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

Detailed Monitoring Protocol

2.0 Monitoring Study Design

PDF files can be viewed with the Acrobat® Reader®

The study design presented here is based on the hypothesis that MSAT concentrations are greater than background concentrations near a heavily traveled roadway, and fall to near background levels within 300 meters (m) (approximately 1,000 feet) from the roadway.1-3 To meet the objective of these studies, a suitable monitoring schedule must be adopted to appropriately characterize representative conditions and temporal patterns in pollutant concentration, established monitoring techniques must be used to reliably measure the pollutants of interest, and siting of the monitoring stations must allow for spatial characterization of the pollutants near the roadway. In general, the recommendations call for continuous monitoring of a few select surrogate species throughout the monitoring study, augmented with integrated sampling of specific MSATs following a pre-determined sampling schedule. The following discussion briefly describes the sampling schedule to be adopted for studies conducted under this Protocol, the criteria for selection of the study locations, and the requirements (in terms of data quality indicators) of the measurement methods to be employed. Recommendations for the selection of monitoring site locations and monitoring methods are made in Sections 3 and 4 of this Protocol

2.1 Sampling Schedule

A number of potential sampling schedules were considered for implementation in the studies to be conducted under this Protocol. These options included the collection of various numbers of samples per day, various sampling period durations, as well as intensive seasonal sampling or routine periodic sampling. After considerable discussion, the FHWA and Sierra Club agreed upon a single sampling schedule that was felt would meet the requirements of the Settlement Agreement. The sampling schedule agreed upon by FHWA and Sierra Club calls for the collection of nine (9) 1-hour samples during a 24-hour period on a 1-in-12 day schedule. During each sampling day two of the 1-hour samples should be collected within the three-hour period centered on the morning peak traffic period. For all sampling days, one of these samples should be collected during the hour corresponding to the typical peak in the morning traffic. The second sample should be collected during the hour either preceding or following the peak hour, on an alternating basis every other sampling day. The other seven samples should be collected on an equally spaced basis (i.e., every third hour) over the other 21 hours of the sampling day. Furthermore, the sampling schedule should rotate each sampling day such that the collection times shift by one hour for the seven samples collected outside of the morning peak traffic period. Table 2-1 shows an example of this sampling schedule for the first five sampling days, in which the morning peak traffic period was assumed to be 7:00-8:00 a.m. This same schedule is shown illustratively in Figure 2-1 in which the times in the first column indicate the starting time for sample collection and each block represents one hour. In this figure, if a sample starts at 0:00 the sample end time will be 1:00. For convenience, the borders of the assumed morning peak traffic period have been made bolder than those for other hours.

Table 1. Example Rotating Sampling Schedule for 9-samples-per-day Plan
  Day 1 Day 2 Day 3 Day 4 Day 5
Sample 1 0:00-1:00 1:00-2:00 2:00-3:00 0:00-1:00 1:00-2:00
Sample 2 3:00-4:00 4:00-5:00 5:00-6:00 3:00-4:00 4:00-5:00
Sample 3 6:00-7:00 7:00-8:00 6:00-7:00 7:00-8:00 6:00-7:00
Sample 4 7:00-8:00 8:00-9:00 7:00-8:00 8:00-9:00 7:00-8:00
Sample 5 9:00-10:00 10:00-11:00 11:00-12:00 9:00-10:00 10:00-11:00
Sample 6 12:00-13:00 13:00-14:00 14:00-15:00 12:00-13:00 13:00-14:00
Sample 7 15:00-16:00 16:00-17:00 17:00-18:00 15:00-16:00 16:00-17:00
Sample 8 18:00-19:00 19:00-20:00 20:00-21:00 18:00-19:00 19:00-20:00
Sample 9 21:00-22:00 22:00-23:00 23:00-0:00 21:00-22:00 22:00-23:00

For each study conducted under this Protocol, the exact sampling schedule should be tailored to specific traffic patterns in the respective monitoring study locations. At least one year of historical data should be used to identify the peak hour in the morning traffic pattern for each monitoring study location prior to implementation of the monitoring study. An inspection of the continuous surrogate monitoring data should be conducted within the first month of monitoring to verify that the peak traffic period was properly selected and that the study-specific monitoring schedule is appropriate for the monitoring study location.

This sampling schedule allows for the measurement of MSAT concentrations with a high time resolution (hourly) during two hours of the morning traffic peak while also allowing measurement of MSAT concentrations on a routine basis throughout the remainder of each sampling day. By rotating the sampling schedule, data are collected for each individual hour of the day at several times throughout the year.

Illustration of rotating sampling schedule in which 24 1-hour blocks are shown as rows in a chart. For each of the five example sampling days shown, the nine samples collected during the respective sampling days are represented as columns in the chart. The hours during which the samples are collected are highlighted in the chart. For the first sampling day, the nine 1-hour samples are collected starting at 0:00, 3:00, 6:00, 7:00, 9:00, 12:00, 15:00, 18:00, and 21:00, respectively. For the second sampling day, the nine 1-hour samples are collected starting at 1:00, 4:00, 7:00, 8:00, 10:00, 13:00, 16:00, 19:00, and 22:00, respectively. For the third sampling day, the nine 1-hour samples are collected starting at 2:00, 5:00, 6:00, 7:00, 11:00, 14:00, 17:00, 20:00, and 23:00, respectively. For the fourth sampling day, the nine 1-hour samples are collected starting at 0:00, 3:00, 7:00, 8:00, 9:00, 12:00, 15:00, 18:00, and 21:00, respectively. For the fifth sampling day, the nine 1-hour samples are collected starting at 1:00, 4:00, 6:00, 7:00, 10:00, 13:00, 16:00, 19:00, and 22:00, respectively.
Figure 2. Illustration of Rotating Sampling Schedule for 9-samples-per-day Plan

2.2 Site Selection Criteria

To meet the study objective it is important to select study locations that are representative of common highway locations and are likely to yield useful data to establish pollutant concentration variations related to highway traffic. Criteria that should be used during selection of study locations have been established for the following parameters:

Section 3 of this Protocol describes the criteria that should be adopted for each of these parameters. Once an appropriate study location is selected, it is important to position the monitoring sites optimally.

Monitoring should be conducted at each of three monitoring sites located at distances of 0-10 m, 100 ±50 m (approximately 300±150 feet), and 300 ±50 m (approximately 1,000 ±150 feet) from the roadway, and at one additional monitoring site, chosen to serve as a background site, located approximately 1,000 m (approximately 3,000 feet) from the roadway and not located near any major pollutant source. It was assumed in the development of this Protocol that appropriate study locations will be selected and that monitor siting will be done such that there is minimal influence on MSAT concentrations at the monitoring sites from sources other than the roadway. It is assumed that this minimal influence is regional in nature and influences all the monitoring sites uniformly.

2.3 MSAT and Surrogate Monitoring Methods

Mobile sources emit a wide variety of pollutants. Of particular interest for this Protocol are the PM2.5 and the following MSATs: acetaldehyde, acrolein, benzene, 1,3-butadiene, formaldehyde, and diesel particulate matter (DPM). However, since monitoring of these species is relatively labor intensive and costly, this Protocol calls for a combined approach in which selected MSATs are monitored on a routine periodic basis using time-integrated sampling techniques, and several surrogate species that are indicators of vehicle emissions are monitored continuously.

The successful completion of these monitoring studies relies on appropriate monitoring of MSATs and surrogate compounds. The monitoring methods recommended for the characterization of most of the MSAT concentrations are standard monitoring methods that have been adopted by the USEPA. However, since there is no recognized method for monitoring of DPM, these studies will include surrogate monitoring for characterization of DPM concentrations. Table 2-2 lists the MSATs that must be measured during each of the monitoring studies and the recommended monitoring method for each.

Table 2. MSATs and Recommended Monitoring Methods
MSAT Recommended Monitoring Method
Acetaldehyde DNPH cartridges-HPLC analysis (USEPA Method TO-11A)
Acrolein DNPH cartridges-HPLC analysis (USEPA TO-11A)
Benzene Canister sampling-GC/MS analysis (USEPA Method TO-15)
1,3-Butadiene Canister sampling-GC/MS analysis (USEPA Method TO-15)
Diesel particulate matter Surrogate monitoring
Formaldehyde DNPH cartridges-HPLC analysis (USEPA Method TO-11A)

In addition to the methods listed in Table 2-2 for monitoring MSATs, additional monitoring will be conducted for the characterization of several key surrogate species. Carbon monoxide, nitrogen oxides, black carbon, and PM2.5 will all be measured to provide supporting data regarding vehicle emissions.

Section 4.1 describes the MSAT and surrogate monitoring to be conducted during the studies conducted under this Protocol.

2.4 Meteorological Monitoring

Meteorological monitoring during the studies conducted under this Protocol should be conducted to characterize the ambient conditions during each study. At one of the monitoring sites away from the roadway, monitoring should include the measurement of wind speed, wind direction, ambient temperature, barometric pressure, relative humidity, solar radiation, and precipitation. Additionally, it is recommended that wind speed and wind direction be monitored at each of the sites.

Section 4.2 presents the recommended specifications for the meteorological sensors to be used during the studies conducted under this Protocol.

2.5 Traffic Monitoring

Traffic monitoring must be conducted during the studies conducted under this Protocol, in order to collect real time hourly traffic data during the same periods that air quality monitoring data are collected. These data include traffic volumes, speeds, and vehicle classifications. Preferably, the locations selected for these studies have appropriate traffic monitoring systems already installed. Every effort should be made to select study locations that have good existing traffic monitoring collection systems in place, and available for the studies to be conducted under this Protocol. However, if sites without suitable traffic monitoring systems in place are selected, the contractor conducting the monitoring study(ies) must insure that appropriate traffic monitoring systems are selected and installed prior to beginning the study(ies). Consequently, the intended use of the traffic monitoring data must be considered.

Vehicle counting and classification data are used for a variety of purposes including measurement of the capacity and usage of roadways and highways as well as in the assessment of maintenance requirements. The most prevalent data currently collected include traffic volumes, vehicle speeds, and vehicle classifications. A number of options are available in selecting equipment for sampling these traffic data.

Vehicle detection equipment in use today can be characterized as either non-intrusive or intrusive. Non-intrusive equipment does not require the installation of the sensor directly onto or into the roadway surface. The sensors for non intrusive technologies are mounted overhead or on the side of the roadway. Offsets from the mainline or edge of pavement are variable depending on the manufacturer. Non intrusive technologies include video image processors, microwave radar detectors, active and passive infrared sensors, ultrasound sensors, and passive acoustic array sensors.

Intrusive technologies are devices that are installed directly on the pavement surface, in saw-cut or holes in the road surfaces, by tunneling under the surface, or by anchoring directly to the pavement surface. Intrusive technologies include fiber optic sensors, inductive loops, magnetometers, micro loops, pneumatic road tubes, piezoelectric cables, and other weigh-in-motion sensors.

Section 4.3 presents a summary of types of traffic monitoring equipment that may be used in the studies conducted under this Protocol.

Updated: 07/06/2011
HEP Home Planning Environment Real Estate
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000