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.
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.