A recent study by a Berlin-based team of scientists has revealed that during heat waves, trees in a city can actually contribute to higher levels of air pollution. As many cities around the world become greener with plant-clad towers, vertical gardens and tree-filled urban developments, the study highlights the need to take into account the location and types of greenery being planted.

We know that there are significant benefits to planting trees in cities, from their ability to store carbon to the way they keep urban areas cool, but we also know that trees can release more than just oxygen into the atmosphere. Trees and other plants are known to release volatile organic compounds (VOCs). In the presence of sunlight, VOCs and nitrogen dioxide (NOx) react to form ozone, a key pollutant that when produced at ground level causes smog and respiratory problems in humans.

Galina Churkina and her team at the Institute for Advanced Sustainability Studies and Humboldt University, used computer models to study air pollutant concentrations in an area of Berlin. They looked at the data from a heat wave in summer 2006 and compared it to a more typical seasonal stretch in the summer of 2014. Knowing that plants release higher levels of VOCs when it is hot, the team was looking at whether those areas of greenery in the city showed higher levels of ozone formation during warmer periods.

The team concluded that ozone levels did indeed spike in areas of urban greenery during the heat waves. Their simulations estimated the VOCs contributed to anywhere from six to 20 percent of the ozone formation during the heat wave, with spikes of up to 60 percent at some points.

The study highlights the hidden complexities involved in managing our urban air environments. Simply planting more trees can sometimes result in increases of certain types of air pollution, despite the other benefits of greener urban spaces.

The solution of course, is not to stop planting trees but rather make sure several factors are considered as we further develop our city spaces. Churkina and her team have identified that certain species of trees emit greatly reduced volumes of VOCs – birch trees, for example, release over 15 times less VOCs than black gums.

Where to locate high-densities of trees needs to be considered as well, such as avoiding planting a high-VOC emitting species such as black gums near high-traffic streets.

But the biggest solution to this counterintuitive conundrum is to simply work on reducing NOx emissions, which should decrease as the numbers of electric vehicles on the roads increase. The broader benefits of bringing in greenery to cities is evident, so a goal of reducing emissions from vehicular traffic should be paramount, and also conducted in tandem with any urban tree-planting campaign so as to maximize the environmental benefits rather than creating ozone-producing smog traps.

The team's research was published in the journal Environmental Science and Technology.

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