City trees may worsen air pollution in hot weather

Hotter summers and more frequent heatwaves due to global warming are likely to worsen urban air pollution from the greenest of all city features: trees and other vegetation.

While many studies have highlighted the multiple benefits of urban greening – it reduces temperatures, controls storm water, sequesters carbon, and improves physical and mental wellbeing – a team of German researchers suggest that future greening programs need to more fully account for the exposure of vegetation in urban areas to the urban heat-island effect as well as human sources of air, soil, and water pollution. 

In particular, the researchers, led by Galina Churkina of the Institute for Advanced Sustainability Studies, in Potsdam, warn of pollution stemming from the higher quantities of volatile organic compounds (VOC) released by vegetation during hotter weather.  

When these VOCs interact with human-caused nitrogen oxides – such as the nitrogen dioxide (NO₂) emitted by the combustion of fossil fuels in motor vehicle engines – the chemical reactions can lead to ozone and particulate matter, both associated with breathing difficulties and aggravating conditions such as asthma and bronchitis.

The research team’s warning is based on studies of air pollutant concentrations in the Berlin area. The German capital is one of the greenest cities in Europe, with forests, parks and agriculture covering a third of its area. 

Using computer modelling to compare pollutant concentrations between heatwave conditions in the summer of 2006 and more typical summer temperatures in 2014, the team’s simulations show VOCs from urban greenery contributed to between 6% and 20% of all ozone formation, spiking to 60% during the 2006 heatwave.

While plants release hundreds of different VOCs, the researchers note in their study, published in Environmental Science & Technology, that only a few have a substantial effect on air quality. 

“The most important reactive biogenic VOCs are isoprene, monoterpenes, and sesquiterpenes. Emissions of isoprene mostly contribute to the formation of ground level ozone, while monoterpenes and sesquiterpenes can lead to an increase in particle number and mass.”

The paper also notes that while the new results align with other studies (covering Asia, Europe, and North America) pointing to the potential of urban trees worsening air pollution through VOC emissions, “other studies of the same phenomenon complicate the picture”. 

The results don’t amount to a call to cut down trees, or indeed to halt urban-greening initiatives. Rather, the researchers note that tree-planting campaigns simply need to be accompanied by traffic reduction in order to truly benefit urban dwellers.

Sarah Bekessy, of the Centre for Urban Research at RMIT University, in Melbourne, reinforces the need for caution.

“When it comes to sustainability strategies there are rarely any simple solutions that have no downside, but in this instance urban greening has so many benefits that dwelling on the negatives would be utterly ridiculous,” says Bekessy, who is also a member of the Australian Research Council’s Centre of Excellence for Environmental Decisions.  

“Planting trees in cities is one of the best ways of to lower urban temperatures. Trees within cities can sequester as much carbon from the atmosphere as a tropical forest; and numerous studies have shown the benefit of urban greening as a public health intervention.

The sensible options, Bekessy counsels, are to look at the types of vegetation used in urban environments, and at the types of city design more susceptible to air pollution: “Careful planning and regulation for urban design, and guidance around the right type of trees and vegetation to plant could mitigate the problem.”