Background: Passage of the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA) and the Clean Air Act Amendments of 1990 (CAAA) have significantly changed the way in which transportation-system and project-development decisions contribute to air quality improvements. Transportation improvements must demonstrate conformity with national air quality standards. This is true for pollutants that impact the entire region as well as those that have more localized impacts in areas adjacent to transportation facilities. Carbon monoxide (CO) and particulate matter (PM) are considered microscale pollutants (their concentration can change significantly over a relatively small distance). In a nonattainment area, EPA regulations require that a project can not cause or contribute to a new violation of a national ambient air quality standard (NAAQS) and must reduce the concentrations of existing violations. To avoid a nonattainment designation, a project expected to cause a violation of the standards may not be advanced in an attainment area.

The air quality impacts of proposed transportation plans, programs, and projects are estimated and evaluated through the application of various required computer models. In the case of microscale pollutants, projects are assessed by so-called dispersion models that predict concentrations at specific locations either directly or indirectly affected by the project. The dispersion models take into account existing pollutant levels and present transportation characteristics at the specific location and meteorological conditions experienced at the site, in order to predict future concentrations of pollutants. Modeling techniques for CO have become more conservative, making it more difficult for projects to meet conformity tests. This is most apparent in the use of conservative default values (i.e., model inputs used where specific observed data are not available) that may overestimate potential pollution concentrations associated with a transportation improvement project. The justification for the use of conservative default values is to avoid future violations of air quality standards that might be caused by underestimating current pollution factors. Actual monitoring data seem to indicate that CO concentrations are being reduced. Because of the limited number of monitors and their sites, it is not clear whether monitoring results are giving a completely accurate picture of the entire situation. There appears to be a discrepancy between modeled and observed maximum pollutant concentrations, under current conditions, which can lead to vastly different assessments of future pollution impacts.

In cases where CO models predict possible violations, project development actions must be stopped while additional monitoring at the site is carried out. This monitoring is designed to obtain site-specific input data in order to calibrate the dispersion model to more accurately describe the current conditions at the project site. Such monitoring can add years to the project development cycle just to confirm that the high concentration predictions are due to inaccurate or overly conservative model input assumptions.

Research is needed to determine if a short-term CO and PM measurement procedure (e.g., day, week, month, 3-month) can be developed that will give reliable input data for use in models to estimate the potential to exceed the standards at a project location. If this short-term monitoring procedure can be linked to modeling as a validation tool for conformity and other environmental determinations, then unnecessary project development delays can be avoided.

Objective: The objective of this project is to develop a short-term monitoring procedure that can produce more accurate input data for air quality dispersion models and can do so in a manner requiring less data collection and less time to complete than do current monitoring requirements. This effort will require a determination of what monitoring and data are necessary to reliably estimate peak emission concentrations of carbon monoxide and particulate matter near proposed roadway improvements. Also, it will require examination and documentation of the conditions that produce observed high concentrations of CO and PM near roadway facilities, in cases where there are discrepancies between modeled and monitored concentrations describing current conditions. It should result in the development of a procedure that can accurately assess the validity of peak CO or PM predictions emanating from air quality models. This procedure should be based upon observed present conditions as opposed to modeled values and should provide an assessment of the differences between predicted and monitored concentrations appropriate for improving the reliability of model impact predictions.

Tasks: Phase I--Literature and Practices Review: (1) Review and document current literature and research on regulatory monitoring and modeling practices associated with project-level air quality analysis. (2) Review and document the current monitoring and modeling guidance employed by federal and state agencies. This review should include: comparisons of modeled and monitored data; identification of key factors and variables affecting concentrations; and a review of existing and evolving modeling and monitoring requirements. (3) Compile recent CO and PM monitoring data from a variety of sources that can serve as the basis for further evaluation. Potential sources may include: EPA's National Air Monitoring Stations (NAMS); State and Local Air Monitoring Stations (SLAMS); Special Purpose Monitoring (SPM) network; other special state studies; and other data sources. Assess and compile, as appropriate, concurrent and comparable hourly traffic and meteorological data. (4) Based on the data compiled in Task 3, develop criteria for integrating and evaluating CO and PM data, meteorological data, and traffic data; propose possible test locations for which data are available to use in developing a short-term monitoring procedure; and propose an analytical approach for examining the data in order to develop the short-term monitoring procedure. (5) Identify a task group of experts to carry out the review identified in Task 10. (6) Submit an Interim Report, summarizing the findings of Tasks 1 through 5 and presenting an updated work plan for Phase II, and obtain NCHRP approval to proceed to Task 7.

Phase II--Data Analysis, Procedure Development, and Testing: (7) Analyze monitored CO and PM data. The following two steps should be conducted in this task: (a) Assess traffic, meteorological, and other relevant conditions leading to high observed CO and PM concentrations. As a starting point, compare and contrast the worst case factors (e.g., low wind speeds, low ambient temperatures, wind direction, and atmospheric stability) employed in a project-level analysis to those same factors observed during high concentration events. At a minimum, this analysis should include: (1) an examination of relationships between high concentrations and key variables; and (2) an evaluation of relationships between both CO and PM violations across NAAQS averaging time periods (e.g., the 1- and 8-hour standards for CO, or daily and annual standards for PM). (b) Develop a procedure for analyzing monitored data at selected sites identified in Task 4 to: (1) establish probability distributions of CO and PM concentrations; (2) determine the probability of exceeding NAAQS; (3) evaluate, at each site, the relationships between worst case model input assumptions and observed high concentration conditions; and (4) determine the validity for a short-term monitoring approach that can accurately predict the frequency and probability of violating concentrations. (8) Document the short-term monitoring procedure for predicting CO and PM exceedances. At a minimum, apply and demonstrate the procedure in at least three locations, comparing the results of the short-term monitoring technique and the worst case EPA-approved modeling. (9) Submit an Interim Report summarizing and documenting the procedure and its development in Tasks 7 and 8. Obtain approval for proceeding to the next tasks.

Phase III--Review and Final Report: (10) Convene the expert task group to obtain feedback for further development and refinement of the monitoring procedure and recommendations regarding further field study of the procedure. (11) Refine and document the short-term monitoring procedure based on input received during the review. (12) Prepare a final report that documents the entire project and presents the recommended plan for further field study.

Status: The revised final report has been published.

Product Availability: The report is available as NCHRP Report 479.