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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.