Thanks to Cool Green Science for this:
BY ROB MCDONALD
Every report has a genesis, an initial conversation that sprouts an idea that grows into a research study. For me, one of those moments was a phone interview I had with a professor at King’s College in London, about the somewhat goofy idea of gluing pollution to roads.London, like a lot of cities, struggles with its air quality, especially having too much particulate matter (PM) in the air. PM is essentially small particles (dust) that can be inhaled into the lungs, where they do major damage, contributing to asthma, strokes, and heart attacks. Outdoor PM causes 3.2 million deaths every year.
London got so desperate to get their PM concentrations down (which they needed to do to avoid fines from the European Union) that back in 2012 they were experimenting with spraying sticky substances on roads, and hoping that enough PM stayed there to remove atmospheric concentrations. It didn’t work too well, apparently.
During the conversation, I brought up the role of trees in potentially reducing PM concentrations. A lot of empirical studies in particular sites had reported a reduction in PM downwind of trees, which essentially function like giant filters. PM will settle out on the leaf surfaces, just as dust will settle out on furniture in your house. The professor, who was trained in environmental health, was a bit dismissive of the potential for trees to meaningfully clean the air. To be convinced they might have a role, he would have to see evidence that trees had enough scope to make a difference, and that tree planting was cost-effective , relative to other ways cities try to remove PM (e.g., putting scrubbers on smokestacks).
Now, here it is four years later, and The Nature Conservancy is putting out a new report, entitled Planting Healthy Air, that tries to answer these questions. The Planting Healthy Air report, done in collaboration with the C40 Climate Leadership Group, tries to estimate the potential for trees to clean and cool the air. For 245 major cities globally, we evaluated the current and potential future removal of PM by trees, as well as the mitigating effect on temperature.
Trees cool the air in two main ways. They cast shade that keeps the sun’s rays from hitting concrete and asphalt, which can absorb and later release heat (the so-called urban heat island effect). They also transpire water, which cools the atmosphere, just as you feel cooler on a hot day when your sweat evaporates…
Read the whole article here.