Field evaluation of vegetation and noise barriers for mitigation of near-freeway air pollution under variable wind conditions

Traffic-related air pollutants are a significant public health concern, particularly near freeways. Previous studies have suggested either soundwall or vegetation barriers might reduce the near-freeway air pollution. This study aims to investigate the effectiveness of a combination of both soundwall and vegetation barrier for reducing ultrafine particles (UFPs, diameter ≤ 100 nm) and PM_2.5 (diameter ≤ 2.5 μm) concentrations. Concurrent data collection was carried out at both upwind and downwind fixed locations approximately 10-15 m away from the edge of two major freeways in California. This study observed that the reduction of UFP and PM_2.5 was generally greater with the combination barrier than with either soundwall or vegetation alone. Since there were no non-barrier sites at the study locations, the reductions reported here are all in relative terms. The soundwall barrier was more effective for reducing PM_2.5 (25-53%) than UFPs (0-5%), and was most effective (51-53% for PM_2.5) when the wind speed ranged between 1 and 2 m/s. Under the same range of wind speed, the vegetation barrier had little effect (0-5%) on reducing PM_2.5; but was effective at reducing UFP (up to 50%). For both types of roadside barrier, decreasing wind speed resulted in greater net reduction of UFPs (i.e., total number particle concentrations; inversely proportional). This trend was observed, however, only within specific particle size ranges (i.e., diameter < 20 nm for the soundwall barrier and 12-60 nm for the vegetation barrier). Out of these size ranges, the reduction of UFP concentration was proportional to increasing wind speed. Overall findings of this study support positive effects of soundwall and vegetation barriers for near-freeway air pollution mitigation.