Technology When trees pollute more than cars

When trees pollute more than cars

Los Angeles, April 2019 (FREDERIC J. BROWN / AFP)

Trees produce oxygen, bring us freshness, but also pollutants. At a time when many cities want to plant trees to adapt to climate change, it is better to choose them! In a study published last month, researchers at the University of Berkeley in California, showed the responsibility of the vegetation of the city of Los Angeles in the spikes of pollution.

In the city, drastic measures have made it possible to lower polluting emissions from vehicles over the past twenty years, such as that of benzene, or those of fine particles, divided by two. But they didn’t understand why there were always high pollutant levels on hot days. Indeed, Los Angeles no longer knows its pollution mists of the 1980s, except when the temperature exceeds 37 °. Ozone pollution then becomes a real problem.

Are trees involved in pollution peaks? In part, since it is because of a combination of factors. Trees emit volatile organic compounds like isoprenes, which are like little rubber balls. Combined with gases from exhaust pipes and agricultural emissions, this forms pollutants that irritate the eyes and lungs of residents on hot days. Researchers counted that 18 million Mexican palm and oak trees were responsible for 25% of aerosol emissions in the city on hot days. Vegetation therefore has a non-negligible responsibility in these pollution peaks, which are problematic for health.

Cutting everything would be too simple and rather counterproductive. Trees still produce a lot of oxygen, they filter certain pollutants, they cool the city with their shade and are shelters for birds, among others.

But the researchers think that we need to diversify the plantations more. This message concerns in particular the cities which plan to vegetate to adapt to climate change, or even to plant urban forests, as in Paris or Lille. German researchers had already looked at more or less emitting species in 2014. Those to be avoided in order not to aggravate the pollution are oaks, plane trees or poplars. On the other hand, it is better to favor conifers such as yews, hairy trees such as beeches or even small-flowered tamarisk.

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