Are Some Trees Adding to Urban Pollution?

New York, NY (June 1, 2014) – A recent article in Scientific American reports on research out of the Institute for Advanced Sustainability Studies in Potsdam, Germany that describes how some trees and greenery can actually raise the ozone level. Here’s why and how this might impact cities with ambitious tree planting campaigns.

poplar trees in cityAlthough trees produce oxygen, some, like a poplar or black gum tree, can also release compounds that can react in the air to create lung-damaging ozone.

According to Galina Churkina, a senior fellow at the Institute for Advanced Sustainability Studies in Potsdam, Germany, who studies urban tree emissions, when certain trees dominate a street they can raise the ozone level considerably. At ground level, ozone is an oxygen molecule that is linked to asthma, bronchitis and other respiratory illnesses.

Similar to vehicles and power plants, trees emit airborne chemicals called volatile organic compounds (VOCs), which in the presence of sunlight react with nitrogen oxides in vehicle fumes to form ozone, one of the components in smog that makes it a health threat.

Trees emit VOCs in part to repel insects and to attract pollinators. Species such as birch, tulip, and linden release very low levels of VOCs, but others such as black gum, poplar, oak and willow produce a lot, leading to ozone levels that can be eight times higher than those linked to the low-impact trees.

This table lists average VOC emission rates for several popular urban trees under standard conditions of temperature and light. The unit for measuring VOC emissions from trees is micrograms per gram of leaf mass per hour.

Tree species Avg. VOC emissions rate (μg *g-1*hr-1)
Black gum 77
Poplar 70
Cedar 11
Birch 5

Standard conditions is defined as 30 degrees Celsius, with photosynthetically active solar radiation of 1000 mol*m2*sec-1. (Data courtesy of Galina Churkina/Institute for Advanced Sustainability Studies)

Because sunlight is needed to form ozone, and the reaction is more vigorous at higher temperatures, cold, cloudy cities have fewer worries than warm, sunny ones. The ozone production process will, however, intensify with climate change, since the warmer it is, the more VOCs the trees will emit.

So should cities stop planting the top tree emitters? No, according to Churkina. Even the worst offenders are not a concern if they are scattered on city streets. Understanding that a linden tree is better than a poplar, however, can help metropolitan areas avoid problems. In urban areas with large tree planting campaigns this should also be a consideration in what tree species to select. And since VOCs need NOₓ to form ozone, cities should probably avoid planting high-emitting trees along streets with heavy traffic.

The Paradox of Pollution-Producing Trees,” Scientific American
Not All Tree Planting Programs Are Great for the Environment,” CityLab