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FHWA → ENVIRonment → Air Quality → Air Toxics → Research And Analysis → National Near-Road Study → Las Vegas

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.

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

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

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

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 NO_X

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.

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

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

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

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.

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.

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.

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

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

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.

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

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

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

Figure 36 and Figure 37 show the mean hourly NO_X concentrations by site from all wind directions and winds from road, respectively. The mean NO_X 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 NO_X 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 NO_X concentration by hour for all stations when winds are from the road.

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

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

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

Long term averages for NO, NO₂, NO_x, 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.

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).

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).

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.

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

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

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, PM_2.5 and PM Coarse. Summaries of PM₁₀, PM_2.5 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, PM_2.5 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, PM_2.5 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, PM_2.5 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.

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

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

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

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.

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

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

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

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

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

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

7.5.5 Particulate Data (FRM Filters)

A summary of PM_2.5 averages and confidence intervals are shown in Table 24. Figure 56 shows box-whisker plots PM_2.5 integrated filter samples. As shown in Table 24 and Figure 56, the 20 m site (Station 1) has an observed higher mean PM_2.5 concentration than the other sites. Station 1 is approximately 19%, 11%, and 9% higher than Stations 2, 3, and 4 respectively. Figure 57 shows PM_2.5 data by date and site.

Table 24. PM_2.5 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.

Figure 56 Box-Whisker Plot for PM_2.5 for all stations, all sample times, all wind directions.

Figure 57 Bar chart for PM_2.5 (µg/m³) for all stations, all sample times, all wind directions.

Updated: 6/28/2017