Sensitivity through the Modeling Chain for Project Level Air Quality Analyses

Focus Area

Air Quality


Air Quality






1-2 years

Research Idea Scope

This research will provide information on the sensitivity of ambient air quality to project-level traffic and other key inputs in the traffic, emission and dispersion modeling chain including meteorological inputs. The research approach will be to create a set of hypothetical project situations and test the sensitivity of modeled air pollutant concentrations to key inputs in the modeling chain, beginning with a variety of traffic data input configurations, such as different ways of characterizing speeds and link source types for the same study area, as well as to uncertainties in meteorological inputs such as wind speed and direction and mixing height. The configuration scenarios or range of inputs tested should reflect real-world situations, drawing on real-world project data as much as possible. The research should prioritize pollutants subject to project-level conformity requirements, especially particulate matter (PM), but may also consider MSATs. Tasks: 1. Summarize existing findings on the sensitivity of emissions and ambient air quality to changes in project-level traffic and other key inputs in the traffic, emission and dispersion modeling chain including meteorological conditions and data. 2. Propose a set of hypothetical project situations and a structured set of tests with proposed real world data sources for the sensitivity of modeled ambient air quality to traffic and meteorological inputs for these situations. 3. Prepare data and conduct the air quality sensitivity tests. 4. Assign qualitative (e.g., low, medium, high) cost estimates for developing or obtaining each of the identified high priority inputs. To the extent feasible, develop recommendations for the most cost-effective inputs on which modelers should focus to conduct project-level analyses for each key input in the traffic, emission and dispersion modeling chain. 5. Document findings in a report that provides guidance for practitioners on which data and analysis items are most important for influencing ambient air quality near transportation projects under different circumstances.

Urgency and Payoff

A number of recent projects have assisted practitioners with improving inputs for project- and regional-level emissions modeling. NCHRP Report 765 (Project 08-83) provides guidance on methods, data sources, and procedures for producing travel forecasts in project-level analysis. NCHRP Report 210 (Project 25-38) provides guidance on developing MOVES inputs, including traffic and activity inputs. NCHRP 25-25 Task 96 is developing a quick reference guide for traffic modelers detailing best practices and guidance on preparing traffic inputs for project-level air quality analyses. NCHRP Project 25-48 is developing an interface between traffic, emissions, and air quality models. An area that is still lacking is the understanding of the relative impacts of key input assumptions throughout the modeling chain on local air quality estimates. A previously submitted topic at the TRB ADC20 Project-Level Subcommittee, on Resource Allocation for Project-Level Analyses, identifies the need to help practitioners understand the sensitivity of key inputs throughout the traffic, emission and dispersion modeling chain, which they can then use to prioritize the level of effort they spend on developing the most important inputs, i.e., the ones to which the results of the modeling chain are most sensitive. Some sensitivity analyses on MOVES inputs have been conducted, including work by the Volpe National Transportation Systems Center for FHWA, NCHRP Report 210, for NCHRP Project 08-101, and for the Coordinating Research Council. These have covered project-scale inputs to varying degrees. However, more detailed analysis of the impacts of traffic inputs would be helpful for traffic and air quality modelers tasked with prioritizing and collecting local input data. Also, the effect of changes in traffic data inputs and assumptions on local air quality has not been well documented. These effects can be estimated through the use of dispersion models. Transportation and air quality planners are interested in understanding the extent to which changes in various emissions and dispersion model inputs might influence an area’s ability to meet National Ambient Air Quality Standards (NAAQS) or other indicators of pollutants of concern such as mobile source air toxics (MSAT). NCHRP 25-25 Task 70 examined changes in MSAT pollutant concentrations (including diesel PM) for two hypothetical projects, but did not perform a detailed assessment of sensitivity to the full range of traffic and meteorological inputs that might be prepared. Traffic and air quality modelers face a range of choices in developing inputs for project-level air quality analysis. Average speeds or drive cycle or vehicle-specific power (VSP) distributions? If average speeds are used, for what disaggregation of link segments? In which cases should idling be explicitly considered (e.g., for port and terminal projects)? For traffic volume and source type data, what level of detail should be collected – just light vs. heavy vehicles, or should additional data collection focus on link-specific distributions of passenger cars vs. light trucks, single-unit vs. combination trucks, buses, etc.? Does the source type data affect all pollutant concentrations equally? How does the impact of uncertain meteorological variables compare with traffic variables? Modelers need to know which inputs are likely to most influence ambient air pollution concentrations to determine how to prioritize the limited resources available for data collection and analysis.

Suggested By

Chris Porter Cambridge Systematics 617-354-0167

[email protected]