations, all sample times, all wind directions.

Bar chart for PM2.5 (Aug/m3) for all stations, all sample times, all wind directions - Description: This graph compares 300 meters downwind, 100 meters downwind, 20 meters roadside and 100 meters upwind. There is no compareable difference in the values.

Figure 57 Bar chart for PM2.5 (µg/m3) for all stations, all sample times, all wind directions.

Updated: 05/21/2013
HEP Home Planning Environment Real Estate
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000