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FHWA and EPA National Near-Road Study Las Vegas

3 Study Design

The objective of the research study is to determine pollutant concentrations and the variation of pollutant concentrations as a function of distance from the highway (Figure 11). Additional important considerations of the study include establishing relationships between pollutant concentrations as related to highway traffic characteristics including traffic count, vehicle types and speeds, and meteorological conditions such as wind speed and wind direction. This study provided detailed concentration data and distributions of motor vehicle emitted pollutants including regulated gases, air toxics, and particulate matter.3

3.1 Detailed Monitoring Protocol

FHWA's "detailed monitoring protocol" outlines a uniform approach to conduct all studies for evaluating mobile source contributions to air toxic compounds and PM2.5 and their dispersion patterns2. A more detailed examination of the monitoring protocol indicates that for each city, continuous monitoring and integrated sample collection were required at four monitoring sites (Figure 11). In addition, wind speed and wind direction was required at each site. Moreover, monitoring for the complete suite of meteorological parameters was required at the monitoring station positioned 50 to 150 m from the roadway (100 m downwind). Table 1 summarizes the measurements taken at each monitoring site and Table 5 summarizes measurement parameters, sampling approach, and instruments.

Illustration of Monitoring Site Locations - Description: Shows the road where the monitors are setup and the orientation in which they are perpendicular to the flow of traffic.

Figure 11. Illustration of Monitoring Site Locations.

Table 5. Summary of Measurement Parameters, Sampling Approach and Instruments.

Measurement Parameter

Sampling Approach

Instrument Data




Detection Limit

Sample Type and Frequency

Carbon Monoxide (CO)

nondispersive infrared

EC 9830T

± 5% 0-1000ppb

0.5% of reading

25 ppb


Serinus 30

< 1%

20 ppb or 0.1 % of reading

40 ppb

Oxides of nitrogen (NOx)


EC 9841B

< 1%

0.5 ppb

0.5 ppb

Black Carbon (BC)


Magee - Aethalometer, Models AE16 and AE20

1:1 comparison w/ EC on filters

Repeatability: 1 part in 10,000

0.1 μg/m3 w 1 min res.


PM2.5 FRM method



24-hour integrated

1-in-12 day schedule

1 sample each day at each road-side location



Thermo TEOM - 1405DF


±2.0 μg/m3 (1-hour ave), ±1.0 μg/m3 (24-hour ave)

0.1 μg/m3


(5 minute)


PM Coarse


USEPA Method TO-11A

Atec 2200 Cartridge Sampler

± 2 %

± 2 %


1-hour integrated

1-in-12 day schedule

9 samples each day at each road-side location



USEPA Method TO-15

Entech 1800 Canister Sampler

± 2 %

± 2 %




Wind Speed

sonic anemometer

RM Young Model 81000

±0.05 m/s

std. dev. 0.05 m/s at 12 m/s

0.01 m/s


(5 minute)

Wind Direction

± 5°

± 10°


Air Temperature

temperature probe

Vaisala HMP45D

Vaisala HMP45A

±0.2°C at 20° C

0.1 ° C

0.1 ° C

% Relative Humidity

relative humidity sensor

±2%RH from 0…90% RH)

1% RH

1% RH

Rain Gauge

rain bucket

Ecotech Rain Gauge

+/- 5% at 25-50 mm/hour

± 1mm

± 1mm

Solar Radiation

solar radiation

MetOne 394 Pyranometer

±5% from 0…2800 watts meter2

±1% constancy from -20°C to +40°C

9 mV/kwatt meter-2, approx



Extech 407764

±1.5dB (under reference conditions)





Axix 223M

Vivotek SD7151



(15 minutes)


Vehicle Count


NDOT Data and Equipment



Vehicle Speed


Vehicle Type


1. Accuracy and precision in terms of ultrafine particle concentration is difficult to determine in the field due the lack of particle concentration standards. However, particle counters are routinely verified in the field for accuracy in flow rate. Precision was estimated in this study by collocating UFP samplers prior to use of instruments in the field.

3.2 Study Design Enhancements

Enhancements to the study protocol included 3-additional CO monitors and 3-additional aethalometers to more fully characterize the freeway "cut-section" and sound meter and video camera to monitor train and plane activity from the railroad and nearby airport, respectively. The video was also used to validate traffic count information received from NDOT.

3.3 Site Location

The site selection process consisted of a series of seven steps: (1) determine site selection criteria2; (2) develop list of candidate sites and supporting information; (3) apply site selection filter ("coarse" and "fine"); (4) site visit; (5) select candidate site(s) via team discussion; (6) obtain site access permission(s); and (7) implement site logistics. This process resulted in the selection of a location along I-15, just south of the "Las Vegas Strip".3

This site was considered the optimal site of all the candidate monitoring sites3. This site has high AADT (206,000 AADT for 2006), no noise walls, meteorological and traffic data availability, manageable site logistics including ROW access, and favorable wind direction3. This location is shown in Figure 1.

Updated: 5/21/2013
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