The Federal Highway Administration (FHWA) agrees to obtain funding for and implement a research effort to characterize concentration levels in the ambient air adjacent to major highway facilities that are attributable to motor vehicle emissions of PM2.5 and the 6 priority mobile source air toxics: diesel particulate matter plus diesel exhaust organic gases; benzene; 1,3 butadiene; acetaldehyde; formaldehyde; and acrolein. PM2.5 will be studied only for highway locations in PM2.5 nonattainment or maintenance areas. FHWA will study the atmospheric behavior of these emissions at five highway locations (unless, pursuant to Part III of the Agreement, there are less than five locations) throughout the U. S. to characterize the emission plume surrounding the highway and its dispersion pattern away from the roadway.
FHWA identified the study of ambient air concentrations in proximity to highways as a priority in the FHWA Air Toxics Strategic Workplan. The need for proximity studies have grown out of new research suggesting that the "plume" from the roadway can disperse over varying distances and these distances are a function of the pollutant, microscale meteorology in proximity to the highway, mix of engine types and fuels among the vehicles using the highway, and traffic characteristics, among others. Emissions from new vehicles are decreasing compared to older vehicles which may also result in reductions in aggregate emissions from a highway segment and concentration decreases in the ambient air as older, higher emitting vehicles are replaced. EPA national trend data has shown that aggregate vehicle emissions have been decreasing over time even as the traffic volume has increased. Whether national trends are appropriate to predict the rate of change in emissions at the corridor level depends on local factors such as the rate of local trip growth in a corridor, the rate of vehicle replacement, and local changes in vehicle engine and fuel type (e.g., adding more diesel engine vehicles to a highway segment over time may increase local emissions).
Another major uncertainty is the distance at which the resulting concentrations in the ambient air decline to background levels. To determine this, proximity studies are needed as outlined below. The best method of evaluating this is to establish monitors to sample PM2.5 and the toxic compounds, the meteorology, local vehicle fleet mix and the traffic characteristics.
The primary focus of the research studies will be to look at both ambient concentrations of mobile source PM2.5 and air toxics and the vehicle traffic that generates this mobile source component. This will require determining a sampling plan (frequency - hourly, daily) and location of monitors (proximity - both longitudinally along the highways as well as adjacent to it) to determine a concentration profile and the plume dispersion boundaries.
Within 180 days following the effective date of the Settlement Agreement, FHWA will develop a Detailed Protocol that will govern the design and implementation of studies to assess the dispersion of PM2.5 and MSATs emitted from motor vehicles and gain insights into the atmospheric processes in proximity to major highway facilities. Within 30 days of the effective date of the Settlement Agreement, the Sierra Club may comment to FHWA as to what it believes that the Detailed Protocol should provide. Before finalizing the Detailed Protocol, FHWA will send the Detailed Protocol it has developed to a noted expert in the field, selected by FHWA in consultation with the Sierra Club, for review and comment on the technical approach and specifications contained therein. Also, no less than 30 days prior to the application of the Detailed Protocol to any highway location or contract for consultant services, the Sierra Club will be provided an opportunity to comment on the proposed final Detailed Protocol.
The Detailed Protocol will include criteria for 1) the selection of study sites within the highway segments selected pursuant to the Settlement Agreement; 2) a sampling plan that will prescribe the study objectives to be satisfied by the selection of monitoring sites, the frequency and duration of sample collection, the chemical species to be monitored, and the measurement method(s) to be applied for each chemical species; 3) the final procedures for quality assurance and quality control regarding monitor operation, sample collection and handling, and laboratory procedures; and 4) the statistical methods to be applied for the purpose of performing analysis of the data obtained from the monitors. The Detailed Protocol shall, at a minimum, include the following practices and procedures.
The selection of monitoring sites at each study location will be consistent with the Protocol that will prescribe, to the extent possible, uniform criteria for selecting the distance between the monitor locations and the edge of the highway right-of-way. The Protocol will provide for as many as five monitors at various distances between the roadway and 300 meters, including one adjacent to the highway, one at approximately 300 meters away from the highway, and one to three monitors in between, depending on local conditions. The monitoring stations directly adjacent to the highway will be consistent with EPA's siting criteria for a microscale monitor (40 CFR Part 58, Appendix E ¶ 8.3). In each metropolitan area where a monitoring study is undertaken, FHWA will also collect data at a distance significantly away (at least 1000 meters) from any known source of MSATs to determine urban background concentrations for comparison purposes.
The 6 priority MSATs (see below), PM2.5 (if applicable) and, to the extent feasible and practical, additional chemical markers selected for the purpose of apportioning emissions between diesel and gasoline engines will be measured at each monitoring site. A suitable surrogate must be selected for diesel particulate matter (DPM) since no method has been developed to measure DPM directly. A suitable measurement method for acrolein must also be identified and implemented since recent EPA data raises doubt that laboratory analysis of monitored samples is adequate. Carbon monoxide concentrations may also be monitored to correlate ambient toxic compounds and PM2.5 with motor vehicle emissions.
Data will be collected in such a way as to define variations throughout the day and to determine the likely sources (mobile, stationary, area) to the extent possible. Collection of MSAT and PM2.5 monitoring data will be coordinated with other data collection activities, such as meteorological, vehicle type and traffic data, to achieve consistency in time periods and data measurement methods to ensure that ambient air concentrations can be effectively correlated with these major variables. Ambient air data will be collected to provide hourly concentrations so that concentrations can be correlated with changes in traffic loads, time of day, vehicle type, meteorology, seasons and other factors that may affect ambient concentrations.
| Pollutant | Measurement Methods and Considerations [Methods need to be identified for each pollutant or source tracer] | Comments |
| Acetaldehyde | Uncertain, may only be able to obtain 3 or 24 hour average | Reactive |
| Acrolein | Not reliable | Reactive |
| Benzene | Yes | |
| 1, 3-Butadiene | Yes | Reactive |
| DPM | No | Must use surrogate |
| Formaldehyde | Uncertain, may only be able to obtain 3 or 24 hour average | Reactive |
| TRACERS for apportionment between gasoline and diesel emissions | [SEE ABOVE. TO BE DONE TO THE EXTENT PRACTICAL AND FEASIBLE.] |
Each study site will employ a full complement of weather instruments to collect data on wind speed and direction, temperature, relative humidity, and probably solar radiation measurements. Meteorology is the most significant driver of the dispersion of emissions in the ambient air and as such, must be part of any analysis effort designed to study the atmospheric concentrations of pollutants. This is especially true for toxic air compounds because some of them are reactive and thus both chemical reactions in the atmosphere and physical mixing will affect the concentrations measured.
If three or more locations are studied, at least one site will be selected where the local prevailing wind is close to the highway alignment, and one site where the prevailing wind is across the highway alignment.
Traffic data will be collected, at a minimum, during periods when monitors are being operated. At a minimum, traffic volumes, vehicle speeds by time of day, classification as to the vehicle fleet mix of gasoline and diesel vehicles along with other vehicle operating characteristics will likely be important considerations. Traffic data will be collected with automated traffic monitoring equipment (including a visual recording method) to maximize the collection effort which will enable continuous measurement of traffic volume and speeds and, where feasible, classification of the vehicle fleet. The availability of this data will be one of the determining factors for site selection in this study effort.
Five study locations will be selected as described in Part III of the Settlement Agreement. The Protocol will contain specific criteria for the selection of highway segments as study locations and the selection of monitoring sites within the study location, including the average annual daily traffic (AADT) on the highway segment, geometric design, topology, geographic location within the US, availability of traffic data, climate and meteorology.
Data will be collected in the smallest (most frequent) time interval as reasonably possible so that resolution of the sources can be made. This generally refers to making continuous (or semi-continuous) measurements hourly or aggregated into 1 hour intervals. The time interval selected must be consistent for the air quality, traffic and meteorological instruments. The study length or sampling period will be defined relative to the start and completion of the project. On the US 95 project, if selected, data will be collected before expansion of the current roadway is complete, controlling for other emission sources stemming from construction or other equipment. Data will be collected after the expanded roadway is open to traffic as well, requiring many months or years. A plan to specify periodic sampling will be developed to cover a pre-defined schedule including all times of the day and night through all seasons of the year.
The estimation of baseline and future highway emissions at each monitoring study site will be modeled.
The Parties agree to encourage independent third parties to compare modeled estimates of MSAT and PM2.5 concentration levels with monitored concentrations for various time periods, meteorological conditions, traffic conditions and other variables. The analysis should attempt to determine conditions or variables that most strongly affect model performance, including whether variance between monitored concentrations and concentrations simulated by the model is consistent over time, or differs depending on changes in variables measured in the monitoring studies.
