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Document Type

Article

Publication Date

2019

Publication Citation

94 Indiana Law Journal Supplement 79 (2019)

Abstract

This Article tells a story that is true but seems completely wrong: Trees can make air pollution worse. Smog and ground-level ozone require two chemical ingredients to form: nitrous oxides (NOx) and volatile organic compounds (VOCs). On a warm, sunny day, these two precursors combine to form smog and ground-level ozone, a pollutant. While NOx are pollutants that are largely human-created, VOCs can originate with plants. In fact, emissions of just one type of VOC from trees exceed all human-caused emissions.

This Article presents new research on the impact of plants, especially trees, on air quality. The science is complicated and evolving, but some conclusions are possible. Different species emit greater or lesser amounts of VOCs, and emissions vary through the year. Some plant species also consume atmospheric VOCs, enough to outweigh their own emissions and thus remove VOCs from the air on net. Trees generally have an outsize impact as compared with other plants because trees are large plants and therefore large emitters. Thus, the mix of species in a given area has an impact on air quality. Building on new science, this Article argues that choosing the right trees can improve air quality. Governments should encourage the planting of trees that clean the air, while discouraging or restricting the planting of trees that contribute to air pollution. Many cities are already encouraging tree planting for a variety of environmental and other benefits, but planting the wrong trees will worsen air quality.

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