Nghe TOEFL

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Narrator: Listen to part of a lecture in an environmental science class. Professor: Today we're going to talk about a surprising but important factor in urban pollution: trees. Now, we often think of trees as beneficial, and they are, but they can also contribute to air quality problems. Specifically, certain species of trees release volatile organic compounds, or VOCs. These VOCs, when combined with nitrogen oxides from car exhaust and sunlight, form ground-level ozone, which is a major component of smog. So in cities with heavy traffic, planting a lot of these trees can actually increase ozone levels, especially on hot, sunny days. For example, in Los Angeles, researchers found that certain types of oak trees were significant contributors to ozone formation. However, this doesn't mean we should stop planting trees. Instead, city planners need to choose tree species that emit fewer VOCs, and place them strategically. Also, reducing car emissions remains the most effective way to tackle smog. So, the key is not to vilify trees, but to understand the complex interactions in urban ecosystems.