How Urban Trees Cause Air Pollution During Heat Waves

City trees in the German capital Berlin

Who would have thought it? Trees are able to worsen the air quality in cities. We may have read several studies proclaiming the benefits of urban greenery. Today we become acquainted with the downside of leafy shrubs and trees. Berlin-based scientists found out that urban trees cause more air pollution in case of a rising ambient temperature. Although it has been known before that there is an existing potential of emissions from urban greenery in the presence of anthropogenic emissions, it is kind of sobering to hear these outcomes. This relationship is particularly important during hot summer days or urban heat island effects as it leads to a higher concentration of ozone and particulate matter.

Urban Trees Cause Air Pollution Which Increases With the Ambient Temperature

During their investigation in 2006, Berlin suffered the impact of a heat wave. Ideal conditions to examine the influence of temperature on urban air pollution. The recently published results show that by plants released volatile organic compounds (VOC) undergo a chemical reaction with, for example, car exhaustion.

Contribution of Biogenic VOC Emissions to Ozone Formation Increases up to 60% During Heat Waves

The higher the temperature, the higher the VOC quantities emitted by green leafs. In this case, chemical reactions with nitrogen dioxide (NO₂), emitted by conventional driven cars, are rising. The end products are ozone and particulate matter (PM) affecting the heart and lungs and causing serious health effects.

The scientists at the Institute for Advanced Sustainability Studies and Humboldt University point out the necessary reduction of conventional motorized vehicles or, more in general, the anthropogenic sources of NOx, VOCs, and PM. They, of course, don’t recommend to reduce the numbers of trees in urban agglomerations. The study also makes very clear that sustainable urban development is a complex task. Many interactions must be considered. Holistic approaches and interdisciplinary considerations can be essential.

For their investigation, the scientists were using the Weather Research and Forecasting Model with atmospheric chemistry (WRF-Chem) and a computer program called MEGAN. This air modeling software enables to estimate the emission of gasses and aerosols from terrestrial ecosystems into the atmosphere.