California Air Resources Board

Principal Investigator/Author: Suzanne Paulson

Contractor: University of California, Los Angeles

Contract Number: 13-306

Project Status: Completed

Relevant CARB Programs: Atmospheric Processes

Topic Areas: Field Studies, Modeling, Sustainable Communities

Research Summary:

Traffic-related air pollutants are a significant public health concern near freeways. Previous studies have suggested that soundwall and/or vegetation barriers (defined here as any substantial installation of vegetation on either side of the sound barrier, trees or tall bushes etc.) may reduce near-freeway air pollution, but the literature is inconsistent, and data for vegetation and other conditions common in California are very limited. This research project combined mobile and stationary measurement and modeling approaches to evaluate the impact of various barrier configurations at four sites in California, and made pair-wise comparisons of the following configurations: no wall, sound wall only, vegetation only, and combined soundwall-vegetation combinations (eight study locations in total, each with a perpendicular transect). Chosen study sites were located along major highways in Santa Monica, Encino, Sacramento and Riverside, and if present, trees were substantially taller than the solid barriers. Three of these sites were chosen as test sites for daytime conditions (Sacramento, Encino and Riverside) and one was chosen for nighttime and early morning conditions (Santa Monica).

Mobile and stationary measurements were conducted on transects perpendicular to the main roadway. Stationary measurements for wind, ultrafine particles (UFP) and oxides of nitrogen were made on both sides of the soundwall. Mobile measurements were made using an electric vehicle for several hours each day during one to three week field campaigns, and included high time-resolution measurements of UFP, oxides of nitrogen, carbon dioxide, and PM2.5 (particulate matter with diameter of 2.5 µm). A dispersion model was developed and applied to analyze data from two of the field studies at Riverside and Sacramento, and provides technical and planning guidance on the effectiveness of soundwall-vegetation combinations for mitigating near-roadway air pollution.

The study concluded that dense vegetation that is taller than the barrier appears to be a clear benefit during daytime as well as early morning. The benefit is significant especially very close to the barriers, but it decays quickly, such that other mitigations might also be worth considering, especially for sensitive receptors.

Final Report & Model: Please email research@arb.ca.gov to request the final report and Model for Impact of Roads with Barriers (MIRB) generated by this research contract.