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Evidence for Near-Road Air Pollution Abatement by Tree Cover
Citation:
Baldauf, R., G. Hagler, H. Brantley, AND L. Jackson. Evidence for Near-Road Air Pollution Abatement by Tree Cover. ACES - A Community on Ecosystem Services, Washington, DC, December 08 - 12, 2014.
Impact/Purpose:
To present near-road vegetative buffer research findings at ecosystem services conference.
Description:
Urbanized areas represent concentrated demand for ecosystem services to buffer hazards and promote healthful lifestyles. Urban tree cover has been linked to multiple local health benefits including clean air and water, flood and drought protection, heat mitigation, and opportunities for physical activity, social engagement, exploration, and play. Sensitive and vulnerable populations have been shown to benefit disproportionately from nearby ecosystem services; these populations include those who live, work, and go to school near busy roadways.Globally, populations with regular exposure to the near-road environment exhibit more respiratory and cardiovascular illnesses, adverse birth outcomes and developmental problems, cancers, and premature mortality. Vehicular emissions elevate near-road concentrations of many air pollutants including carbon monoxide, nitrogen oxides, and fine particles. Near-road research has begun to explore tree cover for its ability to block or trap toxic particles, and to absorb or neutralize harmful gases. Laboratory experiments under idealized conditions have demonstrated the potential for tree leaves to collect particles, and for tree buffer strips to divert gaseous plumes into the upper air column, where concentrations are diluted through turbulence.Preliminary field studies in central North Carolina and Detroit, Michigan, used stationary and mobile air monitors to compare particle concentrations behind near-road tree buffers with concentrations in nearby roadside clearings. Data revealed large variability in the clean air benefits of near-road tree cover, even showing some higher particle concentrations behind tree buffers than in open roadsides. Wind speed and direction were the most significant mediators. In downwind and low wind conditions, the effect of tree buffers included up to 30% lower concentrations of ultrafine particles (< 0.5 ug) and up to 22% lower concentrations of black carbon (an indicator of diesel exhaust). Evergreens were more effective buffers than deciduous trees.Findings suggest that tree cover can significantly moderate ambient concentrations of harmful airborne particles adjacent to busy roadways. However, effectiveness is highly dependent on site conditions, which can negate or even reverse expected clean air benefits. In addition, buffers may merely deflect roadway particles from the immediate near-road zone to more distant areas or back onto the roadway itself, transferring rather than reducing health risks. This area of research is still emerging and more analyses are needed. Buffer species, height, depth, and porosity are critical design variables that require further study. Epidemiology research is also recommended to assess health outcomes in near-road populations under buffered versus open roadside conditions.Although this material was reviewed and approved by EPA, it does not necessarily reflect official Agency policy.