Urban planners have a balancing act to perform.

On the one hand, green spaces in cities provide an obvious low-tech way to help remedy the growing problem of air pollution. But on the other, they are costly to maintain and space-intensive when inner-city land is at a premium.

Image: Green City Solutions

So a single ‘tree’ that has the environmental benefits of a small forest has the potential to be a game changer - and that’s exactly what CityTrees are hoping to be.

Created by start-up Green City Solutions, the freestanding, vertical units are covered in moss and lichens which have a massive surface area to absorb particulate matter, nitrogen dioxide and ozone.

Image: Green City Solutions

The trees’ inventors estimate that a single structure has the environmental benefits of 275 urban trees, but taking up 99% less space and at just 5% of the cost.

Growing on units four metres high and three metres deep, CityTrees are powered by solar panels and collect rainwater in troughs. They also require little maintenance: via the internet of things, soil humidty, nutrient levels and general plant health are monitored and adjusted accordingly.

The carbon footprint of producing a CityTree is four tons, which is more than offset in the space of a year: the manufacturer estimates each unit will capture 240 tons of carbon dioxide annually.

So far, the product has been rolled out in several cities in countries including Norway, France, Germany, Belgium, Macedonia and Hong Kong.

Although each tree costs around $25,000, the cost can be mitigated by the potential for advertising on the trees via logos and screens on the unit.

The invention is part of a growing trend towards using technology to tackle pollution.

From air vacuum cleaners to smog diamonds, innovation to combat the problem is at the cutting edge of science.