Menu of Mobile Source PM2.5 Control Strategies, Including Evaluation of Costs and Effectiveness

Focus Area

Air Quality


Air Quality




Over $750,000


1-2 years

Research Idea Scope

State and local transportation and air quality agencies have limited knowledge of the full range of options to reduce PM2.5 from transportation sources. PM10 nonattainment areas around the country have implemented local-level transportation-related control strategies for PM, such as, use of roadway surface treatments and road construction dust controls. Control strategies in use in PM10 non-attainment areas are targeted towards the coarse mode fraction and are unlikely to be sufficient or appropriate for control of PM2.5 in many parts of the country designated as nonattainment under the new PM2.5 standard.  Only a limited understanding exists of the costs and effectiveness of these PM control strategies particularly in terms of how these measures contribute toward meeting the PM2.5 NAAQS since areas have not had to address this smaller size fraction.

This project would involve a set of case studies of existing transportation PM control strategies to identify the full costs of implementing these measures and to measure effects at reducing PM10 emissions, PM2.5 emissions, and PM-precursor emissions. The estimated reductions would be compared with the reductions required within the SIPs. Emission reductions would also be compared with estimates of the effects of improvements in diesel fuels and vehicle technologies to assess the relative contribution of these strategies.

This project involves developing a menu of control strategies to help inform decisionmakers about the full range of options available for PM control. The emissions effects of each option will be assessed quantitatively using emission models in a range of different geographic settings (e.g., areas with different temperatures, road types, vehicle fleet mixes). Issues of concern to transportation decisionmakers will also be identified, including estimated costs, implementation issues, effects on other transportation goals (e.g., safety, mobility), and locational factors.

TERI Administrator Note (June 2007): Research Underway
NCHRP 25-18: Particulate-Matter (PM2.5 and PM10) Apportionment for On-Road Mobile Sources
(Project has been expanded to include evaluation of control strategies see note at end of description.)

Background: The Environmental Protection Agency (EPA) has adopted new standards for particulate matter (PM) with potentially broad effects on the transportation sector. These new standards are based on particles 2.5 microns in diameter and smaller (PM 2.5); the previous standard was based solely on particles 10 microns in diameter and smaller (PM 10). The new particulate standards have been established on the basis of health impacts but were created with limited knowledge of emission sources. Regional- and project-level modeling of particulates are likely to be required by federal regulations. However, current methodologies are inadequate and uncertain for estimating PM 2.5 and PM 10 emissions. The EPA has stated that full implementation of the conformity rule will be deferred until acceptable methodologies are developed and approved.

Research is needed to determine emission factors from transportation-related sources. Particulates directly emitted by motor vehicles (e.g., vehicle exhaust, tire, and brake wear) as well as indirectly emitted particulates (e.g., dirt or other materials from vehicles) need to be characterized and quantified. Re-entrained road dust also needs to be quantified. Large variations in particulate emissions occur because of various factors such as vehicle type and condition, roadway type, and climate. These important factors affecting emission rates must be quantified. Once PM 2.5 and PM 10 emission rates are known, dispersion-modeling algorithms may need to be revised to more accurately predict ambient particulate concentrations.

Objective: The objectives of this research are as follows: (1) to apportion, from among major sources, the contribution of on-road mobile sources of direct and indirect emissions that contribute to ambient PM 2.5 and PM 10 concentrations near roadways and (2) to determine appropriate PM 2.5 and PM 10 emission factors for use in estimating emission rates with microscale dispersion models. This research should focus its primary emphasis on PM 2.5 and is confined to paved road surfaces in urban environments.

Tasks: The research objectives will be accomplished through the following tasks: (1) Literature Search–Throughout the study, the research team is expected to perform critical reviews of relevant regulations and completed and ongoing research. A database of literature and other information will be maintained throughout the study. (2) Research Design–A detailed study design for Tasks 4 and 5 will be prepared that provides for identification, collection, analysis, control, and measurement of key variables leading to the emissions apportionment and determination of emissions factors for PM2.5 and PM10. The research team will prepare a quality assurance/quality control (QA/QC) plan for data collection, handling, and analysis including both roadway-monitoring data as well as laboratory data where appropriate. Attention must be given to regional and temporal variations in emissions due to climate, vehicle characteristics, geology, roadway types, and roadway maintenance. (3) Interim Report–Prepare initial findings from Tasks 1 and 2 and recommend any work plan modifications for panel review and approval. (4) Data Collection and Database Development–Based on the approved work plan, characterize mass emission rates, chemical composition, and size distributions of particles associated with traffic operating on paved urban roadways. Both re-entrained dust and vehicle exhaust emissions will be characterized to determine sources of PM 2.5 and PM 10. Compile a database appropriate for data validation and analysis. (5) Data Analysis and Emission Factor Development–Analyze the data and develop emission factors suitable as input for dispersion modeling. Validate data according to the QA/QC plan in order to establish the accuracy of the developed emission factors. (6) Panel Presentation–After initial analysis and preliminary development of emission factors, meet with the panel to present findings of the data collection and analysis and to discuss approaches for further development of emission factors and modeling input. (7) Dispersion-Modeling Application Test–Using existing dispersion models, develop a protocol for testing within a dispersion-model application the emission factors developed in Task 5. Evaluate the suitability of existing particulate matter data for this purpose, such as the National Air Monitoring System (NAMS and State and Local Air Monitoring System (SLAMS), and determine the need for new concentration-data collection. The protocol should be designed to compare actual monitored concentrations with modeled concentrations, adjusted for background. (8) Final Report–Prepare a final report that clearly presents the findings and recommendations from the previous tasks. If possible, identify and recommend possible transportation control strategies that may be effective in reducing and controlling PM 2.5 and PM 10 emissions and concentrations.

Status: AASHTO’s Standing Committee on Research approved additional funding ($450,000) to support a continuation of this research to investigate and quantify the effectiveness of transportation control measures in reducing particulate sources. An amended schedule has been proposed to allow for further field work. A no-cost time extension will be required and requested to carry out the work under the revised schedule. The final report for the initial phase has been submitted and a publication decision is pending. A second final report covering the continuation work will be prepared upon completion of that research.


Urgency and Payoff

This project will give transportation and air quality planners information to help them select the most appropriate control measures for transportation sources if required.

Suggested By

FHWA Strategic Work Plan for Particulate Matter Research