Entrepreneurs Fight Air Pollution With CO2-Reducing 'CityTrees' (cnn.com)

CNN is reporting on "CityTree", a unique 10-foot tall mobile installation which removes pollutants from the air." An anonymous reader quotes their report: Berlin-based Green City Solutions claims its invention has the environmental benefit of up to 275 actual trees. But the CityTree isn't, in fact, a tree at all -- it's a moss culture. "Moss cultures have a much larger leaf surface area than any other plant. That means we can capture more pollutants," said Zhengliang Wu, co-founder of Green City Solutions.

The huge surfaces of moss installed in each tree can remove dust, nitrogen dioxide and ozone gases from the air. The installation is autonomous and requires very little maintenance: solar panels provide electricity, while rainwater is collected into a reservoir and then pumped into the soil... "We also have pollution sensors inside the installation, which help monitor the local air quality and tell us how efficient the tree is." Wu said. Its creators say that each CityTree is able to absorb around 250 grams of particulate matter a day and contributes to the capture of greenhouse gases by removing 240 metric tons of CO2 a year... Wu also argued that the CityTree is just one piece of a larger puzzle. "Our ultimate goal is to incorporate technology from the CityTree into existing buildings," he said.
So far they've installed 20 CityTrees -- each of which costs about $25,000.

Comments

Silly

[XXongo]

$25000, for moss?! How silly. They should have just planted trees.

By the way, surface area is irrelevant if there isn't air flow past the surface, like there would be for an actual tree.

Re:Silly

[Wycliffe]

So the day came and they released all those plastic balls and the TV cameras were rolling and what did we see? BLACK BALLS. Black plastic balls rolling into the water.

No one involved with the project had the foresight to consider the color of the balls. Black balls absorb a lot more sunlight and get hotter and increase water temperature, leading to more water evaporation. It would've been trivial to add white pigment to the plastic balls and the cost difference would've been negligible.

This is the problem with being an arm chair engineer. Do you have any proof that white balls would have been better? Besides preventing evaporation, the other goal of the project was to block sunlight and UV rays to prevent the formation of Bromate. The reason that black balls are warmer is that they absorb more sunlight than other colors. Opaque white balls might have been more effective or coating the black balls in something reflective before releasing them (which may add to the production cost and/or durability) but neither you nor I would have any idea without a lot more data.

Re: CO2 is a global problem, not a city problem

[Entrope]

Particulate emissions and NO2 levels are largely local problems, especially when you look at health effects versus distance from source. You should want to clean up air in cities because there are lots of harmful emissions there, and because a lot of people live there. Especially in developing nations, along with China and India, it's wildly expensive to adopt the kind of environmental controls that the US and Europe use. I'm not sure that this gadget provides $25,000 worth of benefit, but I agree with the overall idea.

Where does the carbon go?

[sbaker]

Photosynthesis does: CO2 + Water => Sugar + O2...then the plant takes that sugar and turns it into biomass by converting it to starches and structural materials for the plant itself.

Carbon has a molecular weight of 12 and Oxygen is 16...so CO2 is 25% carbon by weight. So to absorb 240 tons of CO2 per year - it's got to be generating (at a minimum) 60 tons of extra plant material per year - and more likely (because dead/living moss isn't all carbon) it's at least twice that.

There is only just so much space in that concrete container - which means that a literal truckload of dead/living moss has to be removed from it every single week! Then, that biomass has to be disposed of in some way that doesn't simply re-release it into the atmosphere when it decays...you'd have to bury it or something.

This is a ridiculous claim - it can't possibly be true. Even 24 tons a year wouldn't be credible - and 2.4 tons a year would seem high...the entire installation would haves to double in size every year to keep up even that more modest amount.

What I'm sure happened here is that it's plausible that the moss has vastly more surface area than a tree - but moss is much more slow-growing than trees are - so the amount of CO2 it absorbs cannot possibly be as much per-unit-area as the leaves of a tree. So I'm betting that they did all of their math from surface area alone - and didn't stop to think beyond that.

This is B.S.