Pollution Removal estimates the amount of air pollution removed by trees each year as pollutants deposit on leaf surfaces or are taken up through leaves. The value is reported as ounces per year.
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Poor air quality is a common problem in many urban areas. It can lead to adverse effects on human health, damage to landscape materials and ecosystem processes, and reduced visibility. The urban forest can help improve air quality by lowering air temperatures, directly removing pollutants, and reducing energy consumption in buildings, thereby reducing air pollutant emissions from power sources. Trees also emit volatile organic compounds (VOCs) that can contribute to ozone formation. However, integrative studies have shown that increased tree cover reduces ozone formation (Nowak and Dwyer 2000).
Pollution removal¹ by trees at Purdue was estimated using field data and pollution and weather data from the time of calculation. Pollution removal was greatest for ozone.
Emissions vary among species based on species characteristics (e.g., some genera such as oaks are high isoprene emitters) and the amount of leaf biomass. 38% of the urban forest’s VOC emissions were from Northern red oak and Pin oak. These VOCs are precursor chemicals to ozone formation.²
¹ PM10* is particulate matter less than 10 microns and greater than 2.5 microns. PM2.5 is particulate matter less than 2.5 microns. If PM2.5 is not monitored, PM10* represents particulate matter less than 10 microns. PM2.5 is generally more relevant in discussions of the effects of air pollution on human health.
² Some economic studies have estimated VOC emission costs. These costs are not included here, as there is a tendency to add positive dollar estimates of ozone removal effects and negative dollar estimates of VOC emission effects to determine whether tree effects are positive or negative relative to ozone. This combining of dollar values to determine tree effects should not be done; estimates of VOC effects on ozone formation (e.g., via photochemical models) should be conducted and directly contrasted with ozone removal by trees (i.e., ozone effects should be directly compared, not dollar estimates). In addition, air temperature reductions by trees have been shown to significantly reduce ozone concentrations (Cardelino and Chameides 1990; Nowak et al 2000), but are not considered in this analysis. Photochemical modeling that integrates tree effects on air temperature, pollution removal, VOC emissions, and power-plant emissions can be used to determine the overall effect of trees on ozone concentrations.
Ecosystem service estimates were generated using i-Tree Eco, a peer-reviewed urban forest assessment tool developed by the USDA Forest Service and partners. Learn more at i-Tree Eco.
