Narrator: Listen to part of a lecture in an environmental science class. Professor: Today, let’s talk about a surprising factor in urban pollution: trees. Now, you might think trees always improve air quality, and in many ways they do—they absorb carbon dioxide and release oxygen. However, research shows that in certain conditions, trees can actually worsen ground-level ozone pollution. This happens because trees emit volatile organic compounds, or VOCs, like isoprene. When these VOCs react with nitrogen oxides from car exhaust in the presence of sunlight, they form ozone. So, in a city with heavy traffic, planting certain tree species could increase smog. But not all trees are equal. Oaks and poplars emit high levels of VOCs, while species like maples and pines emit much less. Urban planners must therefore choose tree species carefully. They also need to consider the local climate and wind patterns. In a hot, sunny city like Los Angeles, the ozone reaction is more intense. So, the key is to balance the benefits of shade and carbon storage with the potential for ozone formation. This makes urban forestry a complex puzzle.
❓ Question What is the main point the professor makes about trees in urban areas?
A. Trees always improve air quality by absorbing pollutants.
B. Some trees can increase ozone pollution due to VOC emissions.
C. All tree species emit the same amount of VOCs.
D. Urban planners should plant more oaks to reduce smog.
✅ Correct Answer
Some trees can increase ozone pollution due to VOC emissions.
TOEFL Listening Tips
Skim the questions first to predict the content
Watch for signal words in lectures and conversations (however, actually, in fact)
For main idea questions, focus on the beginning; for detail questions, focus on examples and data
For attitude questions, pay attention to the speaker's tone and word choice
Take notes on keywords; don't try to write down every word
