The session focused on 1) assessing the current understanding of the variation (spatial, temporal, and chemical) of ambient PM concentration across the United States and 2) the causes and processes that lead to elevated PM periods. The review and assessment spans spatial scales from national to state to local level.
Participants highlighted the need for research to focus on collecting more data from the limited number of sites that have a comprehensive data collection and analysis effort in order to better understand the variation in PM concentrations. The identified research needs applicable to the transportation community included:
Gain better understanding of spatial, temporal and chemical variation of PM emissions within and across classes of vehicles,
Develop strategies to identify and measure high PM emitting vehicles,
Update motor vehicle PM source chemistry profiles to account for changes in fuels, brake pads, new engine technologies, etc., for both on-road and off-road vehicles, and
Gain a better understanding of the relationship between air toxics and PM emissions for transportation sources.
Researchers also noted that broad interagency coordination is needed across all of the affected community, as no single source type (mobile, utility, industrial, and fugitive) is likely to dominate the source contribution for any location.
Contribution of Off-Road Emissions and Other Combustion Sources: Although both groups saw on-road mobile sources as significant contributors to the PM problem, off-road mobile sources were seen as a contributor that could easily rival mobile source contribution and have a significant impact on the chemical variation of PM. Identifying the contribution of off-road mobile sources was therefore identified as an important priority.
Need linkages to EPA's New Supersites: Researchers believed that an opportunity exists to improve understanding of local transportation measures by collecting information such as vehicle counts, fleet mix, age distribution, and facility types during planned and ongoing studies, for at least some of the new EPA's PM Supersites. Collection and analysis of PM and transportation data at the PM Supersites should be undertaken with an awareness of transportation networks to further the understanding of the temporal and spatial distribution of PM.
Examination of the Speciation Network: The PM speciation network (just now in the early stage of data collection) should be examined or supplemented for a better understanding of transportation source contribution. Improvements might include a higher sampling frequency and an expanded list of compounds for analysis.
Research Topic #1: Undertake a Critical Analysis of New PM2.5 and PM10 Monitoring Network. Both sessions identified examination of the monitoring network as a very useful way to better understand the spatial, temporal and chemical variation of PM across the nation and also locally. Currently, only a few areas across the country have an adequate database to describe temporal, chemical and spatial variations. Examination of the national network would include exploring both the mass and speciated-monitoring network in combination with key transportation data (VMT, facility type, fleet mix and age distribution) to determine the transportation related source contribution to the PM problem.
Research Topic # 2: Gain a Better Understanding of Variations Among Emissions Within Classes as Well as Across Classes and Strategies to Tackle the High Emitters: Current research strongly suggests that 80 percent of the exhaust PM is emitted by 20 percent of the vehicles, either because of age or improperly functioning emission controls. Further research is needed on how to identify these high PM emitters. In addition, diesel-fueled vehicles are seen as significant contributors to exhaust PM; they typically account for only 5 percent of the VMT, but more than half of the exhaust PM. However, the use of cleaner diesel-fueled vehicles introduced since 1994 has increased the PM exhaust contribution share from gasoline-fueled engines. Further research is needed to study PM emissions across vehicle classes.
Research Topic # 3: Develop Better Estimates of all Vehicle Miles Traveled. Current estimates of total VMT contain a degree of uncertainty, particularly for arterials and even more so for local roadways. An incomplete assessment of VMT adversely affects the certainty in the PM emissions estimate. This uncertainty will have broad implications on any type of conformity analysis based on PM emission inventories. Furthermore, a lack of understanding of VMT by facility type affects the estimated spatial and temporal distribution of PM emissions, and as a result, may hamper the implementation of effective control strategies.
Research Topic # 4: Update Motor Vehicle PM Source Chemistry Profiles for Changes in Fuels, Brake Pads, New Engine Technologies, etc. Much of the underlying basis for source contribution is based on PM source chemical profiles. However, much of the motor vehicle PM source profiles are significantly dated and in need of updating. Updating the PM source profiles is needed in particular for motor vehicles with new engine technologies as well as for brakes and tires.
The overall discussion focused on answering questions about the level of uncertainty associated with the contribution of transportation sources to primary and secondary PM. This included discussion of the characterization of PM by location and the spatial extent to which the PM monitoring sites can be considered representative.
Attendees noted the important gap between research efforts that have been conducted to date on PM emission factors and the emission factor models that are used for regulatory purposes, that have not been updated to reflect the results of this research. Thus, an important practical gap in improving the current emission factor models requires updating the current regulatory models with the latest information and also expanding their functionality. Conducting validation studies and improving the packaging for end users are other important needs to improve the use of these models.
An overarching theme was the need to integrate the air quality planning with the transportation planning process. The two processes require frequent feedback both in the regulatory setting, and in expanding the understanding of local PM source contribution.
Incorporating the Research into Modeling Tools: Both groups identified a number of studies that have been undertaken but whose results have not yet been incorporated into current emission models (PART5, EMFAC7G). This lack of integration hinders some of the current planning efforts because the current tools are based on dated information, apply to limited situations and are not comprehensive. Further, the models need to be validated for a broad range of settings (e.g., pavement types, silt loading, levels of VMT, humidity).
Continue to Improve the Understanding of Transportation Source Contribution: It is important to the transportation community to better understand its contribution to the PM problem because of the implication for infrastructure development (e.g., new freeway construction or increases in VMT may lead to regional PM exceedance problems). To that end, methods and models need further development to improve the reliability of the source apportionment.
Research Topic #1: Undertake Source Fingerprinting of Mobile Sources. Special studies are needed to develop more up-to-date source fingerprinting profiles for gasoline and diesel-fueled vehicles. This will increase the robustness of conclusions drawn about source contribution of transportation versus stationary sources. Also, as part of the special studies, detailed data collection should be conducted to collect traffic information such as fleet mix, volume, age, fuel-type and speed. These data can be used in combination with mobile source modeling to corroborate source apportionment studies based on PM monitoring.
Research Topic #2: Collect Transportation Data in Connection with EPA's Supersites Monitoring Program. EPA's PM Supersites will soon be collecting a comprehensive suite of data including PM size distribution, mass, number and composition. Supplemental transportation data (VMT, fleet mix, age, fuel-type, activity and speeds) should also be collected so that analyses can be performed to determine transportation related source contribution at these intensively monitored sites. Analysis of these supplemental data will help in identifying the contribution from mobile sources and likely reduce the uncertainty in the source contribution.
Research Topic #3: Develop Better Understanding of Secondary Aerosols: Mobile source NOX and VOC emissions may at times contribute significantly to nitrate and secondary organic aerosol formation. Photochemical models are used to simulate the complex nitrate and secondary organic aerosol formation process. Based on these simulations the most effective control strategy can be developed. Currently, some locations have knowledge about local sulfate and nitrate formation, but very little about secondary organic aerosols, which at times may be a significant contributor to PM2.5. Research should be undertaken to better understand secondary organic aerosol distribution in both time and space. The research should also include further study to improve the understanding of the process leading to secondary organic aerosol formation from gas phase precursors.
Research Topic #4: Improve the Understanding of the PM Gas/Diesel Split: Much of the focus of primary PM emissions studies IS on diesel-fueled vehicles. However, recent model year diesel-fueled vehicles have drastically reduced their base PM emission rate and contribute only about 5 percent of the total VMT. Thus the fraction of gasoline to diesel contribution to PM is changing, and, with impending Tier II controls will continue to change. Research should focus both on the current gas/diesel PM split, and on the implications of future mandated changes and control technologies.
This session focused on identifying the current state of technology with respect to PM measurement methods and identifying those areas, that need further research and development. The areas identified included: the missing semi-volatile mass in routine measurements, the temporal and chemical resolution in the acquisition of PM data, and procedures needed to assure a consistent approach to the measurement of ultrafine particles.
Participants noted that transportation related parameters are frequently missing in PM study efforts. Both sessions identified the following overall gaps/weaknesses in current work:
Routine PM measurements are generally not helpful in understanding transportation related source contribution because the complex changes in source contribution mix over a 24-hour period,
Semi-volatile aerosols are probably significant and are generally underestimated with current measurement practices,
PM Supersites need additional traffic-related measurements, and
Improved measurements methods and calibration for PM ultrafine particles are needed.
Limited Understanding from Routine PM Measurements: Although analysis of the current PM10 (and future fine PM2.5 study) network does provide information on overall trends, it does not provide information about the source contribution or chemical characterization of the PM. Thus better understanding of the composition of PM will come as a result of studies directed at the PM speciation network, the Supersites and through special studies. Currently, only the New York Supersite is focusing efforts in order to study the relationship between transportation and PM.
Underestimation of Semi-Volatile Aerosols: Particles which are not measured as a result of deposition onto filters because of their volatile composition are probably significant at certain times and locations. Research methods need to be developed that can both quantify the underestimation and clarify when and where their contribution is significant.
Portable Sampling Devices: Current methods for assessing site representativeness as well as for identifying maximum concentration use saturation monitoring techniques. These techniques only provide 24-hour integrated concentrations. Research efforts are needed to develop methods that will give higher resolution (e.g., half-hourly or real-time) PM concentrations so that more definitive conclusions can be drawn about source contribution. These devices will need to be cost effective because of widespread deployment.
Ultrafine Measurement Techniques: Some studies have identified the fact that traffic contributes to a significant increase of ultrafine PM in urban environments and that ultrafine particles are more effective at reducing pulmonary volume exchange than larger particles. As a result, ultrafine particles are of significant interest. However, for ultrafine particles less than 40 nm, calibration is a serious problem. Calibration is the critical next step. In addition, the necessity for standardization in measurement approach is needed so that studies performed by different researchers may be compared.
Research Topic #1: Improve Measurement Method to Apportion PM Contribution from Mobile Sources. There is a need for measurement methods to better apportion the PM contribution from mobile sources, particularly for diesel-fueled vehicles. Special studies are needed to develop more up-to-date source profiles for gasoline and diesel-fueled vehicles. This will increase the robustness of conclusions drawn about source contribution of transportation. Also, as part of the special studies, detailed data collection should be conducted to collect traffic information such as fleet mix, volume, age and speed. These data can be used in combination with mobile source modeling to corroborate source apportionment studies based on PM monitoring.
Research Topic #2: Develop Techniques for Assessing Semi-Volatile PM: Currently, significant loss from semi-volatile material (e.g., ammonium nitrate, organic compounds) may occur due to filter artifacts (gas adsorption and chemical reactions). This loss of semi-volatiles may be significant, accounting for upwards of 20 percent of the total PM2.5 mass. A significant portion of the precursor semi-volatiles (NOX and VOC's) originate from combustion of gasoline and diesel fueled vehicles. Research is needed to develop practical methods, which can measure the loss of semi-volatile material during and after sampling. In addition, further work is needed to assess the positive artifact that may result from adsorption of gaseous organic compounds on the filter medium.
Research Topic #3: Supplement Targeted PM Supersites for Traffic Related Measurements: Currently, PM Supersites are focused on PM measurements by identifying source characterization (or characterizing sources) based on chemical mass balance. Additional measures should be made to corroborate findings based on direct measurements of high resolution CO, NO and NO2, and traffic information such as fleet mix, volume, age and speed. Analysis of these supplemental data will help in identifying the contribution from mobile sources and likely reduce the uncertainty in the source contribution.
Research Topic #4: Standardization and Calibration Techniques for Ultrafine Particles. It is likely that most of the exposure to ultrafine particles are from transportation related fuel combustion. Current techniques for ultrafine measurements are not consistent in their approach (e.g., some techniques use dilution and some do not use dilution). The need for a standard approach is a necessity so that concentrations may be compared. In addition, to be able to compare different measurement techniques a calibration or reference is needed. Current efforts in this area of research are currently in the formative stages.
The session focused on development of emission factors and emission inventories for PM emitted directly from motor vehicles and gas-phase pollutants that form particles in the atmosphere. Emphasis was also placed on the development of methods to better quantify secondary particulate formation and improvements to chemical mass balance models.
Participants in the session noted that the current weaknesses in developing emission inventories will hamper emission inventory-based control strategies (e.g., conformity) because the emission budgets will change as improvements to the inventory occur. The key weaknesses and data gaps identified by the group include:
High degree of uncertainty associated with exhaust emissions factors and emission inventories for all transportation sources.
Poor estimates of re-entrained road dust.
Poor estimate of emissions for brake and tire wear.
Limited tools and testing of models for secondary particulate formation.
Weakness in Emission Inventories: An accurate emission inventory is needed to assess control strategies and emission budgets for conformity. Unfortunately, the current emission inventories have a very high degree of uncertainty. Historically, primary emphasis has been placed on receptor modeling to determine source contribution, that relies on accurate and comprehensive emission source profiles, which change with new technologies and new fuels. In addition, receptor models cannot be used to project future source contribution. In order to assess future-year control strategies and emission budgets for conformity, accurate emission inventories are needed. Areas of important research to the transportation community include: improvements to emissions factors for both the on-road and off-road emission sources across all engine technologies, engine size, fuel, real-time driving cycles, altitude, and ambient temperature. PM emissions should be identified by particle size, as well as chemical and gaseous composition, especially for diesel-fueled engines.
Re-entrained Road Dust: Current modeling methods used to estimate re-entrained road dust emissions both for PM10 and PM2.5 are limited as to their range of functionality. They are based on a fairly limited data set of re-entrained road dust measurements and do not cover all conditions. Improvements are needed for the range over which the models are truly applicable.
Brake and Tire Wear Emissions: Emissions from brake and tire wear may be significant contributors to PM concentrations at roadway intersections or at freeway off-ramps. In addition, recent improvements in brakes and tire technology have led to smaller size PM and to changes in PM composition. A better understanding of the size and composition of today's tires and brakes is needed so that this information can be properly incorporated into emission inventories and modeling studies.
Secondary Particulate Modeling: Mobile source NOX emissions may at times contribute significantly to nitrate aerosol formation. Photochemical models are used to simulate the complex nitrate formation process. Based on these simulations, the most effective control strategy can be developed. Currently, a better understanding of the chemical and physical formation process is needed to improve reliability of existing models. In addition, the process leading to secondary organic aerosol formation from gas phase precursors is not well understood.
Research Topic #1: Develop Improved Estimates of Direct PM Emission Rates from Transportation Sources. The session identified that direct PM emission factors are the most important research area associated with emissions inventories. The primary purpose for the research will be to improve all the factors that affect PM emission rates for mobile sources. Current factors used in EPA's PART5 model are out of date. Thus the first step in improving the inventory will be to improve the base emission factors for diesel and gasoline fired engines for a variety of conditions, including:
Engine condition (age)
In addition information for the PM emission rate should include the particle size distribution and chemical composition. The PM emission rates should be based on "real-time" monitoring of mobile source PM. Besides PM emission rates, emissions efforts should also be directed at developing refined estimates of mobile source ammonia emissions because of ammonia's importance to secondary PM formation.
Research Topic #2: Develop an Improved Model for Re-entrained Road Dust. Current approaches to the modeling of re-entrained road dust are limited as to their range of applicability. A wider-range of "correction factors" are needed that characterize paved road surface conditions, including soil type, meteorology (wind speed, humidity, rainfall), traffic volume, road surface, road treatment, and maintenance. Ideally, these corrections will be based on first principles, but a statistical approach may also be feasible. Further research is needed to develop a full range of "correction factors" applicable to all conditions observed nationwide for both PM2.5 and PM10.
Research Topic #3: Improve Understanding of Today's Emissions from Brake and Tires. EPA's current emission factor model (PART5) for tire and brake wear is based on light-duty vehicles only and is also quite dated, not reflecting today's brake and tire wear compositional changes. The areas of further research for emission factor development include:
Expanded light-duty vehicle testing as well as heavy-, medium-duty vehicle testing,
Temperature effects on brake and tire wear,
Equipment variations (e.g., disc brakes, studded tires), and
Chemical composition and size distribution characterization.
Research Topic #4: Develop Improved Secondary PM Models. Currently, the relative contribution from transportation sources to secondary PM is poorly known. In many instances it is likely that gases from mobile sources are major contributors to secondary PM, which in turn may be a significant portion of the total PM inventory. Research is needed to reduce the level of uncertainty in determining source contribution and hence provide useful control strategy information. Along with continued improvement in the emission inventory, the areas that need further research for PM modeling include:
Improved understanding of the growth in particle formation, starting with smallest size particles (nanometers) to accumulation mode size particles and their incorporation into a PM model,
A highly detailed accounting of the physical and chemical processes leading to fine PM formation,
High time resolution information on particle size and chemically speciated sampling, and
Improved modeling and computational software -- Modeling is constrained by typically available computational resources. Future modeling systems should be evaluated against future high resolution data, and be built flexibly enough to allow for use now, but also for easy scaling to higher resolution or sophistication as computational resources improve.
These sessions focused on 1) developing a better understanding of impacts of existing control strategies, 2) undertaking a critical evaluation of the accuracy of emission estimates, and 3) examining those future control strategies most likely to be needed at the federal, state, and local levels.
Participants highlighted the relative lack of knowledge about other PM-related issues as a challenge to conducting research on control strategies. Weaknesses highlighted by participants included:
Lack of understanding about secondary versus primary PM,
Inaccuracies in the mobile source PM inventory,
Geographic diversity of PM problems, and
Weaknesses in travel and emissions models and data inputs used to evaluate controls.
These concerns led participants in both sessions to place a priority on conducting research in other research areas that may provide relevant information on control strategies.
Coordination Among Partners: The morning group participants, in particular, suggested that while FHWA has a different set of concerns from the traditional research community, FHWA must look for partners in research. Members of both groups expressed surprise about the existence of the NCHRP project presented at the introduction session, and concern that this research initiative was not more widely known. Clearly, coordination must be strengthened across the research community.
Weaknesses in Travel Models/Travel Data: The afternoon group discussed interactions between travel models and emission models, and concluded that there are weaknesses in the ability to gather necessary travel data (e.g. truck classes, VMT by category, etc.) for purposes of modeling PM emissions from transportation sector. MPO representatives talked about the limited applicability of travel models to analyzing PM issues. The group indicated that research into better data collection, improved travel models, and stronger interfaces with emissions models are all needed.
Weaknesses in Emissions Models: Basic weaknesses in models and data inputs make accurate estimates of control strategy effectiveness difficult. Particularly, failure to account for factors such as ammonia from catalyst-equipped vehicles.
Control Strategy Categorization: The a.m. group suggested that there are a number of alternative ways to categorize transportation related control strategies: 1) by implementation level, i.e. federal, state, or local; 2) by behavioral changes -- e.g., car pooling, spare the air days; or 3) by type, e.g. engine technology and inspection programs, fuels. There was a divergence of opinion about which types of control strategies should be pursued.
Research Topic #1: Review of Existing Control Strategy Impacts. Both session participants agreed that a valuable starting point for future research is a comprehensive review of PM control strategy reductions that are achievable with existing PM regulations.
The purpose of such a review should be to determine, to the extent possible, whether existing control strategies can achieve reductions likely to be required under current and future standards, and if not, to provide an assessment of the extent of the "gap" that must be filled. Control strategies suggested for inclusion in this review included:
Low sulfur diesel,
Recent (1994 on) transit bus regulations,
Heavy duty vehicle NOX/PM standards,
Tier II standards for light duty vehicles (diesel and gasoline), and
Off-road engine controls -- locomotives, marine, and others.
Research Topic #2: Critical Assessment of Accuracy in Control Strategy Estimates. Both the a.m. and p.m. groups raised concerns about the accuracy of existing control strategy reduction estimates and agreed that a critical assessment of the validity of control strategy reduction estimates is needed. The p.m. group discussed some of the reasons for weakness in the current estimates. The focus of this discussion was on:
Inaccurate data on travel, particularly for trucks, that limits accuracy of modeling,
Lack of knowledge about secondary PM, and
A poor understanding of in-use deterioration rates.
Research Topic #3: Identify and Examine Key Control Strategies Most Likely to be Required (e.g., Inspection & Maintenance & On-Board Diagnostics, After Engine Treatment Technologies, and Fuels). The morning group suggested that given current uncertainties in our understanding of PM, control strategy research should be directed to those control strategies that are most likely to be required. General consensus among both morning and afternoon groups was that I/M & OBD are both strong candidates for such research. As members of the afternoon group observed, emission control equipment may experience significant deterioration in actual vehicle use, and I/M or OBD would help address that concern. Other potential research topics include after treatment technologies, and fuels, reformulated gas, pricing & market strategies, conformity, and freight related "TDM" style measures.
Research Topic #4: Improve Travel and Emissions Models. The afternoon group discussed at length the inadequacies in existing models used to evaluate control strategies and develop inventories. The data underlying these models are often of poor quality -- with inadequate or inaccurate information about vehicle mix, etc. In addition, the design of the models is not sophisticated enough to address issues such as secondary PM formation.
Research Topic #5: Interactions Across Pollutant Control Strategies. The afternoon group indicated that controls on NOX and VOC might have antagonistic effects. For example ammonia from catalyst equipped vehicles may increase PM problems, but relatively little scrutiny of this issue has occurred to date.