Cleaning the Air through Ecosystem Interventions
By: Hyphae Design Laboratory
Tags: Ecology, Air

Traffic sourced air pollution is a major contributor to human disease, with children and the elderly being most vulnerable to its toxic effects. While efforts to reduce this pollution at the source by implementing emissions controls at the automotive design stage have yielded great progress in the last 100 years, global health-care costs caused by traffic sourced air pollution are still in excess of 2 trillion dollars per year (Roy & Axel Braathen, 2017). Thus there is a need to mitigate air pollution after it is already created as a complement to emissions regulations. In indoor environments this can be accomplished by using recirculating air filters, and many schools use these to protect children from air pollution while they are inside at school. However, these do not protect children during outdoor activities, when exposure risk may be higher due to increased respiration rates.

Is it possible to clean pollution out of outdoor air?

In fact, numerous studies have found that trees and other vegetation can mitigate air pollution by capturing it on their leaves, by diluting it by facilitating vertical mixing, and by reducing air temperature. In order to quantify the health-care cost return on investment for planting trees, Hyphae and our partners on the Green for Good (G4G) team (Institute for Healthy Air Water and Soil, University of Louisville, Washington University in St. Louis, Louisville Metro Government Office of Sustainability, Owsley Brown II Family Foundation, and the Funders’ Network for Smart Growth and Livable Communities) sought to test the amount of air pollution that could be mitigated by trees at Saint Margaret Mary K-8 school in Louisville, KY, and to measure the effects of the tree planting on health biomarkers in the students and teachers at the school. This school is located directly downwind from a busy roadway, and so Hyphae designed a roadside vegetated air barrier to maximize the interception and dispersion of traffic-sourced air pollution. Figure 1 shows a basic graphic of the measurement area for the intervention.

Figure 1: Measurement area before and after with control and bio-filter zones

Existing literature on air pollution mitigation by roadside vegetation includes the observation that as much as 37% of road sourced ultrafine particulates can be captured or dispersed during crosswind conditions when the trees have dense foliage and are planted close together (Al-Dabbous & Kumar 2014). However, inconsistent effects are seen when vegetation is sparse and the vegetated buffer discontinuous (Hagler et al. 2012). Tong et al. (2016) simulated particulate deposition and dispersion on roadside vegetated buffers, and found mitigation increased with increasing leaf area density and buffer depth, with 12 meter deep buffers performing much better than 6 meter deep buffers (where depth is the horizontal dimension perpendicular to the roadway length). Fuji et al. (2008) conducted wind tunnel experiments with different species of tree foliage and consistently found higher particulate capture by conifers than the tested broadleaf species.